This is my first time attempting a counter EMF circuit.

The project is a Marine Autopilot Motor Drive. It uses a reversable 18amp, 12vdc brush type motor. The LED's are to show pump direction. I wish to control voltage spikes coming back to the digital controller and other system components. The relays are rated 30 amps, with 12v coils, with built in surge supression resistors. I do not have the coil resistance or how much power they need.

The attached image shows two back to back zener diodes across the motor terminals.

My questions are:

Will the surge resistors protect the polarity lights?
Are the two 20v zener diodes sufficient to absorb spikes from the motor windings?

Thanks for any help and comments.

 

Welcome to AAC!
1) Are you sure the relay coils have surge supression resistors? It is common to have built-in diodes for back-emf suppression (so coil polarity is important).
2) The LED current will be close to 30mA, which is the rated maximum for many LEDs. It is good practice to run components at less than their rated maximum to ensure a long working life.
3) If 18A is the normal running current of the motor the starting/stall current will be much higher. Personally, I'd go for beefier relays/contactors or preferably solid-state switching (which would eliminate clatter from relays/contactors).
4) With that size motor it will be important to bring it to a halt before changing the direction of rotation.

 

The zener diodes will have to be able to cope with a surge current of 18A. When the relays switch, the inductance of the motor will continue to drive the motor current somewhere - in this case through the zeners. You would probably be better to use the standard four diodes arrangement.

 

Hi Alec_T-

1) Yes I double checked my relay supplier, and they have resistors built in.
2) I can change the resistor value to help that, thanks!
3) I am looking at Mosfet circuits for the H-bridge now, but need a design that uses 12v switching for the 4 transistors. Any sources appreciated.
4) The motor has a fair amount of resistance from the hydraulic pump, so I think it should stop turning pretty fast. Fast enough? I don't know..

AlbertHall- Your circuit looks nice and simple, do you have any tips for sizing the capacitor you put across the motor leads?

 

The capacitor is for interference suppression. You can worry about that, if you need to, when it's all working.
Maybe the 'resistor' is a varistor which would have the advantage over a diode that you can connect power to the coil either way round.

 

I don't see how a resistor in parallel with the coil can provide any surge suppression, unless it's a varistor .
I suggest you connect a reverse-biased diode across the coil as belt-and-braces back-emf protection.
If you opt for a MOSFET bridge instead of relays, the two high-side MOSFETS will need to be P-type, unless you have a voltage source higher than 12V to ensure the gates are driven high enough to turn the MOSFETS on fully.