How to trigger on "loss of signal" to troubleshoot in-circuit relays

[DeadBattery0Ah]

Trying to find root cause of a big 2pole joslyn clark contactor that intermittently "chatters". I've verified it works fine when using isolated 240Vac coil supply. So I'm looking to use my hand-held hantek 2d42 scope to try and diagnose. My question has to do with how to somehow trigger on 1) intermittent loss of the 240vac coil voltage. And/or 2)intermittent control relay contacts opening. Any info (or fresh idea's are welcome!) Thank you!

[bob91343]

This is tricky because you don't want to connect your test equipment directly to the power line.  This is not a high speed situation, as the 240 V would have to be absent for a few cycles to allow the contactor to drop.  So you would first add a small transformer to isolate from the power line to the test probes.  Then rectify the secondary voltage with a diode and small capacitor, say 100 pF.  Observing the voltage across this capacitor, you should see a positive swing (assuming positive rectification) when power is applied, and a drop in this signal when power is removed.  Trigger on this drop.  You may need a discharge resistor, say 10k.

If that doesn't trigger reliably you may need to add a voltage comparator to convert the sine wave to a square wave.

[Manul]

I agree with bob91343, first thing what you need is a transformer. You could use 240V primary 12V secondary or something similar. Isolation and lower voltage, but still the same waveform.

If you want to record the waveforms one idea is to use line in on a computer. Audio card is perfectly adequate for such low frequencies. You can record hours of mains frequency and analyse later. Just build a resistor divider for required signal level (after low voltage isolating transformer, not directly!).

[perieanuo]

trigger on 1) intermittent loss of the 240vac coil voltage

before that contactor you have some trigger device (relay, pcb, whatever), if you 'scope' that contact or transistor (when he's closed, you have the coil powered, when it's opened you have contactor not powered), so it's simple to trigger that device (you have small voltage for contactor coil ON and high for OFF)

for the 2'nd, in your place i'll use some calculated resistor on one relay contact, here you get voltage when contactor opens his contacts for whatever reason like coil not energised or bad contact, BUT the oscilloscope probe voltage capabilities must be respected, maybe use a separation transformer. the load AFTER the contactor contacts is energised via your monitoring load for contactor switch, so maybe you should disconnect the contactor load just to filter what you want, if it's coil or contact problem. of course if you disconnect the contactor load(s) you can connect some test load like a light bulb, and that in conjunction with the resistor test load will give you what you need to trigger. hope you get the schematic idea.

i know, everybody says the separation must be on DUT side, you're a beginner, so be advised. i use sometimes on the oscilloscope side, but i take care not to fry my precious hands  because you can get mains power on oscilloscope ground clip for example