Popping sound when switching microphone with relay

[davesquaredconsulting]

Hi,

I designed a small PA system for a client. The system uses a Shure dynamic balanced mic going through a Radio Design Labs STM-1 preamp. I'm feeding the Hi-Z output to the NO legs of a Kemet EC2-24NU relay. The NC legs are connected to a female 1/8" stereo jack for aux audio source. The common legs of the relay feed a YFK BBE Rev 8.0 tone control/volume board, which then feeds out to power amps. The relay is triggered by a momentary push button on the base of the microphone, and connects the negative leg of the relay coil. All components are powered by the same 24vdc regulated power supply.

It works fine, except that when I release the mic button, and deactivate the relay, I get this noticeable crack/click/pop sound in the audio. My research seems to indicate the relay needs a snubbing zener diode across the coil. However, I'm having difficulty understanding all the factors that go into selecting the proper diode. Do you agree that this is the proper solution? If so, could you please recommend the proper diode to use in this application. Help is greatly appreciated, as I'm way overdue on getting this up and running for my client.

Thanks for helping.

David

 

[duke37]

A zener diode across the coil seems to unnecessary, they are used to switch the relay off rapidly. Are you worried about a few milliseconds?

An ordinary diode will limit the pulse voltage to very little. Almost any diode will do, the common range is 1N400x and 1N4007 will do for many other puposes also. If the relay takes a small energising current, then even a 1N4148 would be adequate

 

[AnalogKid]

The nominal coil current is less than 9 mA. Go with the 1N914 / 1N4148. It has a lower conduction voltage and much faster switching speed than the 1N400x series. Both of those things should increase its effectiveness.

ak

 

[Externet]

Or try two of the three sections of a CD4053 integrated circuit instead of the relay. But use a 12VDC supply instead of 24VDC.

 

[duke37]

A capacitor across the switch may help also, I would try a 0.1μF.

 

[davesquaredconsulting]

A zener diode across the coil seems to unnecessary, they are used to switch the relay off rapidly. Are you worried about a few milliseconds?

An ordinary diode will limit the pulse voltage to very little. Almost any diode will do, the common range is 1N400x and 1N4007 will do for many other puposes also. If the relay takes a small energising current, then even a 1N4148 would be adequate

I'm not worried about switching times in this application. I thought the zener was preferable because of the research that talked about the delay in contact separation could cause degradation of the contacts themselves, and possible sticking. I'm assuming that is not a concern since I'm only putting line level audio through them?

 

[davesquaredconsulting]

Thanks for the quick replies everybody. Another question I forgot is about the diode rating. When I looked them up, they have different voltage ratings. Do I need to match that to the supply voltage rating?

 

[AnalogKid]

I thought the zener was preferable because of the research that talked about the delay in contact separation could cause degradation of the contacts themselves, and possible sticking. I'm assuming that is not a concern since I'm only putting line level audio through them?

Correct. 20 A, maybe. 10,000 times less, no.

ak

 

[davenn]

My research seems to indicate the relay needs a snubbing zener diode across the coil.

research should have shown you that it is a normal diode, not a zener diode ....
The diode is to stop the back EMF created when the coil de-energises from damaging anything connected to the coil ... often a switching transistor

It won't solve the issue you are having ..... switching the input to an amplifier on and off will cause clicks etc
maybe googling microphone click suppression may turn up something useful



Dave

 

[duke37]

The diode is to stop the back EMF created when the coil de-energises from damaging anything connected to the coil ... often a switching transistor

Here the relay is controlled with a switch which will arc when turned off thus giving a click. As davenn says, a diode will suppress this arc. A capacitor across the switch may also do this. It is cheap enough to try both diode and capacitor.

A zener diode cannot be connected across the relay coil without an extra series diode.

 

[AnalogKid]

A zener diode cannot be connected across the relay coil without an extra series diode.

???

Depends on the zener voltage. A 1N4001 is a rectifier rated for 50 V PIV (peak inverse voltage). But it does break down eventually if you raise the reverse voltage high enough. In effect, it is a 100 V zener with horrible accuracy, about +/-25%.

For a 24 V relay coil, a 30 V or 33 V zener would work just fine as a suppression diode. True, it is not the standard approach, but there is nothing wrong with it.

ak

 

[duke37]

For a 24 V relay coil, a 30 V or 33 V zener would work just fine as a suppression diode. True, it is not the standard approach, but there is nothing wrong with it.

So, how would it be connected?
If it is connected to action the zener voltage during switch off, then it will short the relay coil when the relay is energised. Adding a diode in series will stop the short and limit the voltage on the coil during switch off.

An alternative connection is to connect the zener from collector to ground to limit the collector voltage. I have never tried this. In this application, speed is not a problem. I am thinking of trying various configurations to drive an electromechanical counter for use on a coil winder where speed may be important.

 

[AnalogKid]

I have used you alternate connection in automotive applications after seeing that TI built that technique into some of their power driver ICs. It works well, and keeps any residual inductive kick effects off of the power rail; not a big problems in cars, but it could matter in an instrumentation application.

But I might have misunderstood your original comment. I'm saying that a zener diode can be placed directly across the relay coil with the same polarity of a standard rectifier, as long as the zener voltage is greater than the coil operating voltage. As above, not the usual component for this application, but if that's all you've got...

ak

 

[eetech00]

Hi

If switching audio, Use a relay with make-before-break contacts Instead of the standard break-before-make contacts.

 

[davesquaredconsulting]

I greatly thank all of you for your responses. I apologize that I remain confused. I know it would help me if I understood exactly why this is happening. I just tested if the popping happens with no signal present, and confirmed that is the case. Now I'm getting that popping when I engage and disengage the relay. I just tried using batteries to trigger the relay independent of the main power supply, and the popping is still as prominent. So, with no audio signals present, and a separate power source triggering the relay, what is causing this issue?

 

[(*steve*)]

Does your microphone use phantom power?

If there is a DC path through the microphone it is quite possible that current flows through the microphone to charge the input capacitor. When disconnected the charge slowly leaks away (no click) but when it is reconnected a pile of current flows (click).

 

[Wireaddict]

Can you manually push in the relay armature so as to transfer the contacts (I.E., without energizing the relay coil)? If so, try this & see whether or not you still get the pop. That'll show you what's causing the noise. IMO the coil's causing at least some of your noise.