# Resistor / Diode Sizing



## Wardenclyffe (Jan 11, 2019)

A *flyback diode* is any diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. It is used in circuits in which inductive loads are controlled by switches, and in switching power supplies and inverters.

This diode is known by many other names, such as *snubber diode*, *commutating diode*, *freewheeling diode*, *suppressor diode*, *clamp diode*, or *catch diode*.[1][2]




https://en.wikipedia.org/wiki/Flyback_diode


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## splatz (May 23, 2015)

You can get relays with the snubber diode built in, that's the easiest way to go. These relays have a diode  an MOV accessory you can add on, leave the engineering to them. 

All in under $20 at Automation Direct
Ice Cube Control Relay: 15A, DPDT, socket mount (PN# 782-2C-12D) | AutomationDirect
Relay Socket: for 782 and AD-70S2 series cube relays (PN# 782-2C-SKT) | AutomationDirect
AutomationDirect Protection Diode Module: 5/pk (PN# AD-BSMD-250) | AutomationDirect wrong one that's for DC 
MOV Module: 5/pk, 120 VAC coil voltage, for 782 series relays (PN# AD-BSMM-120) | AutomationDirect
and you'll have four spares.


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## joe-nwt (Mar 28, 2019)

What is the contact rating of your relay?


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## backstay (Feb 3, 2011)

Most lighting contractors are only powered to change state. They are not powered on all the time.


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## splatz (May 23, 2015)

backstay said:


> Most lighting contractors are only powered to change state. They are not powered on all the time.


I am trying to think which is worse for the inductive kickback. That occurs when the power is removed from the coil. 

If the lighting contactor is magnetically held, the coil is energized until the control power is cut; at that time, the collapsing field in the contactor coil surges onto the control wiring. Simple enough. 

If the lighting contactor is mechanically held, I am not sure they all work alike. I am thinking there's two solenoids one operates to turn the lights on, the other to turn the lights off. When you turn on, the coil is briefly operated, then turned off, and a kickback occurs. Same when you turn off. So with a mechanically held contactor, there's twice as many kickbacks for the same number of on/off cycles.


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## micromind (Aug 11, 2007)

What really are you using? 

Is it the coil or the contacts that burn up?


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## Almost Retired (Sep 14, 2021)

power said:


> Hello to all. I have a problem at a Department Store whose lighting panel (LED and incandescent only) is switched by a large 3Ø 208v contactor (Nema Size 3), and contactors coil is 120vac.
> 
> I want to switch the 120vac coil with a 12vdc relay, but I keep burning out (or at least I think I am) the 12vdc relay!
> 
> ...



what is your source of 12VDC ?
a less than robust source will allow too much voltage drop and not operate the coil properly


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## frankendodge (Oct 25, 2019)

Looking at your drawing, the diode should be reverse biased across the 12v relay coil, not across the contacts.. and that would only be to protect the electronics in the 12v control circuit from back emf.
If you are using a 12v power supply, and a mechanical switch to control the relay it shouldn't be an issue..
The 12v relay contacts are breaking 120vac not 12vdc. There shouldn't be a resistor or diode across them. You are certain you have a 12vdc not vac coil in the relay?

You say you THINK you are burning out the relay. What are the symptoms, and what is the circuit doing that it shouldn't, or not doing that it should?


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## splatz (May 23, 2015)

frankendodge said:


> Looking at your drawing, the diode should be reverse biased across the 12v relay coil, not across the contacts.. and that would only be to protect the electronics in the 12v control circuit from back emf.
> If you are using a 12v power supply, and a mechanical switch to control the relay it shouldn't be an issue..
> The 12v relay contacts are breaking 120vac not 12vdc. There shouldn't be a resistor or diode across them. You are certain you have a 12vdc not vac coil in the relay?
> 
> You say you THINK you are burning out the relay. What are the symptoms, and what is the circuit doing that it shouldn't, or not doing that it should?


Right the diode is missing from the coil of the 120VAC lighting contactor, but if you put it across the contacts of the 12V relay, it should protect the relay contacts.


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## CMP (Oct 30, 2019)

I would use a MOV metal oxide varisistor directly across the terminals of the AC coil to shunt the turn off surge.





__





a/c coil transient suppression


Hi folks, I'm new to the group and to stamps, so thanks in advance.




forums.parallax.com








https://www.digikey.com/en/products/filter/tvs-varistors-movs/141?s=N4IgTCBcDaIAoEMB2CDOB7JBLAxgAgC8kAnEAXQF8g





https://api.pim.na.industrial.panasonic.com/file_stream/main/fileversion/1994


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## frankendodge (Oct 25, 2019)

Inrush on a size 3 contactor is around 700-800va from the first few datasheets I found. 
I suspect the contacts on an ice cube type micro relay would be pushed pretty hard at that.

I'd try a larger relay. Something like:









A diode across the 120v could would be a half wave dead short.. low buck smoke machine.
Across the contacts of the 12v relay would essentially pass a half wave to the 120v coil at all times.. what effect this would have on the coil I'm not sure. Not something I've ever seen(doesn't mean it's a bad idea.. splatz, is this commonplace on these large contactors?)
My thinking is the coil would be drawing excessive current through the diode as it wouldn't clamping the contactor cores together?
An mov makes some sense, but isnt it eventually destined to fail? Wouldnt it's failure cause problems down the line?

To the op: is it possible to replace the lv controls with a line voltage countdown timer switch, or is the lv wiring in place and buried?


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## joe-nwt (Mar 28, 2019)

frankendodge said:


> Inrush on a size 3 contactor is around 700-800va from the first few datasheets I found.
> I suspect the contacts on an ice cube type micro relay would be pushed pretty hard at that.
> 
> I'd try a larger relay. Something like:
> ...


Get a G7L-1A-BUB-DC12. Skip the diode/MOV/snubber/whatever.

The rest of your post is not helpful.


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## CMP (Oct 30, 2019)

Finding one could be the issue. Major vendors showing 39 to 61 weeks tentative lead time.
It's kind of pointless, if there is no feedback from the OP. There are other ways to go about it, but with no input, I don't see the reason to waste time.


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## paulengr (Oct 8, 2017)

If OP feeds a 120:12 transformer then rectifies it and feeding a big capacitor you do NOT get 12 VDC. You get 1.414 x 12 V = 17 V.

Even a DC coil has an inrush. Once the armature lands the coil impedance increases about 10x In counter EMF greatly decreasing the surge. DC coils on off state especially big ones need surge suppression but the coil isn’t usually the vulnerable one it’s the transistor outputs.


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## splatz (May 23, 2015)

frankendodge said:


> A diode across the 120v could would be a half wave dead short.. low buck smoke machine.
> Across the contacts of the 12v relay would essentially pass a half wave to the 120v coil at all times.. what effect this would have on the coil I'm not sure. Not something I've ever seen(doesn't mean it's a bad idea.. splatz, is this commonplace on these large contactors?)


You're right, I should have said


splatz said:


> Right the diode snubber is missing from the coil of the 120VAC lighting contactor, but if you put it a suitable snubber across the contacts of the 12V relay, it should protect the relay contacts.


There are different snubber circuits, some are suitable for AC, some for DC. Snubber circuits may use flyback diodes, resistors in series with capacitors, MOVs, and I'm sure other things. 

To the OP's point - if you roll your own snubber circuit, you would have to have to make sure all those electronic components are listed and rated for the circuit, and can handle the kickback pulse. My point was there are ready to use snubbers available with the proper ratings, so you can leave the engineering to the manufacturer. Or actually, try to use relays / contactors with this protection built in, but if you're stuck with one without, for simplicity buy a ready to use product rather than roll your own. Similar to the load resistors put in to resolve issues with LED dimmers etc. You could use a properly rated resistor, but you might be better off using the one Lutron makes, if an inspector asks about it, no long conversaion if you can show the Lutron part number and spec sheet. 

This one from Red Lion ought to work for this lighting contactor, it is the RC (resistor-capacitor) type. It's under $20 at Allied.and you could put it across the relay contacts, or across the contactor coil.


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## splatz (May 23, 2015)

Now here's a question. I can see how the MOV snubber works with AC - the MOV closes on a spike in voltage. How does the RC resistor-capacitor snubber work? 

I am thinking the collapsing magnetic field produces a DC pulse, even though the coil is supplied with AC. The polarity of the pulse depends on whether the supply was interrupted in the positive portion or the negative portion of the AC cycle. 

Capacitors have polarity. Wouldn't you need an RC in both directions, one if the contacts open in the + part of the pulse, one if the contacts open in the - part of the pulse? Otherwise you're only protected half the time.


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## paulengr (Oct 8, 2017)

splatz said:


> Now here's a question. I can see how the MOV snubber works with AC - the MOV closes on a spike in voltage. How does the RC resistor-capacitor snubber work?
> 
> I am thinking the collapsing magnetic field produces a DC pulse, even though the coil is supplied with AC. The polarity of the pulse depends on whether the supply was interrupted in the positive portion or the negative portion of the AC cycle.
> 
> Capacitors have polarity. Wouldn't you need an RC in both directions, one if the contacts open in the + part of the pulse, one if the contacts open in the - part of the pulse? Otherwise you're only protected half the time.


1. Only some electrolytic capacitors have a polarity. Most do not.
2. A rapid change in voltage across a capacitor causes dead short as it charges. This goes both ways so bad itself it does nothing but create a pulse delay. Combined with series inductance or resistance creates a filter that lowers the peak and stretches it out over time.


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## emtnut (Mar 1, 2015)

paulengr said:


> 1. *Only some electrolytic capacitors have a polarity*. Most do not.
> 2. A rapid change in voltage across a capacitor causes dead short as it charges. This goes both ways so bad itself it does nothing but create a pulse delay. Combined with series inductance or resistance creates a filter that lowers the peak and stretches it out over time.


Bit of a typo? - Only SOME electrolytic capacitors are non-polarized (Bi-polar) ... MOST DO!



paulengr said:


> 2. A rapid change in voltage across a capacitor causes dead short as it charges. This goes both ways so bad itself it does nothing but create a pulse delay. Combined with series inductance or resistance creates a filter that lowers the peak and stretches it out over time.


I don't quite understand what you're saying here. But the easy way to think of this is you "can't change the voltage instantaneously across a capacitor". So any feed back surge is shock absorbed by the cap.

Also in the diagram splatzy posted, note that there is a polarized cap in one, and a normal bi-polar in the other. The first across the load is polarized, the one across the contacts is bi-polar.


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## power (Feb 27, 2012)

backstay said:


> Most lighting contractors are only powered to change state. They are not powered on all the time.


This is a non-latching Nema size 3 contactor (3-pole). Form A contacts. Out of box state is NO. Energize the coil (maintained energize), contacts close.


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## power (Feb 27, 2012)

micromind said:


> What really are you using?
> 
> Is it the coil or the contacts that burn up?


In my original drawing, it's the contacts in the 12vdc relay that burn up. I was wondering if a resistor/diode (snubber) across the coil (of the 12vdc relay) would prevent the relay from "chattering" on a power-down, thus avoiding the contacts from rapidly open/closing several times a second. As these contacts are subjected to DC currents, it is hard on them to open them under load, especially several times a second!


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## micromind (Aug 11, 2007)

If the relay contacts chatter and thus, the 120 contractor chatters, it will indeed burn up the relay. You'll need to figure out why it chatters. What is causing the 12DC to turn on and off and on and off........

Start by positively removing the 12DC from the relay coil and if it doesn't chatter, then the problem is in the 12DC system. 

It's possible that the flyback from the 12DC coil is causing something to turn off and on and off and on. If so, a diode across the coil may very well fix the problem. 

Be careful with the diode, it needs to be such that it conducts on reverse voltage and not normal voltage. If it is installed the wrong way, it'll short out whatever is feeding the coil.


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