# spike protection using zener diode rather than RC snubber for 220vac contactor

#### Tinsae

Joined Jan 8, 2015
110
Dear all,

I am building a PLC system which control 24vdc relay and the relay finally control 220vac coil 380vac magnetic contactor. For the medium size contactor RC snubber became okay but for the bigger and other coil device it need tuning. So I am looking for zener diode way. But the only available zener diode is 200V and other low voltage 24 and below in my location.

question
1. Can I connect those different voltage zener diodes (200v, 24v and 12v) in series to clamp the voltage say at 270Vac
2. Do I need resistor in series to limit the current?
3. What wattage of the zener diode is recommended for AC3 11kw contactor

#### Tinsae

Joined Jan 8, 2015
110
Dear LowQCab,
MOV degrade each time the contactor switches that is why I am thinking zener

#### LowQCab

Joined Nov 6, 2012
2,351
You don't have enough Energy built-up in that Contactor-Coil to degrade a MOV.

If You want to suppress the Spike even further, simply add a Power-Resistor across the Coil.

You can also swap-out the Coil for a lower-Voltage version from the Contactor manufacturer.

A fourth option is to use a Solid-State-Relay (SSR) that has a Zero-Crossing Output to switch the Coil
then there would be little, or no, Spike to deal with in the first place.
Also, You can eliminate the intermediate Relay, because the PLC can Switch it directly,
as it only has a LED to light-up instead of a big Coil.
A 5-Amp, 230-Volt version is around ~$20.oo. ( this is not a Zero-Switching version ) https://www.digikey.com/en/products...5/752093?s=N4IgTCBcDaIMJzARgOwDYC0A5AIiAugL5A . . . . Thread Starter #### Tinsae Joined Jan 8, 2015 110 You don't have enough Energy built-up in that Contactor-Coil to degrade a MOV. If You want to suppress the Spike even further, simply add a Power-Resistor across the Coil. You can also swap-out the Coil for a lower-Voltage version from the Contactor manufacturer. A fourth option is to use a Solid-State-Relay (SSR) that has a Zero-Crossing Output to switch the Coil then there would be little, or no, Spike to deal with in the first place. Also, You can eliminate the intermediate Relay, because the PLC can Switch it directly, as it only has a LED to light-up instead of a big Coil. A 5-Amp, 230-Volt version is around ~$20.oo.
( this is not a Zero-Switching version )
https://www.digikey.com/en/products...5/752093?s=N4IgTCBcDaIMJzARgOwDYC0A5AIiAugL5A

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Dear LowQCab,
1. the company where I bough the plc doesn't like using mov on contactor (degrading issue)
2. you pointed wonderful option "If You want to suppress the Spike even further, simply add a Power-Resistor across the Coil."
I like this idea. since i don't care about compactness of the system, all what i want is reliablity so i like this idea and i am thinking putting a resistive load like 60w bulb across the contactor coil what do think? i know this is bulky but i dont care if it is reliable.
3. or you can advise me on the zener diode way
4. there is no solid state switch for inductive load on my location

Joined Jul 18, 2013
25,654
I have always used R/C snubbers across contactor coils with success.
Why the apprehension for their use?
Better still, use DC coils with Diode.

#### Tinsae

Joined Jan 8, 2015
110
If you remember you were the one who gave me initial value for the R and C when I deal noise problem in atmega project. Based on your suggestion I solved that problem (for contactor). But when I tried to apply for solenoid on other case it wasnot as good as for contactor. RC values should be calculated for each coil or should be tried with the help of oscilpscope but I cant have scope and cant measure inductance of coil. Recently I show a video on automationdirect.com from where I imported the plc that one RC values cannot work on other coil it even add oscillations instead of reducing spike. That is why I am trying with zener diode.
Can I do it with zener diode?
At which voltage should I set the zener circuit for 220vac coil?

#### Tinsae

Joined Jan 8, 2015
110
Dear all,

#### LowQCab

Joined Nov 6, 2012
2,351
Good Video,
but they completely left-out using a Power-Resistor,
which can be extremely effective, without affecting release speed as much as a hard clamp will.
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#### Tinsae

Joined Jan 8, 2015
110
Good Video,
but they completely left-out using a Power-Resistor,
which can be extremely effective, without affecting release speed as much as a hard clamp will.
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By the way I have tried with power resister way. I use 5w incandescent bulb (there is no almost nothing in my place market that is why I use bulb) and I saw hope using it. I am thinking using 60W for better effect.

#### LowQCab

Joined Nov 6, 2012
2,351
A Light-Bulb is probably the cheapest solution.
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Joined Jul 18, 2013
25,654
If you are designing the system as you say, you should be using DC contactor coils as a preference together with diode across the coil.!
Rather than the AC version.

#### cmartinez

Joined Jan 17, 2007
7,764
Dear all,

I am building a PLC system which control 24vdc relay and the relay finally control 220vac coil 380vac magnetic contactor. For the medium size contactor RC snubber became okay but for the bigger and other coil device it need tuning. So I am looking for zener diode way. But the only available zener diode is 200V and other low voltage 24 and below in my location.

question
1. Can I connect those different voltage zener diodes (200v, 24v and 12v) in series to clamp the voltage say at 270Vac
2. Do I need resistor in series to limit the current?
3. What wattage of the zener diode is recommended for AC3 11kw contactor
One word: TVS ... Transient Voltage Supressor ... it's basically two zeners back-to-back. I've been using them for years and they've gotten rid of all my spike-surge nightmares ... they're available at ratings of thousands of volts.

I.E. I've been using TVS' 90VDC motors with a rating of 380V to protect mosfets rated at 500V, and so far no problems after more than 15 years.

#### Tinsae

Joined Jan 8, 2015
110
DC contactor and TVS are nice but the are not available in my location. My mistake was I didnt import them with the PLC. What is available now is 82V zener diodes and bulb. I am going to try with them. Yesterday I tried one 120V and two 82V zener diode in serie as one zener and connecting another same group back to back. When I tried it, it burned. I realized the sum of one group was 284V, but I didnt account peak voltage of 220V

#### kaindub

Joined Oct 28, 2019
100
Do you have a real problem or just guessing at this stage?
Having over 30 years experience with PLC I can say that if you are using a relay output from your PLC to drive a 220VAC coil, no suppression of any sort is needed. The company I once worked for sold RC suppressors for coils, but we sold very few if any. So spikes on AC systems do not seems to be a problem
Only once did we encounter a problem with a coil interfering with other circuits and an RC suppressor fixed the problem.

#### prairiemystic

Joined Jun 5, 2018
290
AC contactors have high inductance once the armature pulls in and the magnetic circuit has no airgap.
This is necessary for their impedance so they don't get too hot. But inrush current is also very high with them because inductance is much lower when the armature is up, until it pulls in which takes a long time.
With high inductance, it means their energy storage is high and it is customary to use a MOV to limit back-EMF, protecting switches or relays. MOV's don't age terribly unless they are abused/undersized. Your contactor isn't activating that often.

If you do the math, a TVS or back-back zener clamp is challenged to dissipate the pulse energy, the voltage is very high. You would have to know the contactor's inductance and series resistance to do simulations to know if the part is sized correctly.
Consider unidirectional 1500W TVS they go up to 600V, without us knowing details of the contactor.

Joined Jul 18, 2013
25,654
AC contactor coils also often have a shading ring which makes them run that much hotter.
The only edge an AC armature/contactor/relay etc has is at switch on, after that, the DC is far better option.
I have always had success with the R/C snubber on AC contactors and do not remember any need to go with any other solution!

#### du00000001

Joined Nov 10, 2020
106
DC contactor and TVS are nice but the are not available in my location. My mistake was I didnt import them with the PLC. What is available now is 82V zener diodes and bulb. I am going to try with them. Yesterday I tried one 120V and two 82V zener diode in serie as one zener and connecting another same group back to back. When I tried it, it burned. I realized the sum of one group was 284V, but I didnt account peak voltage of 220V
As you already learned, the peak voltage of the nom. 220/230/240 V is way higher than the nominal value. Basically you can combine zener diodes in series, but allowable power dissipation might be smaller than what you'd like to have. (This is what TVS diodes are made for: worse "zener behavior", but way higher power dissipation.)
3 x 230 V zeners in series plus another 3 x 120 V zeners reversed might do the trick.

Just had to find out that most of the African continent is not covered by distribution. If you have some power rating specification, you may start a private communication to see what I might be able to do for you.

#### kaindub

Joined Oct 28, 2019
100
Remember that a zener diode uses the reverse breakdown voltage to generate the cutoff voltage. Bias it the other way and it acts just like a diode.
Also zener diodes in themselves cant dissipate a lot of power. in a normal shunt regulator circuit a resistor is used to limit the current in order to limit the current to a safe level through the zener. In an AC circuit there is no current limit resistor so the power through the zener in clipping mode is almost infinite.

#### Tinsae

Joined Jan 8, 2015
110
Do you have a real problem or just guessing at this stage?
Having over 30 years experience with PLC I can say that if you are using a relay output from your PLC to drive a 220VAC coil, no suppression of any sort is needed. The company I once worked for sold RC suppressors for coils, but we sold very few if any. So spikes on AC systems do not seems to be a problem
Only once did we encounter a problem with a coil interfering with other circuits and an RC suppressor fixed the problem.
Dear Kaindub,

I was a guy of DIY, for some times I have been dealing with control system built from scratch using micro-controllers. But I have seen a serious problem (also traumatized ) of spikes when contactor/or solenoid switches. Its problem make the mcu to reset but I tried to handle it with codes seemingly successful but I begin to see another problem so I switched back to prevent it rather handle the spike. Then I tried to add RC snubber, that gave a solution for some type of contactor but not for solenoid. Now I am not doing some DIY stuff, but a serious project with PLC and I should not allow the system to a fail at some times. You dont know when and where the spike could be a problem. I have tested the above diy project about 90 times switching the contactor but that does not guaranty the safety of the system in fact it didn't. Now I didn't see any problem (resetting or mis behaving) in the plc when the contactor switches (with RC selected by me) but the scada pc monitor switched off momentary, this is an indication of spike which must be solved.

I wish my PLC would be robust to the spike!