Quick question about diode snubber for a 12v relay

Sean Halley

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Nov 23, 2008
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Los Angeles, CA, USA
Hey guys - quick question for those who know about standard hookups for relays.

I've got some of the little 12v coil 4-pole relays (rated at 125v AC for 5 amps on the poles) guys use in audio preamps - and I built one into a box on my guitar pedalboard for moving a pedal into two places in the signal chain.

I know that the coil can kick back enough to hurt sensitive circuits (saw it shut down the switcher I was using), so I put a 4007 diode on the two 12v leads of the relay to protect the switcher. The white band end is towards the positive connection, so nothing should travel back down that lead to the switcher and hurt it (the switcher is inside of an expensive MIDI controller I don't want to replace :).

It was working fine with a couple of caveats, but the relay died the other day. Before I simply replace it with another one, I want to make sure I'm doing the right thing.

So a couple of questions:

1. I've got a regulated 400ma 12V tap on my pedalboard powersupply that wouldn't operate the relay. The actual meter reading on the two available 12v taps were 11.9 V and 12.1. Both of those taps wouldn't trip the relay.
2. I have another pedalboard supply with an unregulated 12v tap, that shows up as 12.5v - and it trips the relay just fine. I ended up using a wall wart 12v 300 ma supply that worked great for a few hours, and then the relay died. Is this normal, do you think? Do you think the 11.9/12.1V supply wouldn't trigger the relay because of current or voltage? Kind of curious.

Also, is there a better (or the right!) way to use a diode to protect the switcher from when the relay's coil shuts off? I have bigger diodes lying around, if that helps. I connected the diode right to the leads of the switcher, so perhaps I should harness it differently? Connect it differently?

Anyway, thanks as always for any help in advance - the diode was a guess of mine until I did some reading and found out that it's normal. I just need to know what I'm doing wrong, or if it's just a damn normal relay failure.

Thanks guys!

S..



 

CJ

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relays are all different, i have seen 6 volts pull in a 12 volt relay,

might not be a solid as 12, but you can still hear it click,


so you have some other problem besides the relay voltage,

there should be a minimum pull in voltage listed on the spec sheet,
 

mikeyB

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Apr 12, 2005
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Manchester UK
i'm no expert on relays but just throwing up a few ideas!

Is the wall wart regulated - you mention voltages - try measuring the wallwart off load.
Are the relays latching? ie 12V on and 0V off.
If they are not latching, i know that the coils can take the current pulse to switch the contacts and the off "backlash" pulse can turn them off again - but if you leave the current on the coil it may be burning the coil?!? The coils may be designed light duty just to carry a pilse for a short time - if left drawing current they may bake out!
 

JohnRoberts

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diopro.gif


Google is your friend...  diode goes across the relay coil.

JR
 

gswan

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Sep 18, 2006
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Sydney, Australia
John's diagram shows how the snubbing diaode should be connected. You can use a zener to reduce the energy dissipated in the diode instead, but a 1N4004 usually suffices.
A wall wart supply is often unregulated, so although it might say 12V on the label, it is probably more like 18V in reality under light loading. Usually relay coils are fairly resilient to over voltage, even up to double the voltage, depending on how long you drive them at this level.
 

Sean Halley

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Nov 23, 2008
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Location
Los Angeles, CA, USA
Sorry for my lateness everyone - I've been slammed at NAMN in Anaheim....

Thanks CJ!

John - from my initial post "so I put a 4007 diode on the two 12v leads of the relay to protect the switcher" . Google was already my friend, obviously. I had that scenario happening without the zener.

Mikey - thanks mucho. All of the measurements I mentioned were off-load.

I may need to rephrase my question...

With a relay like this, and with a power supply within spec, if I only connected the 4007 across the coil leads as I mentioned, is there any electrical harnessing reason that it would fail? I'm just wondering if I should swap the relay and see.....


relay.JPG



Thanks mucho!
 
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