Testing Relay Life - Update at 780,000 cycles

Well I still feel a bit disappointed. No dramatic deterioration after 780,000 cycles.

At least I have worked out how to get Box and Whisker plots out of Excel to help assess changes over time. Trying to do anything with Excel for so many observations is about as slow as the time it takes to transfer the contents of the 32 Megabit flash at 9600 baud. Brings back fond memories of SAS on IBM mainframes.  But I digress!

I divided the 782,224 observations into 12 groups. The first 4 groups are from 16 observations per cycle. The last 8 groups are from 8 observations per cycle, the sum of each 8 observations then scaled by 2.

The shift in the readings from group 5 onward is related to ambient temperature. Up until then we were in a strong heatwave. Thankfully it subsided and ambient temperatures dropped some 15-20 degrees. In addition, the air was circulating faster over the heat-sink of the constant current generator because the window was open and an ex tropical cyclone was responsible for a steady stream of air through the window.

Group 15 shows what happens when the phone is not in the office and there air is still. Still some outliers but I can't rule out family members walking past the office with a phone in their pocket.

With a few days of mild weather expected I'm moving the board up to the workshop. No RF, no strong air currents. Hopefully with those potential sources of error removed I will see a large deterioration in the contact resistance. At present the voltage drop across the relay pins gives and estimated contact resistance of 10 milli-ohms.

So I should hit 1.3 million cycles in a few days. Fingers crossed something wears out.

73's

Testing Relay Life - Update at 262,000 Cycles

Well I'm feeling disappointed. I had thought that by now some evidence of degradation would be visible in the contact resistance. But after operating the relay 262,000 times I don't see any strong evidence the end of life is near. The ADC reading for the voltage across the closed contact has been quite consistent over time. The variability could be due to fluctuations in ambient temperature and RF interference from cellular and wifi sources.

I'm going to modify the code so that 8 readings of the contact resistance are taken, instead of the present 16. That will allow me to record twice as many cycles before filling up the 32mb memory chip. It will also reduce the "on" time each second which will reduce the temperature of the LM317 used in the 2 Amp constant current generator. I suspect the LM317 gets hot enough that the temperature protection is causing the current generator to fold back to a lower figure.

But I am starting to wonder if switching 20% of the rated current means the life is governed by mechanical, not electrical, failure. That would be over a million cycles.

73's

Testing Relay Life - Update at 72,000 cycles

 Happily for me the STM8 hardware and STM8 eForth combination is once gain proving to be reliable and useful. The board I had made to test the relay is shown below. 

 

The test is reaching the point where I expected to see some form of degradation. But there is no strong evidence that failure is near. Until it fails I will not really know if results to date could have predicted it. 

 Looking at those peaks the 16 readings which are averaged and plotted for each cycle are consistent. It's not just a few high readings during the one cycle which push the average up. So in hindsight 16 readings per cycle was probably too many.

With a 32megabit flash chip hanging off the STM8003 I can save 4 million bytes. Since I take 16 readings for each relay cycle and only save the lower 8 bits for each reading that means I can save 250,000 readings. Will the relay last that long? Stay tuned...

73's