Diode supression of relay back emf 1

[megahurts]

I am using a 12V relay to operate a DC motor in a motorcoach. I was wondering how important it is in this application to use back emf supression diodes across the relay coil. Do I really need to care about the energy spikes being put back on the 12VDC supply line? I had already put them in my circuit as a "sure, why not" but then realized that I now need to add a full-wave bridge to protect against reverse input polarity. I would like to drop the relays and the bridge if they are not really needed.

 

[IRstuff]

Not to protect the supply!! to protect the TRANSISTOR!!

TTFN

 

[artym]

If you driving the relay with a non-electronic switch (i.e another relay or a toggle switch) the auto environment is so full of spikes that it might be ok to omit the diode.

It IS good practice to supress, and a small, cheap "no polarity" way to do it is a "ZNR" disc . Digi-key sells a n 18vdc rated part for .27 (p7282-nd)

 

[megahurts]

Yes, I am using a toggle switch to operate the motor, sorry, I should have mentioned it in my initial post.

Thanks artym for your suggestion. I have seen surge absorbers used in other applications, but didn't think to use one here. There is a cost increase over a diode, but I'll save on not using the full-wave bridge.

 

[OperaHouse]

You can also use a resistor for suppression and that would not be polarity sensitive. Use one that is between 10 and 20 times the coil resistance. If you have a 1 amp solenoid, use a 220 ohm 1 watt resistor. Whatever the strady state coil current is, multiply that times the resistor value to get the peak spike. Coils will probably produce far less than that. It may sound like a lot, but it is considerably less voltage than without it. Only negative is the circuit draws a little more current.

 

[jbartos]

Suggestion to megahurts (Electrical) Sep 18, 2003 marked ///\\I am using a 12V relay to operate a DC motor in a motorcoach. I was wondering how important it is in this application to use back emf supression diodes across the relay coil.
///Not necessarily back emf suppression diodes across the relay coil. The suppression resistors are most reliable and least expensive.\\ Do I really need to care about the energy spikes being put back on the 12VDC supply line?
///Yes, actually about voltage spikes that contain some energy. However, the voltage spike is actually detrimental.\\ I had already put them in my circuit as a "sure, why not" but then realized that I now need to add a full-wave bridge to protect against reverse input polarity. I would like to drop the relays and the bridge if they are not really needed.

 

[jbartos]

Suggestion: Visit

http://relays.tycoelectronics.com/app_pdfs/13c3311.pdf

for suppression techniques and hardware

 

[Lakey]

Hi,

There is another consideration that should be taken into account if suppress the relay. Doing so will slow the relay’s drop out time. This will reduce the life of the main contacts due to increased arcing and may affect the applications performance.

A balance can be achieved by using a diode in series with a resistor that are connected across the coil terminals.

Regards,

 

[jbartos]

Suggestion: The relay drop-out time and arcing reduction is possible to control by a stronger spring.

 

[Lakey]

Jbartos,

True, but does Megshurts want to be messing about with springs?

If he does, he may have a problem with relay's operating characteristics i.e. pull-in & drop out voltages.

Regards,

 

[Farkel]

Suppression would be good in either way suggested also to
prevent magnetizm build up in the relay with heavy duty cycle.

 

[jbartos]

Suggestion to the previous Lakey posting: True, the spring of an as-built relay is better leave as it is. My posting addressed the problem generically, i.e. that the relay may be engineered, designed and manufactured that way.

 

[CurtB]

Suppression is a must for automotive relays if you ask me. A diode is what I typically use across a relay. It is a possibility that over time, the diode will fail. In order to make it easy to replace I use a plug in type. They can be found at Digi-key.

 

[wired1]

Note that the use of diodes or suppressors on dc relays is considered mandatory enough that some manufacturers design them into the relay.