Sticky relay's

[Mouldy]

Hey all,

Got some 15amp CPU based Electronise speedos in bodger (one of my feathers) and at debenham the 1 side stoped going in forward drive but still went backwords. However the other side seemed normall.
Ive checked for shorts etc and untill today this strange problem dissapeared and its just come back, Exsept this time the 1 side (same side as before) refuses to go backwords. Anyone else had this happen? Or know what it could be?
Thanks!

[grant_ploughbot]

I had this happen(well in venom anyway).
The relays usually tend to stick in the 30 amp, but are known in 15s, best thing to do is get them altered to external relays, the one in them as standard ar often not up to the job.
Also, if you hit them, they usually unstick, hehe

Grant-PloughBot-Team Rc Wars

[blazerbotics]

Alphas done that to 3 sets of controllers, i just keep replacing the relays, cant be bothered to wire up an external set! At the weekend i think i had one black out as i totaly lost foward on one side, so had to drive the start of the fight in reverse! yet halfway through it started working again.

[talulah]

This is a common fault with relays. What happens is when they are switching high DC currents, just as the relay opens, and arc occurs across the contacts, this lifts material from one contact and deposits on the other, leaving a pit on one side and a lump on the other. The next time the contacts close, the lump snuggles into the pit, and the contacts lock together.

Thats all very interesting, but how do you stop it? Heres some alternatives:

1. The relay is probably under-specified. Switching 30A DC requires a relay rated 30A SWITCHING, not just 30A continous current. Use a 30A automotive relay like a Siemens Mini-K or Hella. These are very rugged and designed for switching high current inductive loads.

2. Use a solid-state relay to turn the load on and off instead. These are circuits which look like a relay from teh outside, but actually use a MOSFET to do the switching. However, these are quite expensive.

3. Build your own discrete MOSFET circuit. This isnt too hard as long as you have a convenient 12v supply to turn the MOSFET on with.

Good luck!

Thanks for the advise people, After hitting the robots front scoop into a curved shape via a large hammer (while still atached to the robot) they seem to have started working again...After the weekend I think Ill go for the exsternal relay vershions of the electronise See you all at newark!