I have a circuit where I'm switching the primary of an audio transformer using relay NO/NC contacts. A partial schematic is shown. enter image description here

The problem is that I'm getting inductive kick whenever the contacts close to switch off the input. This can be seen in the scope image below and is audible as a "pop" at the output.

enter image description here

I have the high pass on the secondary and impedances right where I want them, with a corner frequency at 99.6kHz and slight underdamped peak at about 56kHz. That gives a relatively flat response up through 20kHz. Part of the issue is also that the input should be able to handle 3Vrms, and the spikes are not that high (generally 2-4V). I've read that a snubber circuit is commonly used for this, but not sure where to start. If treated as a low pass filter, the large 220uF cap as part of the snubber would need a tiny resistor to pass audio frequencies, and I think would throw off the impedances.

Would appreciate any useful input.

UPDATE: So after considering the responses and researching mute circuits, I realize the above circuit drawing demonstrates what I wanted to accomplish but is horrible in implementation. Even with make-before-break contacts, trying to break both sides simultaneously and have them do so at the same time to prevent inductive kick is wishful thinking, so I've implemented the following circuit:

enter image description here

While a little slow to charge initially, this eliminates pop from DC on on, and inductive kick from off.

Question: Is it an issue if blocking caps are left floating as opposed to discharged when disconnected? In this case, removing a connection at the input doesn't allow the 220uF cap to discharge.

ANOTHER UPDATE: So an upgrade with make-before-break contacts seems to not have solved the issue after all ... the DC seems to be taken care of, but now I have a huge "ON" spike, and I don't understand where it is coming from. Here is the new circuit:

enter image description here

Here is the result at circuit output (beyond transformer secondary) whenever the relay gets switched:

enter image description here

There is a shorting of the transformer primary for a double-digit microsecond duration when switching on, but this wouldn't be the cause, would it? Also there is a much smaller spike when switching "off". Maybe bouncing contacts? Any ideas?

  • \$\begingroup\$ What are you trying to accomplish? zero crossing current switch? why do you need a switch? \$\endgroup\$ – Sunnyskyguy EE75 Sep 5 '17 at 19:31
  • \$\begingroup\$ Side note, I would definitely not trust that measurement (peak voltage). Inductive kicks have a very fast rise time and you're only measuring at 20ksps. Seems there is not enough BW to really capture the spike \$\endgroup\$ – Andrés Sep 5 '17 at 19:47
  • \$\begingroup\$ Also, it seems that the spike occurs when the relay first disconnects, in which case you would have the inductive kick corresponding to the leakage inductance. Once it switches completeley then it's shorted and you'd be safe. \$\endgroup\$ – Andrés Sep 5 '17 at 19:53
  • \$\begingroup\$ TonyStewart: no this is an audio device, transformer is for balanced input, and relays switch inputs on/off. Andres: good point about the peak. And yes, that's right, the spike occurs immediately when it disconnects, and makes an annoying pop noise. It closes quickly, but it's that noise I'm trying to eliminate. \$\endgroup\$ – User7251 Sep 6 '17 at 3:57

It appears from your drawing that the relay shorts the transformer primary circuit in the "Off" position. If the 220 uF capacitor is charged in normal operation, it will discharge through the relay and transformer primary when the relay switches off. This discharge current should cause a spike on the transformer output.

  • \$\begingroup\$ Yeah, this is what I was thinking before I measured it, so tried various resistors in the "short" circuit branch between 3 and 10. Makes no difference. The actual "kick" is coming from the breaking of the relay contacts from 8-9 to 9-10 \$\endgroup\$ – User7251 Sep 6 '17 at 4:00

Several ideas:

1) You can add a muting circuit. Probably the secondary side would be easier, for example across C23. This can be an analog switch that shorts across C23. You would apply the mute circuit momentarily before opening and closing your input relay.

It can be an analog switch, a relay, or even an optoisolator. Just depends on your design specs.

Also, you might want some high value resistance across C22 to bleed off any residual charge on it.

2) You might also consider replacing the transformer with an instrumentation op-amp so you don't get inductive kick back, but still get the common-mode rejection of the transformer. You also would not need the huge 220 uF capacitor.

3) Replace the relay with back-to-back MOSFET switches that you can softly turn on and off with a voltage ramp signal.

  • \$\begingroup\$ Vince: Thanks for the ideas, I prefer to stick with the transformer for now for the better CM rejection and isolation. I like the sound your third idea, basically the relay on/off is controlled by an external contact, then 12V is sourced from this device through the external contact to switch it. Would I use something like an RC circuit to set the ramp on-off time? Would you possibly be able to provide a simple schematic of how you would accomplish this? BTW isn't the closing of the relay contacts enough to bleed off that C22 charge? It's pretty much a short circuit at that point \$\endgroup\$ – User7251 Sep 6 '17 at 4:16
  • \$\begingroup\$ The first idea sounds like it could work too, but seems like it would be more complex than the third since there's no controller or anything to turn a mute circuit on-off. But maybe there's an easy way to implement this? \$\endgroup\$ – User7251 Sep 6 '17 at 4:18
  • \$\begingroup\$ What about just using relays or analog switches with 2 SPDTs with make-before-break contacts? Then the transformer circuit is shorted closed before the input is disconnected, eliminating the kick, assuming both make before any one breaks. \$\endgroup\$ – User7251 Sep 6 '17 at 4:53
  • \$\begingroup\$ Re: 1st idea: Yeah, maybe a make-before-break kind of circuit might work, basically just delay the relay opening so that the shorting/muting circuit is applied first. Re: RC circuit to ramp on/off control, it can work but an opamp integrator would be smoother. Also, the MOSFETs would need a fixed voltage to bias properly so you'd have to put a capacitor on each leg of the primary so now you'd have to use 470 uF caps to get same freq response. I think a muting circuit and putting a simple RC delay on the input relay is best/simplest solution. \$\endgroup\$ – Vince Patron Sep 6 '17 at 16:18

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