A Question About Relays.

[JDNobodi]

I've bought some relays and some have a built in resistors and some don't.
What is the resistor used for and what application are the resistors style relays used for? Is it okay to interchange the relays?

 

[StevoandaRedK5]

resistors create resistance to drop voltage, usually to protect electrical components in the system. i don't any specific applications, but whether or not they would interchange would depend on what they are being used for.

 

[dremu]

Duh ... I don't think you mean a resistor. Post up a pic maybe.

I suspect it's a diode across the relay coil. This would clamp the reverse spike when the coil closes.

The only reason I can think of a resistor -- at least on the coil -- would be to make a 12V coil usable on 24V systems, something like that.

-- A

 

[JDNobodi]

http://terminalsupplyco.com/Store/Product.aspx?pc=332+019+110

From the drawing on the side of the relay, it appear the resistor is parallel with the coil (between 85 and 86). I don't know why the relay is built like that. For StevoandaRedk5 explanation to make sense, the resistor should be series with the coil.

Oh by the way, Bocsh relays are now Tyco.

 

[dremu]

http://terminalsupplyco.com/Store/Product.aspx?pc=332+019+110

From the drawing on the side of the relay, it appear the resistor is parallel with the coil (between 85 and 86). I don't know why the relay is built like that. For StevoandaRedk5 explanation to make sense, the resistor should be series with the coil.

Oh by the way, Bocsh relays are now Tyco.

Gotta be a clamping diode. The diode goes backwards across the coil,so that the back EMF coming out of the the coil when it opens doesn't explode the transistor driving it.

-- A

 

[JDNobodi]

Okay, the use of a diode makes sense. I at one time bought some relay base pigtails with diodes and was wondering what the diode was used for.
But when I search for relays, I find relays with resistors. I don't find relays with diodes.


http://terminalsupplyco.com/Store/Default.aspx?CAT=EC070010#Categories

I wish I didn’t leave the relays at my buddy’s shop. I’m tempted busting one of these $7.00 relay apart a figure what is really in there. I know there is a coil and contacts. I want to see if there is really diode or a resistor in there.

 

[dremu]

Well, huh. They do clearly say "resistor."

Easy enough to test without opening the thing if you have a DVM with diode test. If it has ~600mV drop across the coil one way and some other wildly different number the other, that would be a diode.

If the coil resistance is the same in both directions, then it's a resistor.

It's possible the resistor is there for the same reason. It would be a gentler clamp for the back EMF, plus it's not polarized the way the diode is. If the clamp were a diode and you connected the coil backwards from its intended use, the relay would work ... but the diode would pop in the first half second or so and then never be any good =))

If it's a resistor, while it doesn't instantly clamp the back EMF (*) it would at least dissipate it fairly quickly, and it would work the same in either direction. (The downside is that there's more heat and current draw for the coil drive since the resistor is always part of the circuit, as opposed to the diode which is ordinarily reverse biased and so doesn't do anything. Careful selection of the resistor value would be required, IMO.)

{* = yes, I know, the back EMF would have to ramp over the bias voltage knee of the diode, etc etc... but if you know any of that means, you have a scope on your bench and can watch it yourself }

A little googling lists e.g. Bosch relays with both diodes and resistors. It's possible that folks confuse them, but given the polarity issues, I'm gonna guess that yep, they could be resistors; yep, they do the same thing as the diodes; and yep, I'm nerdy

-- A

 

[JDNobodi]

So these relays are ideal to be cycling on the ground side by something electronic.

 

[dremu]

So these relays are ideal to be cycling on the ground side by something electronic.

Doesn't matter which side is driven, but yeah, the extra crap on the coil is so that a transistor (or other semiconductor) can energize the relay. The back EMF on a regular relay won't immediately kill the transistor right off, at least, not always, but could eventually fry it.

If you're just driving the relay with a switch or other mechanical solution, then it's moot whether there's a resistor, diode, or dead goat across the coil.

-- A

 

[JDNobodi]

Thanks, I thought I’ve bought a bunch of relays which I could not be able to use. Now I know I can use the relays and even use one for an ideal application.