propagation of back EMF

[Simon]

Can you explain further, if I short the scope probe surely it will pick up nothing as the cable is shielded and by shorting nothing will happen. I will try this as it could explain how a common comparator chip is surviving -400 V

It can pickup the noise through the shield current (common-mode noise causes this), see Dave's blog on the strange oscilloscope phenomena-episode. No matter how well he short-circuited the probe, the effect was still visible. You can imagine that there is no field inside the shielded conductor. Now, the inside conductor stays at 0 volts, but the shield voltage changes. Result is that you'll have signal at the scope input amplifier. Note that amplitude of this signal is not changed even if you change the probe attenuation. So larger your probe attenuation, the bigger the spikes will become. Which is logical since the noise doesn't pass through the attenuation resistors inside the probe.

Or alternatively, the ground clip (are you using that?) forms a loop where a rapidly changing magnetic field can couple quite effectively, giving rise to weird results.

Regards,
Janne

what sort of waveform am I looking for ? I'm getting a typical back EMF spike that just shoots straight down to -400V and then back up and dies sown after a bit of oscillating which is no surprise (the oscillating) as there are 8 8amp fans in the circuit

The other thing is if it is picking it up "over the air" it is still coming from the setup

[Simon]

well the plot thickens. I am now quite certain that a substantial voltage is flying around the system based on a very simple fact: I got a shock off of it. I got a shock off the PCB of the thermostat that I was holding in my hand as I had opened it up to probe the comparator IC itself. Now spikes of up to -400V were observed on the power supply of the thermostat. I then opened it and probed the power in to the stat and the power pin of the comparator IC.

See image 52.png now this was all done using the battery neg as ground, the stat got -400V (blue) on it but the IC got -200V (yellow)

Then I used the IC's ground pin as the scope ground and this is where things start to go wonky: see 55.png well yes the IC now only has -15 volts on it but the whole stat still has -400V. It explains how the IC may be surviving but I can't understand how the relationship has changed

BEWARE that image 55 have different V/div scales

[tecman]

Simon:

What you are seeing seems to violate some laws of nature, so:

Where is the flyback diode mounted ?  If it is some distance from the coil, how much wire is involved ?

A flyback diode should contain all of the circulation current between the coil and the diode.  Are there any other inductors ?  How about wire lengths in that "loop" ?

Are there any "sneak" paths that current gets into another coil/inductor ?

What is the inductance of the coil ?

Is there an alternator (dynamo) in the circuit ?  If yes you may be seeing an alternator dump issue, common in battery/alternator systems.

Lastly is your measurement methodology clean ?  Grounds and things can make bad problems look much worse, your shock acknowledged.

Paul

[Simon]

Ah yes violate laws of nature it does, my one certainty is that with respect to ground in the sense of the floor there is a sizeable voltage as I got a shock off it and was not even touching a PCB pin directly.

There are no flyback diodes, there are diodes on the relays but there is a total of 8 fans that pull 8 amps each with no flyback diodes.

All wires to and from parts are about 1.75 metres.

There is no alternator in the circuit but there will be in the vehicle (which I cannot work on as it's over 250 miles away)

as far as i know I've done my best on the measurement side, I do have a 1.5 metre length of wire from the negative of the battery that I use for earth when i have to stray far from the battery with the probe and I am using this as a common ground for all measurements so that I'm working on the same reference for all of the circuit.

The puzzling thing is that although the voltage on the IC was less when i used the stats negative as measuring earth the voltage on the power in was still the same. my only offering there is that there is a diode on the power in (so in theory I should not even see the -15V)

[alm]

As I posted in the other thread, the negative voltage seems really counter-intuitive. There seems to be a lot of ringing going on, but that might be in the setup (sounds like you have tons of inductance and capacitance. That would also be the case in a real car, but might be significantly different, and hard to replicate. I would try to keep ground leads really short to prevent ringing and magnetic pickup.

One simple experiment would be to install a freewheeling diode and see if it changes anything, I'd just use whatever you have in hand, even 1N400x/540x. They might be able to handle the current since it's a short peak, and if you blow up a $0.01 diode, who cares? A freewheeling diode wouldn't help against negative peaks (it would only be reversed biased or breakdown), but should help against EMF that behaves properly according to textbooks .

The peak lasts about 30ns, that's way too fast for a 50/60Hz rectifier diode, so a schottky might work. The frequency seems close to the bandwidth of the scope, so the peak might actually go even lower. I'd be careful with the duty cycle, since a ~30MHz 400V peak is likely beyond the specifications of your probe/scope (even if it's for only 100ns).

Does setting the CH1 vertical setting to 100V/div change anything in the second situation? In that case the voltage might be induced by a current into the probe (magnetic pickup by the ground lead?). Maybe the ~15pF of the scope probe over the IC shunts the spikes?

[Simon]

well as I said there are 6 8 amp fans running all the time, they have built in speed control resistors and indeed things are worse when they run at speed 3 (no limiting resistor) so ringing I expect is natural, what i never expected was the fact that I would get bursts of ringing, basically what you see happens maybe 20+ times but each burst is quite spaced out (like 10 times the length of the main burst) and only the first burst is so large but I guess that is because it is the initial energy that jumped the relay contact gaps. As this relates to a 2 piece military vehicle things certainly will be more hectic than with my little set-up and I've already warned them that these phenomenons are quite a physical thing and can only be faithfully replicated by using the actual installation which is likely to never happen

[Rhythmtech]

Usually when I've gotten to this stage I start thinking Heisenberg's Observation effect - Which abstracted on to what you are doing says that you effect your measurement by simply making a measurement.  Is it possible that some of the voltages you are seeing are being incurred by probe impedance or other measurement variables?