Testing Relay Life - Update at 1.83 Million cycles

Somewhat frustrated that the relay was still clicking away, so mechanical failure had not been reached, I went of to research what electrical failure means. I had assumed it would manifest itself as a sharp rise in contact resistance but another test is contact opening times.

At 20% of the relays switching capacity it appears I was unlikely to reach an electrical failure.

Since I had two opening time observations from early in this exercise, 1820 uS each, and the opening times were now 1910 uS it seemed worthwhile to also log the opening times.  Closing times are a low priority since I had no easy way to measure contact bounce with this test board.

In addition, I gave some thought to how I could increase the 2 Amp test current I was using. The next relay I wanted to test had a 2Amp rating so I didn't want to undertake the heavy modifications to the test board a 10 Amp current would require. I elected to place an RC circuit across the test contact to add 8 Amps across the contacts when initially closing. 

Since the constant current generator had a no load voltage of 10.5V the mean R should be 1.31 ohms (10.5V / 8Amps). I plan to use two 2.7 ohm resistors in parallel for 1.35 ohms.  The capacitor needs to be fully charged in about 400mS.

I am using a constant current generator, not a constant voltage, and a series R. At 2 Amps I lose 2.7V across R, so C charges to 7.8V. Then a constant voltage relationship takes over to finish charging. I will start with a 1000uF capacitor. It=CV so that should charge to 7.8V in 39mS. Then a further 68mS to reach about 10.5V.

Now to consider the extra heat the constant current generator has to dissipate when charging the capacitor. The additional 2 Amps for 39ms plus the tapering current for 68mS is extra heat, but this compares with the 2 Amps being forced through the relay contacts when closed for 50 ms. 

I decided to reduce the on time to 21ms. ( Wait 5ms then take 8 readings at 2ms intervals ) A further pause of 1 second each time the buffer had finished writing to the external flash storage would let me disconnect by "ear" should the need arise and help with the heat dissipation.

Might as well store the closing time, even with no de-bounce checking, since I had bytes to spare. I was filling a 256byte buffer with 32 readings of 8 bytes. Now I was filling it with 16 readings of 8 bytes plus 2 bytes for each time measurement.

We're back in a heat wave now with daytime temperatures rising above 37C. So leaving anything running in the shed would have to wait a few days till the next cool spell. I'll make all these mods and see what happens. 

In the interim this is where I got to:

 

My conclusion is there is no degradation in contact resistance after 1.83 million cycles when switching resistive loads at 20% of contact rating. 

I'm disappointed there was no failure. Next time!