Just wanted to add thanks for all your work testing these meters Joe, really interesting to watch and learning a lot. Also great when looking to purchase some of these meters - especially the ESD type tests.
How much will the transient tests be wearing out the MOVs on these meters? It'd be interesting to see a test on a meter where the MOVs have worn out, and to see whether it changes the way the meter responds to transients.
No problem. Glad to share the little bit I have done. I learn something from each meter I look at and some of them still impress me. There was no way I would have thought that the BM235 would survive at these levels. I know the fan boys (Fluke 87V, UNI-T, etc) hate seeing their meters so low in the standings and will deny it, suggest the test is bad, suggest I had it in for their meter... It's pretty funny really that people can be so attached to their meters. I just run the tests and present the data.
About MOVs wearing out, it's a good question and I don't think it's something I can answer. The MOVs will be rated for some level, some number of hits at some standard. These are normally a direct hit. Now you put a PTC and maybe a resistor in front of it... I have some experience with them being damaged in real life, catching fire, splitting, etc.
Let's consider the EEVBLOG rebranded BM235. I tested five of the function selections (LoZ, V, ohms, mV and diode). I then ran eleven different levels (1KV, 1.5, 2, 2.5, 3, 4, 5, 5.8ish, 6, 8 and 10KV). Each function is tested with five transients at each level in both positive and negative polarity. So 5 * 11 * 5 * 2 or 550 transients total!
But they are not at the same energy levels.... Again, the MOVs are normally taking a direct hit, with a very low impedance source. The small MOV in the BM235 is rated
for 40J with the 8/20us. But in the meter, we have a 1.5K PTC and a 1K resistor limiting the current into the device. We can pretty much ignore the 2ohm source of the generator. Even if the MOV was taking a direct hit from my generators, I test with under 20J. Most meters were damaged with less than 10J available!
So, where does that leave us. What I can tell you is that I have yet to see an MOV fail as part of my testing. What fails are the PTCs, input resistors, high speed clamps, ICs, PCB traces.
Consider the Fluke 107 that 5KY's sent me. That meter has seen the worst of any meter I have tested. You would really have to be stupid and hook it to a MOT or something to damage it. 15KV 100us FWHH to damage it! And what failed, not the MOVs. The high speed clamps and the PCB.