The Joule Smasher LED Flasher - 6 to 8 years from a single AA

First please excuse me for a heading that sounds like click-bait. I'm just very excited at the results to data and wanted to share progress with you.

It is important to remember that during the entire life of the battery there is no change in the rate or the intensity of the flashing. No feeble blinking allowed in my projects!

The Joule Thief was a wonderful circuit for getting light loads to operate from batteries that were essentially flat. I built many of them and was amazed at the results. However, I stumbled across boost convertor IC's which cost cents and delivered even better results for no engineering effort on my part.

Recently I tried a variety of circuits for flashing a LED. My need was to be able to see if the gate to our property had been closed at night without walking all the way there. I didn't want to turn on the driveway lights because one of the benefits of going outside with the dog before bed is the night sky. The driveway lights detract from that. And where's the fun in painting a black gate white!

Testing showed that using a micro to flash a LED was way more efficient that discrete versions of LED flasher circuits.  Building on this example I made some code changes to enable pull-ups on unused inputs and adjusted the timing to suit my needs.

Instead of powering this from a 3V battery I used a AA battery and a boost convertor. It was obvious from the start of testing that this was going to be a step change in how long a AA battery could be made to flash a LED. Several years looked possible so I used an AG3 battery to test the concept.

The quoted capacity of an AG3 cell varies from 25 to 35 mAh. Based on earlier measurements I expected an average current draw of roughly 100uA, say 250 hours or 10 days. I used a brand new cell that was several years old from an unknown Chinese battery manufacturer. Given my contempt for most things purchased via AliExpress I would have been satisfied with 5 days given my expectation of 10 days.

After 15 days the battery voltage was still 1.205 volts. It appears from extrapolating the measurements that the AG3 will will last around 25 days.

I have a Joule Smasher Led Flasher running on a brand new AA battery. That battery still reads 1.555 volts after 15 days. It's a wild guess but if the 25-35mAh capacity of the AG3 is comparable to the 3000mAh of an alkaline AA then the Joule Smasher should flash the LED for 6 to 8 years. That exceeds my wildest expectations and is far longer than I expect to be living at this property. 

There is still room for improvement. I'm boosting the battery to 3.3V because at the time of building that was the part I had to hand. So I'm wasting a significant amount of energy in the current limiting resistor in series with the led. That might represent an increase of 20% in battery life.

Sometime next year I'll build a few more of these to confirm results and check the benefit of boosting to 3V or 2.7V.

Li Ion Battery Charging - Preliminary Test Results

Recently I blogged some guidelines for Li Ion battery use. I have ignored the "Store at 50% charge" folklore I read about everywhere because I store in the charger at a terminal voltage around 4v per cell. No research appears to exist on how harmful this is. But then I couldn't find research that supports storing at 50% compared with say 90%. 

Page 10 in this document has graphs which suggest charging to 4.0V represents a battery capacity of around 80% and a substantial increase in the number of charge discharge cycles available.

Still, I wondered if there was any degradation. To see if this could actually be tested I ran a trial on a few rubbish cells to see if anything happened. The trial is not as rigorous as an academic paper, but reflects my real world use of these batteries.

The method is:

• Each morning remove a battery and discharge to a fixed voltage. 
• Record the capacity
• Put battery back into charger in the same slot.
• No attempt to control temperature or other variables. 
• Some batteries do not get tested every day simply due to time constraints. These sit in the charger at the nominal terminal voltage of 4V.

I say 4.0V but I actually adjusted the charger circuit to be 3.95V for Battery A and 4.05V for the Battery B.

As I said - rubbish cells. Brand new 18650 from China but had been sitting in storage a while. Over the course of the trials I decided to reduced the discharge current since 450ma was more than the capacity of the cells being tested. I also learned there is relatively little energy between 3.1 and 3.0 volts so I adjusted the threshold to 3.1V after 18 days.

 

Battery B initially appeared defective. It seemed to terminate discharge early and I twice had to re-start the trial on that day. With the drop in discharge current the battery magically had reduced variation in each trial and each test ran to the terminal voltage.

Since I haven't seen evidence of degradation after 24 days I will in future check less often. In between trials the battery will remain in the CC/CV charger at the nominal 4.0V.

Led Flasher - Could it really be 4 or more years from a single AA batery?

So I built the ATiny13A led flasher and achieved an average current draw of 30uA over several hours by measuring the discharge of a 0.78F capacitor. This is around 25% of the discrete version.

 

Which is crazy compared to what I was expecting. After poking and prodding I determined that some of the difference was due to not allowing for the internal resistance of the pin driver on the ATTiny13A. Instead of getting 20mA I was getting 7mA of current through the LED. However, this did not explain all of the difference between what I measured and what I expected.

With some more tweaking of software and hardware I achieved a LED that flashes for 10ms every 2 seconds drawing 10mA. That's the rated maximum for a single pin. I was thinking of using two pins in parallel but it seems bright enough for my purposes.

As a sanity check I put a new but several year old AG3 battery onto one of these Tiny flashers on 9:30am on Dec 6th. I believe the battery was rated at 25mAhr when new. I expected this battery would  run the led for 10 days.  Note the makeshift button cell holder below.

 

However, after 10 days of operation it appears there is still substantial life left in the battery. 

   Day  Voltage   
    6        1.35           
    8        1.32           
    9        1.31
   10       1.30    
   11       1.28

I can't explain the observed result which is way better than I expected. What was supposed to be a sanity check has sent me back into my spreadsheet looking for an error. Wherever the error lies I am amazed that the AG3 cell looks like it will run the Tiny Flasher for 20 days.

It would seem that based on the range of outcomes at this point the Tiny Flasher will run for around 4 years from a single AA battery.

 

The only way to be sure is to monitor the one running on a fresh AA battery to see what happens. 

More on this in 2027?

Some notes on Li-ion battery charging

I've used lithium ion batteries in many projects for several years without mishap. I never realised how much mis-information exists in relation to these batteries. Claims such as "You have to cycle the battery 5 times before it reaches it's rated capacity" do nothing but demonstrate how stupid the person is making such a claim.

Many years ago I sighted research which showed that if you can keep a lithium ion battery between 80% and 20% of it's capacity it will last essentially forever. Now I question if that was real research or folklore. Consider that one of my laptops a decade ago actually had a bios setting that allowed you to only charge the battery to 80%. Like all "information" in this field I don't know if this is still, or even was, valid.

Guidelines

However, I have a few guidelines that have not let me down:

• Never fully charge 
• Never discharge below 3.1V (revised from 3.0V after measurement)
  
• charge at C/10 or less.
  

BRCL4054CME

When I have to package the battery and charging circuit into a small enclosure I reach for the BRCL4054CME. Simple, reliable and effective. However, like every charging IC I have looked at it is cumbersome to terminate the charging voltage at less than the designed limit of 4.2V. So when I use this IC I generally don't satisfy the first guideline. 

The second guideline is satisfied by shutting down the circuit using voltage detection and third by selecting the appropriate resistor.  Simple but not quite the complete solution. And it limits you to charging from a 5V source.

Preferred Solution

Where space permits the following circuit is the basis of my preferred approach.

 

Any DC source from 8V to 30V can be used and it has survived mishaps. In my multiple single cell charger I now include a mosfet to isolate each battery during a power outage. That stops discharge through the series resistors that set the voltage. It also means where fitted I can simply switch off the power when leaving the workshop.

My longer term goal is to have enough batteries sitting charged that I only need to switch the chargers on when a cell has been used in a torch or project and needs recharging. I plan to write up the finished project once my battery testing is finished. Preliminary findings will be blogged in a few days or so.

I never had to consider extended power outages before but on reflection of the increasing natural disasters in the world it seems prudent to keep as much charge in the battery  as possible without doing harm until it is needed.

Led Flasher - Some Preliminary Results

 To test the boosted AA flasher concept I used the following methodology:

1. Charge capacitor to around 1.5V
2. Connect to a 3.3V boost circuit
3. Load 3.3V output with a 68k resistor (simulates the 48uA average current draw of concept)
   
4. Measure voltage at periodic intervals
5. Calculate average current drawn

I had previously estimated the average current drawn from the AA battery as 140uA when boosting to 3v.

The results I obtained were quite consistent with this until the voltage dropped below 1V. The divergence after 48 minutes is likely to be the fixed assumption for efficiency. As the input voltage drops more current is required and the losses increase. 

Still, not bad for a thumb suck.

The planned 4ua sleep versus brief 20mA flash may give a different result due to the boost circuits efficiency varying with load. 

It appears this approach will deliver a flashing led with around 3 times the life of the discrete version I built. The next step is to build this prior to Christmas.

Led Flasher - Power Budgets

Here we have a real change of focus. No RF to be found!

I was fishing around in my junk box recently and stumbled across a LM3909 Led flasher I built some 35 years ago in high school. It got me thinking about how one would approach this today. 

There are lots of interesting links if you search for them. It certainly got me thinking.

A 1Farad capacitor proved to be 0.87F after measuring the voltage change while charging through a 8.2k resistor. The capacitor was then charged to 1.5V and the led flasher connected(circuit below). After 10 minutes the voltage had dropped to 1.027V giving an average current draw of 700uA. 

 

 

The fact that a slower flashing version drew slightly more current surprised me. In comparison the LM3909 has a typical current drain of 500uA flashing at 1Hz. 

My suspicion is I can do a lot better than this. There are some claims out there for current draw so low that I question the basis for them. The circuit above can easily be seen 50m away at night. Flashing a LED that can only be seen when it's resting on your eyeball is just nonsense. Rant over.

One of the interesting concepts I saw was using a micro to flash the led. A credible site with useful information on how to do this can be found here. František's approach to calculating the power required agree with mine.

I expect that a micro pulsing a led would consume around 44uA at 3V. 

 

Time On	0.004	secs
Flash Rate	2	secs
Time Off	1.996
Led Current	20	mA
Average Load	0.000040
Micro Sleep current	0.000004
3V current	0.000044

 

With the overhead of a 1.5V to 3V boost circuit the total current draw from a AA battery could be around 140uA.

The Energizer datasheet shows at this low drain around 3000mAh of capacity can be obtained down to 0.8 Volts. Since the boost convertor I plan to use can keep running below 0.8V the life might be slightly higher. Perhaps 2.5 years is possible?

The heat of summer has not started so I'll see if I can test this concept in coming weeks.

10W Class AB with parallel transistors - Almost

Update: I've learned that the cause of my waveform distortion (sharkfin) is most likely the presence of harmonics.

Time for a second attempt. The board was mounted to a CPU heatsink, milled for clearance underneath as necessary. Cautiously I increased the drive. I found I could easily achieve over 10W from 2 - 20MHZ. The distortion on peaks is evident so further work is needed. 

By testing with a single tone I am being harsh on the amplifier.  A two-tone test signal delivering 10 Wpep would have 2.5W in each tone. At 4W with a single tone there is no obvious distortion. I must drag my two tone generator out of retirement.

However, this JBOT approach with parallel SOT89's has merit. The distortion reminds me of what I saw with the 1W class A experiments when the biasing wasn't quite right. Even with 80mA of standing current in each transistor the distortion at 10W was present.

It appears a cheap, stable and effective 10W HF amp can be realised with parallel SOT89's. The application of a two tone test signal is needed before this particular transistor model is condemned.

73's

 

10W Class AB with Parallel transistors - Initial Success

Having drafted a board with parallel SOT89's I sent the files off to have it fabricated.Upon receipt of the boards I found many errors and it transpires I generated the gerbers from an early draft which had not been finished. 

Out came the tools to manually fix the board and I populated the first board as a proof of concept. I ran out of 1210 resistors which I have been using as thermal bridges so I finished the board with 1206 resistors instead.

In my last blog I stated:

Since 2 of these in parallel class A could easily deliver 1W continuously it is reasonable to expect that 10 of them, operated class AB in push-pull, could deliver 10W. 

On reflection it would be reasonable to assume 10 devices could deliver at least 5W. I quickly found I could achieve over 10W but with some distortion. I also quickly found the smoke can escape.

Here is the board mounted on standoff above a heatsink which has a 50Ω resistor mounted to it. All of the emitter ballast resistors had burnt out. 

Presumably one transistor got too hot, developed a collector to emitter short which burnt out one resistor. Now 9 transistors were carrying the load, another transistor failed and resistor burnt out. Now 8 transistors.....Apologies to anyone with "10 in the bed and the little one said, roll over, roll over" now running through their head.

I will have to modify a second board and try again.

73's

Richard

10W Class AB - Next Steps

So I checked, and re-checked, every transistor I could lay my hands on. Nothing was suitable for 10W. Nor does it appear anything can be purchased that might be suitable that has not been tried.

Several transistors in plastic encased TO220's worked fine at a few watts but couldn't cope beyond that.

From here I have stopped looking for a general purpose transistor for a single-ended  amplifier.

A board is being layed out with parallel SOT89's. Since 2 of these in parallel class A could easily deliver 1W continuously it is reasonable to expect that 10 of them, operated class AB in push-pull, could deliver 10W. 

Something to be put to the test in the near future.

73's

Richard

10W Class AB amplifier - Modest Success

After letting the smoke out of yet more transistors I finally managed to get a repeatable 6W out of the amplifier. I made two changes to the amplifier:

1. The output transformer was changed to a larger binocular core, and
2. The input transformer was changed to a 9:1 transformer instead of a 4:1 transformer.

The first change appears to have resolved the issue where a few amps was reducing the return loss. See the post here for what was happening.

After popping yet another transistor I suspected the voltage across the base with the 4:1 transformer was exceeding the Vbe rating of the transistor with fatal results. With the new transformer I have yet to pop the transistor.

However, I still can't achieve a continuous 10W because the transistor, which I believe is rated for 17W,  starts shutting down as the junction temperature rises. I have a large finned heatsink so the cause of this is unknown at present. 

Note that I am measuring power using a single RF frequency, or carrier. Thus the 6W is equal to average power. This gives me hope that with a two tone test signal 12W PEP is possible.

Why the modest label? My goal was to achieve 10W PEP with a general purpose transistor. I had to use a RF transistor and at present I haven't measured 10W PEP though I believe the amplifier can achieve this with a two-tone test signal.

The best of the general purpose transistors available today that I tested was a 2SC6144 which achieved 4W with a single frequency on 80m. However, this was with the smaller output transformer so the full potential of this device has not been tested. If you try one please let me know what you achieved.

There are some obsolete transistors mentioned here that also delivered 4W continuous which shows the junk box can be a useful source.

73's

Richard

10W Class AB Amplifier - More ferrite needed?

 So I replaced the transistor and connected a 5.5Ω in series with a 100nF cap across the transformer. I then used the nanoVNA to look back to the collector from the output port. Varying the collector voltage from zero, and then 8.8v to 16v showed almost no shift in the return loss.

However, adjusting the bias for 2A of collector current really shifted the return loss curve.

Idle current - bottom curve    2Amps top curve

Given the collector voltage was largely unchanged, the transistor capacitance from the collector to base and emitter would have been relatively constant. So the only explanation left is the current changed the transformer's characteristics. 

Next step is to replace the transistor yet again (smoke escaped) and try a larger transformer.

73's

Richard

10W Class AB Amplifier - Further Development Results

To test the conventional 9:1 transformer I blogged here I revised the board. The populated board is shown below:

With scars of many replacement transistors

I started with a 2SC1306, a once common CB output transistor. The smoke escaped all too quickly. I then tried a few general purpose transistors with unsatisfactory results.

One of the key things I noticed was that as the output was increased many transistors had a threshold. When that threshold was exceeded the output quickly dropped. If the bias was too high then thermal runaway also occurred.

The presence of this self-heating effect was noted in all of the transistors I tested, sometimes resulting in transistor failure. TO220 packages were better than TO126 packages presumably because they have a lower thermal resistance.

With only a few 2SC1306 left I switched to a 2SC2146. I was able to get close to 4W (40Vpp) but  eventually the smoke escaped.

I'm going to replace the transistor and see if a larger transformer is needed. 

73's

Richard

10W Class AB HF Amplifier - Initial Testing

I mentioned in this post that while the 2:1 transformers were successful, they still needed to be tested at higher power levels. This post covers that testing and some initial observations on transistors to be used in this amplifier stage.

10W(?) HF Test Amplifier using a CPU Heat Sink

Above is a picture of the amplifier using the two transformers I tested back to back in this post. At 4W continuous the transformer used in the output had no issues at 80m. I didn't get a chance to try other frequencies for an extended period because I was too busy letting out the smoke!

The initial transistor tested was a 2SC1096. From the datasheet we read:

·For audio frequency power amplifier and low speed switching applications
·Suitable for output stages of 3 to 5 watts car radio sets and car stereo

I expected this to work well at 80m and 40m given the Ft of 65MHz and it did so. Gain decreased with increasing frequency but across 160-15m I was able to achieve 4W continuous by adjusting the drive. 

2SC1096 Initial waveform at 1W output on 80m

 

The initial waveform at 1W output on 80m (above) suggested an increase in standing current would beneficial. I pushed the output up to around 40Vpp. 

2SC1096 4W Out

At this point I was not entirely happy with that downward sloping waveform after the positive peak of each cycle. But I pushed the bias too far and popped the transistor. However, my initial goal of testing the bias circuit and 50:12.5Ω transformers was achieved.

I replaced the transistor with a 2SC1226, which is broadly similar to the 2SC1096 i.e. it's not a RF transistor. Again I could get 4W easily.

2SC1226 at 4W Out

Of note is the initial downward sloping part of the waveform is around 0.06uS long, against the 0.08uS of the 2SC1096. Which sounds better though I suspect this is still not ideal.

The 50:12.5Ω transformer was replaced with a Ruthroff Type 9:1 on Small Binocular Ferrite. This gave me the potential to output more than 4W. I could still get 4W or so out but increasing the drive popped the transistor.

So progress has been good. I might have popped a couple of junk box non-RF transistors but I established the development board, biasing and transformers are all working.

Next time I will replace the transistor with an RF transistor so I have something to compare a number of readily available general purpose transistors against. 

73's

Richard

9:1 Conventional RF Transformers 50:5.5Ω

I started with one of these binocular cores simply because it was already on the bench. It's also enjoyable to get something that has no data to work!

Using the braid from a piece of miniature coax cable as a 1 turn winding, I teased two holes in the closed end so I could wind 3 turns through the inside of the coax braid.

1 turn of braid, 3 turns inside braid

Good for 80m to 10m. What struck me some time later is the improved frequency response of this conventional style transformer compared to the 9:1 transmission line transformers I tested on the same core. This was contrary to my expectations that transmission line transformers had wider bandwidths. If I have learned anything in recent months it's that measurement beats folklore.

 I know that compensating caps are used in conventional transformers so I added 18, 22 and 27pF caps in turn across the 50Ω side.

Same transformer with different caps shunting 50Ω winding

The improvement in return loss is clear. I then repeated this on the 5.55Ω side of the transformer. The capacitors ranged from 220pF to 470pF. But these values appeared too large and I was getting inconclusive results. 

It appears a convectional transformer can be wound on these binocular cores. However, I need to do some reading on how to select the compensating capacitor values should they be needed. 

73's

Richard

9:1 Transmission Line Transformers 50:5.55Ω - More Results

Ruthroff Type 9:1 on Two Ferrite Beads

As an alternative to using trifilar windings I tried two of these cores, each core with 6 bifilar turns. These are connected in such a way that a 9:1 transformer is achieved. My expectation was this was just not enough ferrite or wire to work. I was surprised then to see a useful return loss from 160m to 20m. 

Ruthroff 9:1 Transformer Two lots of 6 turns on a single core

Ruthroff Type 9:1 on Small Binocular Ferrite

Switching to a regular binocular core I wound 6 bifilar turns on each half and repeated the measurements.

Binocular core, 2 x 6t bifilar, each winding on separate half of core

Comparing these two cores with the 4 bead 4 trifilar turns blogged about last time shows the 2c beads can be just as good as this binocular core:

Upper: 6 bifilar turns on each of two beads
Middle: 6 bifilar turns on each half of binocular core
Lower: 4 trifilar turns on 4 beads

 

Surprised again I looked at the Smith Chart of S11 for the binocular based 9:1 transformer. Since parallel capacitors are often seen compensating RF transformers I captured a few tests. Unfortunately I lost track of the compensating values used but the impact is clear. As the capacitor value increases the S11 curve rotates clockwise on the Smith Chart:

2 x 6t bifilar on Binocular Core
Upper no compensation, Middle some compensation and Lower More Compensation

 The resulting plot of the magnitude of S11 shows the outcome:

2 x 6t bifilar on Binocular Core
Upper no compensation, Middle some compensation and Lower More Compensation

Compensation can improve the performance of this style of transformer. It struck that a series capacitor could be used to "slide" the uncompensated curve around the Smith Chart. A 1nF capacitor was about right at 14MHz to cancel the series inductance present:

So yes the trace slid around the Smith Chart, but the transformer became narrower in bandwidth:

So if you need compensation to achieve the desired performance perhaps a conventional style transformer with compensation can be used? I'll look into that next.

73's

Richard

9:1 Transformers 50:5.5 - Initial Results

With the success testing 4:1 transformers I examined some 9:1 transformers. A trifilar winding, being a bit thicker than a bifilar winding using the same gauge wire, proved difficult with the small cores previously used. I wondered how a single winding through 10 of these cores would perform. I passed a single trifilar winding through 10 turns, arranged them into a U or long binocular shape, and secured them with piece of heatshrink.

1 trifilar turn through 10 cores

Note the Smith chart response is curving away from the 50Ω origin upwards and to the right. This is the same with a bifilar winding.

Returning to the test results here is the sweep of S11: 

1 trifilar turn through 10 cores

 

Very useful from 160m to 30m applications and so easy to wind. Just one turn. But it looked a bit cumbersome and not as broad as I had hoped. So I tried two turns through 6 cores. This was better but still didn't improve the upper frequency. So I tried 4 turns through 4 cores. 

Comparing 1t 10cores v 2t 6cores v 4t 4 cores

Clearly 4 turns through 4 cores is a better transformer, and would handle the 1W I intend to push through it at 80m. However, not as broadband as I had hoped given the success I had with 50:12.5Ω transfomres using these core.

At present these transformers are destined to be used in the 10W Class AB amplifier for 80m I am planing to drive with the 1W Class A amp I have developed. It will be interesting to see if they handle the power.

Overall, these tiny cores are a useful part. I certainly consider them useful for 4:1 transformers but trifilar windings are more time consuming to wind and solder.

73's

Richard

4:1 Transfomers 50:12.5Ω - Success

I hadn't expected this to work out so easily. I already had a selection of bifilar wound transformers wound on these cores so it seemed like a good place to start. Terminated in 12.5Ω I looked at the 50Ω connection with the nanoVNA:

8t bifilar on a single core

Another single core with 12turns bifilar was tested and it was too many turns. Good for 160m to 40m though. 

Shifting to two cores side by side like a binocular core with 7 turns proved useful for all of the HF bands. Five turns could be a suitable starting point for 6m.

7t bifilar on two cores binocular style

To check the insertion loss I wired two of these transformers back to back. (50:12.5 + 12.5:50) 

The loss was negligible.

Loss across two transformers back to back

With 1W applied to this arrangement for 15 minutes the cores showed no sign of heating, perhaps 1degree above ambient.  

The goal was to find a suitable transformer for matching 50Ω to input of a 10W amplifier. If a 4:1 transformer is the solution then this has been achieved. But a 9:1 transformer may be required so another round of testing is needed.

73's

Richard

4:1 Transfomers 50:12.5Ω - Preliminary

Having resolved which ferrites to use for 200:50 ohm transformers it is time to see if any of the cores I have are suitable for matching from 50Ω to a lower impedance using a bifilar winding. The application in mind is for the input stage matching on a single ended mono band HF amplifier delivering 10W or more. 

While I have built push pull HF amplifiers it always struck me as ironic that semiconductor manufacturers often specify the IMD properties of RF transistors in a single ended circuit. However, that just gets ignored because push-pull configurations are claimed to deliver lower distortion and harmonic cancellation. Maybe that's possible, but I know single ended amplifiers can deliver very good results. And I struggle with the concept that a transistor in the off state can cancel harmonic energy generated by a transistor in the on state. Perhaps the overall result is less about the topology and more about the implementation?

My approach will be to determine which core can offer a good transformation from 50 to 12.5 ohms, then use two back to back to see what the loss is and how well the core copes with 1W of RF. 

73's

Richard

1 Watt RF Amplifier using General Purpose Transistors - Latest Iteration

Taking on-board the test results to date the latest iteration of the 1 watt amplifier has the following characteristics:

• 1 Watt output
• 2SC3356 > 2SD1664 > 2x 2SD1664R in Parallel
  
• First two stages use a bifilar transformer to present the collector with 200 ohms for increased gain.
• Use of resistors to provide a thermal bridge to ground for heatsinking. 

The results of my experiments have overturned my previous position. You do not need a working RF transistor, suitably rated, recovered from another radio to build 1 watt Class A HF amplifiers. 1W HF amplifiers can be reliably made using many of the general purpose transistors available that cost a few cents each.

If there is sufficient interest I'll put together a kit. My back of the envelope estimate is the cost would work out around US$5 for a PCB with all surface mount parts fitted.

Save those working RF transistors for more demanding applications!

73's

Richard

4:1 (200:50 ohm) Transformers - Yet Another Suggested Core

So this core is surprising but bang for buck is pretty amazing. It's quite small but 8 bifilar turns spaced around the core is useful from 160m (just) to 6m.

8 bifilar turns

 Perhaps you have the patience to use more turns of still finer wire?

 

12 bifilar turns

But glue, or hold while you start winding, two side by side and wind 7 bifilar turns using fine wire also results in a good transformer:

Given the extra time it takes to work with these small cores it might be false economy. But it does remind me how useful the nanoVNA is for making such measurements of unknown components. And these ferrites are really low cost.

Details of power handling and loss soon.

73's

Richard

4:1 (200:50 ohm) Transformers - Another Suggested Core

I had some more cores arrive, one of which appears to be as useful as those mentioned in this blog here.  This one is a binocular core from LCSC, part C498903. It is larger than the binocular core in the earlier blog.

Instead of showing you a Smith Chat of the swept response I present a plot of S11 versus frequency. This should make it easier to discern how good the transformer is. Six turns is good for HF, while 7 or 8 turns improves things at the bottom end of HF. I haven't measured a 5 turn transformer but it might be better for 6m.

6t bifilar

7t Bifilar

8t Bifilar

 

73's

Richard

Using 1210 Resistors as Thermal Bridges in 1W Transmitter

I mentioned in this post the concept of using surface mount resistors as thermal bridges. I've had a chance to explore this now and after various tests I'm completely satisfied this is a suitable way to heat-sink SOT-89 surface mount transistors in 1W HF amplifiers.

Balancing cost versus ease of installation  I settled on 4 pieces of 1MΩ 1210 resistors for each transistor. A 1MΩ resistor needs no further modification to isolate the collector voltage from ground. On an extended test the two parallel transistors had a  case temperature of around 130 degrees while dissipating a total of 3.4W and delivering 1W of RF to the dummy load.

 

I hope you find this tip useful.

73's

Richard

ps Please excuse the soldering. I'm using up some solder I would not recommend and and it's difficult to get a good joint without using too much.  That's compounded by it being too thick. 

My preferred solder is presently this one. 

My preferred flux is presently this one which I apply with a nylon type small paintbrush trimmed short to make the bristles a bit stiffer.

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors- Still More Success!

Seems I can't help myself but try different transistors. In the course of testing my resistor as a thermal bridge concept I needed to populate a test board for measurements. After that I finished populating the board and tested numerous transistors with it. 

Initially I had some problems, both a soldering fault and poor biasing due to an error in my spreadsheet. But I learned a lot about debugging this amplifier in the process. I have now tested a large assortment of transistors, both SMD and through hole. Most worked very well at 40m and 80m, some well beyond that. And I never encountered any instabilities.

Even on my narrow board I never let the smoke out of any transistor despite relentlessly abusing them. The only casualty was a 2.2ohm decoupling resistor burning out.

1 watt is about the limit with a 13.8 volt supply. The output signal is 20Vpp. After decoupling and loss in the collector choke you have about 13.2V at the collector. You need a bit over 1V on the emitter to set the current with a 5Ω emitter resistor. So you are left with a margin of say 2V to deal with saturation considerations. Even with everything going well the best you might achieve is an extra 1dB of output. Probably not worth the grief chasing it.

Too much standing current and the 2V margin is reduced. Too little standing current and distortion is present. What I do is calculate the bias component values based on a standing current of say 220mA since the bare minimum needed is 200mA for 1W of output into 50Ω. The base resistor to ground is then rounded up to a convenient value. Once built, check the emitter voltage. If higher than desired put a suitable value resistor in parallel with the base resistor to ground to reduce the emitter voltage to the desired value.

I then examine the emitter voltage (Ve) with my CRO. It must be greater than zero at all times when delivering the required 1W for a Class A amplifier. The exact value needed to avoid distortion varies with the transistor used. The minimum Ve is a function of standing current so if you are bottoming out then a small adjustment to the base resistor to ground is needed.

Parallel output transistor at this power level at HF are easy to heatsink using resistors as a thermal bridge and cheap. I recommend this approach over expensive RF devices in this application.

73's

Richard

80m Transceiver - Weaver Method of SSB : Preliminary

The building blocks are coming together. 

• Low pass transmit filter (a)
  
• Antenna changeover relay (a)
  
• Receiver front end filter (a)
  
• receiver detection, AF filter, AGC and PA  (b)
  
• Microphone amplifier and SSB Modulator (c)
  
• low level transmit amplifier (d)
  
• Power Amplifier (e)
  
• Synthesizer with LCD display - built and to be tested (f)
  

Analysis based on the inductors I had to hand suggested my receiver band pass filter may require changing. To facilitate that I decided to incorporate the three building blocks marked (a) onto one PCB. I will make the filters on a daughter board then fix that to the base board.

Rough layout and progress can be seen below.

73's

Richard

Possible Alternative to Thermal Bridge?

It was ironic that investigating the use of surface mount transistors for QRP amps (see previous posts using the QRP tag) required a relatively expensive thermal bridge to heatsink the SOT89 package. These thermal bridges are made using aluminium nitrite. 

I noticed by accident that some high power surface mount resistors use aluminium nitrite as a substrate. Unfortunately, these are no cheaper than proper thermal bridges.

However, what about regular high power surface mount resistors? Reading various papers I learned they use a ceramic substrate with a thermal conductivity about 1/6th that of Aluminium Nitride. I hastily scratched a break in the resistance film on a few such resistors and constructed a test jig. Sure enough, I had a thermal bridge.

The initial result is very encouraging using two 2512 resistors, open circuit, on each transistor in the parallel 1W amplifier. I cranked up the supply until each transistor was dissipating 2W. The transistor temperature was stable at 125degrees Celsius while delivering 1W continuously. Some more suitable resistors are on the way and a re-design to the pcb means I will be able to properly test this soon.

73's

Richard

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors- Success Again!

After the success I wrote about last time I began poking around. I noticed that the driver, a BCX56, was contributing to the distortion at high output levels. I populated a second board, this time with what was an unknown SOT89 transistor taken from a FM92 sub-circuit. After some investigation it appears this transistor is a BFQ19. Mouser still stock them at around A$1 (US$0.75). However I have no more of them so I will experiment with some alternatives now that the proof of concept is working so well.

I also tried a different pair of output transistors (2SD1664R) and a different binocular core which I will write about later when I have a moment.

I doubt the binocular core was the reason for the different outcome. I could easily achieve 1 watt from 3.6MHz to 21Mhz. At the upper end I had to increase the drive but no distortion crept in. This new transistor costs just A$0.05 (US$0.04) a piece. That price is still a fraction of the cost of the thermal bridge but everything worked so well that this has become my new standard output transistor in this application.

A wider board with hopefully better thermal proprieties is planned. 

73's

Richard

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors- Success!

Having populated the board I previously blogged I applied power. Backwards of course. However, no harm done. Phew.

After noting the amplifier was working with no obvious issues I checked the temperature of the output transistors. Too hot for comfort so I added the thermal jumpers I had brought from Mouser for testing in just this kind of application. Never having used these before it was pleasing to see how they allowed a "hot" transistor tab to be thermally grounded but isolated for DC and AC voltages.

Now for a smoke test. With a 13.8volt supply I could only achieve a clean output of 16.6v peak to peak at 3.6MHz. Increasing the supply voltage to 15.9volts allowed a clean 1 watt output sine wave to appear on the CRO.

Tuning around and writing down measurements meant I noticed that the output of my signal generator is not as flat as you might expect from a HP product. Regardless, at a frequency of 20MHz I set the signal generator output to deliver 10v pp into the dummy load. I then tuned down and the output was quite flat until 10MHz when the output looked like some frequency doubling was happening. Changing the capacitor coupling the emitters together solved this. I had used a 15nF by mistake. But as I reduced the frequency still further the output resumed the appearance of a sine wave and started increasing until it reached 19v pp at 6MHz before declining as frequency was reduced.

I haven't time at present to investigate if that is the signal generator output level wandering around or something weird occurring in the amplifier. So I cranked the supply voltage up to 15.9 volts, set the output for 20v pp at 3.6MHz and walked away.  Later I returned and no smoke of any kind. I did note the output had dropped a few volts due to the temperature rise of the transistors. But no damage done and a few puffs dropped the temperature enough to show the output rise.

By now you  might be wondering what transistors I used. The very first packet I picked up were 2SD1007's. When I returned to the house I checked my LTSpice model and it was in loose agreement with my measurements. eg I had measured 1.4V on the emitter whereas LTSpice was suggesting is should be 1.18V. 

Some finessing is still required, but the proof of concept test board is actually a working amplifier.

 

Conclusion:

Cheap (A$0.10 each) SOT89 transistors can be made to work as HF amplifiers in the vicinity of 1watt when used in parallel.

However, you need a thermal jumper which costs 10 times more than the transistor. And my heat sinking using the ground plane could be larger. In practice that will require a wider board so I will abandon my plan to use the piece of extrusion I have on hand to house the amplifier.

So more thought needed regarding the thermal jumper.

73's

Richard

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors - PCB

 The PCB to test this idea is on it's way. Shouldn't take long to test the concept once it arrives.

I placed the second output transistor on the other side of the board. This kept the traces shorter and appeared cleaner.

The board is long and skinny since I am working towards mounting this inside a standard size of aluminium extrusion with an internal dimension of 38mm.   This plan may have to change depending on what happens with my heatsink test. More later.

73's

Richard

G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors Preliminary

The test results for the 2SD882 showed it had the flattest response but at 160mW it appeared to reach a thermal limit that saw the output drop to near zero. After cooling off it appeared to work normally again. 

A LTSpice model suggested this transistor should have been capable of 1W, and with a much better heatsink and increased standing current perhaps 1.5W. However, achieving this would require a smaller emitter resistor with the result that Zin falls to 38Ω at 3.6MHz and 25Ω at 7MHz. 

Using two 2SD882's in parallel has some drawbacks:

• The gain is lower, so they have to be driven with a higher input
• The input impedance is lower still (30Ω @ 3.6MHZ and 18Ω @ 7MHz)

Looking at a few transistors with LTSpice returned the following  ballpark numbers when operated in parallel:

                                       Zin at 

    Transistor          3.6MHZ    7MHz

    2SD882            30                    18

    MJD44H11         36                  24

   2SC5824              42                  33

SOT89 style package 

     2SD965              43                  41 

    2SD1007           42                   35

    2SC5964             45                  38   

   

Conclusion

General purpose through hole transistors don't appear to be a good solution when operated in parallel. They don't have the frequency response of a true RF transistor and the input impedance is lower than desired for a 50Ω module approach.

Some of the SOT89 style transistors appear promising and if the heatsinking can be resolved may be worth pursuing.

73's

Richard