Transient Voltage question

[DiodeDave]

I have a DC motor controller powered by 2 12v batteries in series. (The controller is a 4qd Pro 150 which powers my locomotive. The leads from the batteries have a 30a fuse.)

I would like to put a relay between the batteries and controller in order to turn off all power to the controller.

I've read enough about transient voltage to be concerned.

Is there any danger in a spike when the relay is opened (or closed)?

Thank you

 

[Arouse1973]

Hello. You would normally connect an flyback voltage suppressor across the relay coil and if needed an arc suppression circuit across the contacts. We would need more details about what relay you intend to use and what type of load it will be switching initially.
Thanks
Adam

 

[DiodeDave]

Adam

I'm not sure what you are asking. I will have a 40a automotive relay in series with the batteries. When the relay is closed, and opened, the controller and motors will not be operating. There won't be any load except for possibly a few small LEDs and a brushless fan from a computer CPU.

The power switching to the 8 amp motors is all done in the controller. Due to the location, it is not possible to actuate the relays in question while the motors are operating.

 

[Harald Kapp]

Adam is referring to two different symptoms:

1. When the current through a coil is interrupted, a high reverse voltage (flyback voltage) develops across the coil due induction: v=L*di/dt with di/dt being very high (dt -> 0). This high voltage can cause havoc in the attached circuit. Therefore it is usually suppressed by connecting a flyback diode in reverse polarity across the coil.
2. When a (relay) contact pair is opened while current is flowing, the air between the contacts can become ionized and an arc can develop. The heat from the arc can keep the air ionized and current continues to flow until the contacts (or another circuit element) have been vaporized and the circuit is finally interrupted. This is mainly an issue with DC currents as they can persist rather long. An AC current has zero crossings which help to cancel the arc since current is null during that time.
   Several methods are known to suppress this kind of electrical arc. If you can ensure that no high current is flowing when the relay is actuated, you don't need a suppressor circuit.

There won't be any load except for possibly a few small LEDs and a brushless fan from a computer CPU.

This shouldn't be an issue for an automotive relay. Note that you need a relay with at least a 24V contact rating. To be on the safe side better find one with a 30V rating or higher.

 

[Arouse1973]

Yes thank-you Harald, you are correct. Also the load that is being driven maybe initialy many times the constant rating. This also needs to be taken into account. A 30 Amp fuse is for a fault condition and will pass many times that current for a very short time, but the relay contact could be damaged in that time.
Adam