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