# Electromechanical Relay & Snubber Circuit

Discussion in 'Microcontrollers' started by Urahara, Feb 9, 2009.

1. ### UraharaMember

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Hi

I intend to use a MCU pin to control, via a npn transistor, a 9V DC relay coil which switches on the primary of a transformer (inductive load?) of a device. Let's say the load takes up either max 4A/240VAC, or 5A/110VAC).

On the DC end, I know it is recommended to use a flyback diode to handle the voltage spike to protect the transistor from damage.

My questions are on the AC end :

1) To protect the relay contacts, it is recommended to use a RC snubber network across the contacts. If I use a relay that can handle 10A resistive load, such as http://www.semics.pl/stykowe/przekaznik_JQC-3FF_HONGFA.pdf is the snubber circuit necessary?

2) Actually my true concern abt the snubber circuit is about 'safety' since I think there is AC current flowing even when the relay is off. Is this a real concern? If a RC circuit of 100ohms-0.1uF is used, how do we calculate the current flowing?

Thks!

2. ### LeftyretroNew Member

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I think you just have to compute the capacitive reactance at the operating frequency (60hz?) for the cap and add the 100 ohms resistor to it. Then add the impedance of the load connected to the relay contacts. Then using ohms law you should be able to find the current flow when the contacts are open.

Lefty

3. ### UraharaMember

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thks! assuming the current flow is abt 10-20mA with the contacts open, is this something one should be alarmed about?

Guess I was "schooled" in the thought of never having anything running when u are dealing with AC, so the thought of having a snubber circuit to fix something but which leaves a small current still running worries me.

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5. ### LeftyretroNew Member

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As it would worry me also. 10-20ma is enough current to hurt someone, so snubbers should probably not be used where anyone could come in contact with the wiring. I would think converting to solid state relays or using over-sized contacts might be a safer approach. By your oversizing the contact rating for the service I think that would be good enough.

Lefty

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Hello,
I'm sorry for my misunderstanding,
How can a current flow through the contacts of the relay, when they are open?

7. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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By placing a capacitor across them - which would be a BAD idea.

Although the current wouldn't flow through the contacts, but around them through the capacitor.

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Now i understand, thanks.

0.1uF capacitor has a quite large reactance, 1 / (2Π*60*0.1µf) = 26.525kΩ.
Isn't it considered a very large stand-by power consumption?

9. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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Yes, and you wouldn't do it - no one here suggested you should.

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11. ### UraharaMember

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Hi Nigel

From google-land, snubber circuit seems to be quite widely used to reduce contact arching on electromechanical relays. Would like to hear your opinion on why you feel it shouldnt be used. Besides derating the relay to handle arcing, do you know of other ways?

Thks!

12. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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As in this application we're switching AC, it's self quenching anyway, as the power drops to zero - which is why relay contacts have far higher AC ratings than DC.

If it was DC then a snubber is likely to be of more use, and there's no leakage problem through it.

13. ### lalesa@silvanlabs.comNew Member

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Snubber CKT problem

Hi,
I am using a relay to control loads and snubber ckt is used to avoid arcing but the problem is when the relay is open the voltage around 185V(AC) will be there at the load......How to avoid this??? is this the problem or normal operation of Snubber ckt???

14. ### MosaicWell-Known Member

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Snub the relay coil driver to the maximum acceptable voltage based on the specs of the coil driver semiconductor. Use a flywheel diode of reasonable speed (1n4001 will work) in series with a unidirectional transorb with a rated max Clamp Voltage under the max breakdown voltage of the relay driver semiconductor. The two diodes are oriented to provide for reverse breakdown of the transorb driven by the relay coil backemf.

This will have nearly a 10 fold reduction in relay response 'opening' time with consequent improvement in contact life as arcing is reduced as well.

Last edited: Nov 17, 2010