Question about diode across relay coil

[R.G.]

Don't worry.. He'll live... :|

Thanks gary. I was worried for a minute there.

Hey, I've been meaning to ask you - what happens when you load up an inductor and then interrupt the current, causing an arc in the switch if there's a great, big loop of wire with a big area that the current's flowing in?

[mooreamps]

Thanks gary. I was worried for a minute there.

Hey, I've been meaning to ask you - what happens when you load up an inductor and then interrupt the current, causing an arc in the switch if there's a great, big loop of wire with a big area that the current's flowing in?

Well, you know the answer as well as I.. But for the sake of the others reading this , lets walk through it..

When you apply a dc voltage to a "DC" coil, it will generate a magnetic field around it. It is this magnetic field that pulls the contacts inside the relay.. Then, when the dc voltage is removed, the coil will discharge it's energy and the magnetic field around it will collapse thus releasing it's pull on the relay contacts.. It is the collapsing magnetic field, also known as the inductive kick that is feed back into the circuit... Hense the flyback diode.. It is inserted reversed biased in the circuit when the voltage is applied. But becomes "forward biased" as a result of the polarity reversal of the magnetic field generated by the coil as the field collapses..

But, inside these little 12 volt dc relays, I don't think the coils are big enough to generate much of an inductive kick. Well, they would spike a 2N2222, but I don't think they could hurt a fairly robust 12 vdc power supply... Still, having a flyback installed across a relay coil, driver transistor or not, is still probably a pretty good idea....


-g

[R.G.]

Well, you know the answer as well as I.. But for the sake of the others reading this , lets walk through it..

When you apply a dc voltage to a "DC" coil, it will generate a magnetic field around it. It is this magnetic field that pulls the contacts inside the relay.. Then, when the dc voltage is removed, the coil will discharge it's energy and the magnetic field around it will collapse thus releasing it's pull on the relay contacts.. It is the collapsing magnetic field, also known as the inductive kick that is feed back into the circuit... Hense the flyback diode.. It is inserted reversed biased in the circuit when the voltage is applied. But becomes "forward biased" as a result of the polarity reversal of the magnetic field generated by the coil as the field collapses..

But, inside these little 12 volt dc relays, I don't think the coils are big enough to generate much of an inductive kick. Well, they would spike a 2N2222, but I don't think they could hurt a fairly robust 12 vdc power supply... Still, having a flyback installed across a relay coil, driver transistor or not, is still probably a pretty good idea....

Good walk. Yeah, I knew that part and I figured you did too.

But I wondered whether you'd tumble to the setup where the voltage arcs the opening switch and generates a pulse of RF which is transmitted out of the loop antenna formed from the - and I quote - "great, big loop of wire". They used to call them spark gap transmitters.

Sure, a random loop of wire doesn't transmit all that well, but for things either in or near that - and I quote - "great, big loop of wire", the RF is easy to pick up. And when that's inside some piece of amplifying equipment, it's really easy to induce RF clicking issues as the RF is detected by the first junction or nonlinear amplifying device (tubes qualify here) it hits. It's very dependent on the energies, the flyback voltage, the clamping voltage, and whatever frequencies the random loop happens to "tune" the arc pulse to. The random nature of the current loop means that the burst of RF isn't always all that detectable. But sometimes it is, especially if you are using high gain, high impedance things like tubes and FETs.

One neat trick for suppressing the arc is to clamp it back into the power supply with a diode. After all, this isn't rocket science, is it?

[Ronsonic]

that may very well be true, but it doesn't mean there's no use in following best practices.

The last time I went shopping for relays, I was finding that it's now almost universal to find a diode built into the package. The important thing to know about them is that polarity is important.

[Dave H]

But, inside these little 12 volt dc relays, I don't think the coils are big enough to generate much of an inductive kick. Well, they would spike a 2N2222, but I don't think they could hurt a fairly robust 12 vdc power supply... Still, having a flyback installed across a relay coil, driver transistor or not, is still probably a pretty good idea....

The diode is a good idea. I once repaired a little relay driver without a diode that was killing 150V transistors so I measured the flyback voltage with a 'scope and it was over 350V.

Dave H.

[guitarmike2107]

Could someone explain why you would choose to switch a relay with a transistor.

Why not just switch the thing itself i.e. via the footswitch or panel switch

[kg]

Could someone explain why you would choose to switch a relay with a transistor.

Why not just switch the thing itself i.e. via the footswitch or panel switch

the only compelling reason i can think of is so that you can work out some sort of more complex switching matrix logic.

[kg]

But, inside these little 12 volt dc relays, I don't think the coils are big enough to generate much of an inductive kick.

to know definitively how large it can get, all you have to do is measure the coil Q.

takes about 10 seconds with an LCR meter.

[Zipslack]

Solid state relays

Just a quick note about the solid-state relays...they are nice in some instances (internal LDR-controlled transistor switching), but they can generate noise and WILL generate heat that must be dealt with. A lot of these units have metal baseplates as a heatsink, and will sometimes require extra heatsinks depending on the current. Although they don't have inductive kickback, they do have leakage current, so they are not "fully" off like a mechanical relay. They also tend to cost more. Just some food for thought...

[GregS]

Originally Posted by GregS
I'm using a 5V relay for switching in an amp I recently built with no diode across it. I have been using it for months without any issues or pops.
Yes, what they said.

What that means is that something is absorbing the flyback energy. Sometimes a driver transistor can eat this kind of abuse, especially if it's at human speed, for lots of iterations without failing immediately. It may even last for a long (by human standards) time if it's not overheated by the avalanches. But it's not good for whatever drives the relay. Generally if what drives the relay is a transistor it lasts longer, as single transistors can take more abuse than the insides of an IC in many cases.

Using a diode means the diode is always shunting the flyback spike back into the power supply, and there are no long term worries (or sudden failures) with the driver. This is why it's just good practice to do this and not count on getting by with with it for an unknown period of time. Good design practice means never (OK, hardly ever ) having to say you're sorry after shipping the design.

I should have been more specific about the circuit I'm using. There are no transistors or IC's to damage. I'm using the heater supply rectified by 2 diodes. There is also a 470 uF capacitor between the supply end of the relay coil and ground. I imagine this would absorb any voltage spikes. The switch used to ground the other end of the coil is a heavy duty toggle switch suitable for use as a standby. It would be rated for hundreds of volts at least so I don't think arcing will ever be a problem with a 5V relay coil. However, if there's something I've missed here and I'm getting away with something I'd like to know.

Also, it is still not clear from the discussion whether or not the flyback diode is needed if there are no sensitive parts like transistors in the circuit. Some members have said it is not necessary other than suppressing pops, some have said it is because of possible arcing in the switch to ground the coil. Again, if your coil voltage is only 5V or 12V and you're using a heavy duty switch rated for 100's of volts is this a concern? Also would that capacitor I have absorb these spikes anyway? Could someone please clarify?

Thanks.

[guitarmike2107]

Strange, Non of the SSR's that I have used have any requirement for heatsinking, I have also mostly used them in shunt switching, so noise hasnt been an issue nor leakage current.

Is your comment from experience or from reading something somewhere?

[mooreamps]

Also, it is still not clear from the discussion whether or not the flyback diode is needed if there are no sensitive parts like transistors in the circuit. Some members have said it is not necessary other than suppressing pops, some have said it is because of possible arcing in the switch to ground the coil. Again, if your coil voltage is only 5V or 12V and you're using a heavy duty switch rated for 100's of volts is this a concern? Also would that capacitor I have absorb these spikes anyway? Could someone please clarify?

Thanks.

Use a 1N4007 as the flyback diode, and a 100pF 1 KV cap across the relay contacts...

-g

[Zipslack]

Experience...but that was dealing with higher-current stuff (20-50A). Lower current stuff probably wouldn't have these problems to a high enough degree to worry about, but it's been a while since I used any, so the technology might be better now.

[guitarmike2107]

I think it has improved, or at these currents many of the issues you have raised are non issues.

Have a look through this paper for some advantages of SSR's over EMR's

http://www.clare.com/home/pdfs.nsf/w...ile/an-145.pdf

[Steve Conner]

Well, the SSRs most people are familiar with are the triac output kind, that might be rated 40A at 600V AC or whatever.

Not the tiny MOSFET output ones under discussion here, used for switching low power signals.