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propagation of back EMF

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Simon:
So as some of you probably know I have been crazy enough to take my precious Rigol oscilloscope to work and go "spike hunting" with it on the air con system we are producing. Well I captured spikes of up to +/- 200 V !!! on a 24 volt supply. now the thing is that I cannot find what is is producing these spikes.

So the situation is that I have 3 24 volt 8ish amp fans running under the control of relays (so should not cause a problem even if they have no diode) and a thermostat controls a compressor clutch (that does have a back diode) and 2 8 amp fans also controlled by a relay. When the clutch + 2 fans are switched off via the relay in the thermostat and at the same time another relay is turned off and this turns on a solenoid valve a +/-200V spike is generated on the power in pin of the control box which is not directly connected to anything but the solenoid that has a diode in it.

Basically the question would be can back EMF propagate up the negative ? I am getting oscillatory bursts that start with a + and - peak of 200 V and dies down over maybe 2 uS

Of course we should have back diodes on EVERYTHING but at the moment I'm trying to state the case to our customer that in it's current state it is not our gear causing the thermostat to blow which is still working miraculously

Kiriakos-GR:
You got a large puzzle to solve .

Lets start with the small things,  is this 24V power source an transformer or inverter ?   

Simon:
two car batteries in series, not quite a replica of the customers military vehicle but as close as I can get with no budget, basically if I'm powering a fan from a relay when it is switched off it is isolated from the positive, so the only way the spikes can work their way around is by coming up through the negatives (In effect revering the power polarity for a short time ?)

ModernRonin:
> So the situation is that I have 3 24 volt 8ish amp fans running under the control of relays (so should not cause a problem even if they have no diode) and a thermostat controls a compressor clutch (that does have a back diode) and 2 8 amp fans also controlled by a relay. When the clutch + 2 fans are switched off via the relay in the thermostat and at the same time another relay is turned off and this turns on a solenoid valve a +/-200V spike is generated on the power in pin of the control box which is not directly connected to anything but the solenoid that has a diode in it.

I would go device by device. Unhook all but one device, turn it on and off, see if you get the spike. Repeat for all devices. If you're lucky, it's only one of them. If you're really lucky, it's just a bad diode, which you can replace and problem solved.

If you're not lucky, well... let's not go there. Test one device at a time in isolation and see what happens.

> Basically the question would be can back EMF propagate up the negative ?

In theory, yes.

Simon:
Yes we have been thinking of that (so I'll have to more more leads)

and Yes basically the question is can back EMF propagate up the negative ?

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