Question about diode across relay coil

[Gaz]

This may be a dumb question, but I was wondering if the diode typically used across a relay coil had to be physically soldered across the relay?

Since the cathode of the diode goes to the power supply, and the anode goes to ground, is it okay to place the diode away from the relay itself and over at the power supply going straight to ground? It seems to me that this is electronically the same, but I'm not sure if my pseudo-science is on today. Thanks.

[loudthud]

The relay coil has significant inductance. When the relay is energized, typically by switching one side of the coil to ground, a magnetic field is created in the coil. When that one side of the coil is disconnected from ground, the inductance tries to maintain current flow as the magnetic field collapses. This causes the voltage at the now disconnected side of the coil to swing positive above the voltage that energized the relay. There are many variables but the voltage can easily swing 50 or 100 Volts above ground for a short time. If you were using a 40V transistor to energize the relay, it can be damaged by excessive voltage. The diode across the coil limits the voltage swing to about 0.7V.

[Gaz]

Thanks for the response, it helps my fundamental understanding of what the diode is actually doing. I'm still unclear about whether the diode can be placed from the relay's power supply (a small transformer in my case) to ground, or if it needs to be physically placed across the relay's contacts.

[loudthud]

It has to be connected across the coil. If it just goes from from the power supply to ground, it's not going to protect the transistor. Try to measure the resistance of a choke. If you are touching the wires from the choke when you disconnect the ohm-meter, you'll feel the jolt.

[Jazz P Bass]

We are talking the speed of light.
Whether or not the diode is physically attached to the coil or one foot away is irrelevant.

[Gaz]

Right, but Loudthud seems to be suggesting that the physical placement does matter. I'll have to draw a little layout to show exactly how I'm doing it...

[Gaz]

Here's a schem/layout that represents the two different options.

[kg]

hey gaz

speed of propagation hasn't anything to do with it.

you want the diode close to the coil it's catching the flyback from.

this is to keep the current loop small.

flyback voltage can hit Q*Vsupply. Q is a function of the reactance vs. resistance of the coil and is intrinsic to its physical design.

a small loop will best serve to contain the interference generated to the smallest possible area.

hth
ken

[Gaz]

Thanks for the cogent explanation, Ken, I'll be moving those diodes over to the relays themselves.

And I have to mention. There was a funny coincidence a while back when my new roommate and I somehow figured out we were both reading your website: I was studying tube amps, and he was downloading hard to find Subaru manuals!!! Haha!

[mooreamps]

This may be a dumb question, but I was wondering if the diode typically used across a relay coil had to be physically soldered across the relay?

The flyback diodes are typically used to protect driver transistors to engage the relay coil , like the old workhorse 2N2222.. I don't see any driver transistors in this print, and I'd don't think what "little" flyback voltage is going to hurt anything... But, if you want to use one, anywhere would be fine...

-g

[Gaz]

I was under the impression that they were necessary with any setup... maybe they're encouraged in non-transistor driven designs to prevent a popping noises ??? - never tried relay without them, so I couldn't say myself.

[mooreamps]

I'm just saying the where's and the why's of when they came from.. I use a 12 volt relay in my preamp channel switcher, and it works fine without it.. Not saying not having them is a bad idea. You make up your own mind.. Just understand why you will always see one across a relay coil, with using either a transistor or a fet driving the coil...

-g

[Steve Conner]

Gary uses a LED instead of a regular diode.

As there is no transistor to kill, the only use of the diode is to snub the voltage spike that might otherwise couple into the signal wiring and cause a pop.

[guitarmike2107]

The diodes will also protect any arcing on your switches too. A few years ago I had switches failing on 12v ambulance lift systems because of arcing from relays and other coils. Ok so its probably not a major concern here but it does help.

[kg]

Thanks for the cogent explanation, Ken, I'll be moving those diodes over to the relays themselves.

it's just good housekeeping and circuit design... and it absolutely DOES reduce popping and switching noise.

And I have to mention. There was a funny coincidence a while back when my new roommate and I somehow figured out we were both reading your website: I was studying tube amps, and he was downloading hard to find Subaru manuals!!! Haha!

obviously i'm going to have to make it a pay site.