Relays, arcs and diodes

[Triton]

Hey (again) all,

I think I need your help on a subject. I searched the forum topics but could not find something relevant so I decided to ask directly.

The question is: Do I need diodes to protect my DPDT relay circuit from counter-EMF?

I have read that the coil of the relays produces a counter-EMF that might be several hundreds of Volts. This counter-EMF may harm other sensitive components of the circuit. Yet, in my circuit there is nothing between the switches and the relay coil to be harmed. Should I protect the switches with diodes? Does the counter-EMF harm the coil itself?

Again, the motors produce a counter-EMF that may cause arcs to the contacts (COM-NO) of the relay thus threatening the relay.

I will try to describe briefly my control circuit for the ROV (Fig1).

My control box has 3 switches to control FWD, STOP and RVS of each motor (only one switch is shown in Fig1). From each switch I send the signal (12V, approx 100mA) to the coils of a pair of DPDT relays (connected as shown in the tutorial of homebuiltrovs site http://www.homebuiltrovs.com/howtorelays.html) through a 100m cable. This 100m cable consists actually of 7 lines of 24 AWG (2x3 DPDT pairs + 1GND). The relays control the current for the motors. The motor circuit is powered by a separate sealed battery on the ROV.

Some technical data:

My motors are L1600 Johnson bilge pumps (12V/24V) with plane props (8’’x4) that draw 5A when running in water (1.2A in air).

The DPDT relays are rated for 8A at 30VDC. Their specifications can be seen in the attached file. I will not pretend that I understand fully these specifications. This type of relay is mentioned as miniature heavy load relays. Is there something in these specifications saying they can handle (or withstand) counter-EMF?

Note: In Fig.1 I have noted (with arrows) the two places on my circuit that diodes might be needed.

Thanks in advance

[derelicte]

only if you are powering the relay's coil with something like a microcontroller or transistor/fet. for switches, you don't need it, but it wouldn't hurt.

[Triton]

Thanks derelicte for the quick answer.
I had imagined so about the coil circuit. I could put flyback diodes in the coils. They are cheap and I already have them.

Any suggestions about the motor circuit?

[derelicte]

the same applies. if you are using something like a h bridge, you need diodes. relays are just switches, so you don't need diodes across the motor in that case.

[Triton]

Thanks again.
I'll assemble and pot everything and I'll go for some diving to see how it works under pressure.
I'll try to add my project to the forum, hopefully in the next 2 weeks, to share things.