OT: I need help from electronics experts on using relays

[MattAnonymous]

Hey guys this isn't for a pedal, unfortunately, but my engineering team and I are finishing up a remotely controlled project.  We need to run about 18v to some motors through a circuit that was originally designed for about 6 volts.  The obvious problem is that the amperage is gonna be too high for the components (for example, it fried a transistor).  One guy in the group came up with the idea to run a smaller voltage (and amperage) to the board to use it as a switch for some relays.  In turn, the relays would take the higher power and distribute it to the motors.  The motors must also run in reverse (the circuit does this by inverting the polarity of the wires).

My questions are:
Would this work?
If yes, what type of relays would we need?

If no, any other ideas?

Thanks a bunch in advance,
Matt

[davebungo]

Relays will probably be unreliable unless you are really careful to avoid contact damage due to flyback currents and such given that you are driving an inductive load.  You really should consider using something like a H-bridge driver chip driving MOSFETs designed for the purpose.  These have very low on resistance and fast switch on times so they don't get warm.  Have a look at the Motorola web-site and search for H-bridge.  I think you will get somewhere following that line.  

I should explain that the H-bridge driver chips generally provide a high current drive to overcome the gate to drain and source capacitances in order to provide a really sharp turn on and off, a charge pump to provide a "high side" Vgs (typically above the supply rail) and also protection against over/under voltage etc.

[Transmogrifox]

Here's my 2 cents:

If the current is less than an amp, you can use this:

www.geocities.com/transmogrifox/relay

If it's a high current load, I recommend using a solenoid---generally you use a relay to fire the solenoid coil, and the solenoid to drive the motor.

I think you could use a system similar to a car ignition system, and probably just go to your local autoparts store and get a solenoid.

For large motors, relays would probably be unreliable, as expressed by davebungo, but solenoids are designed for such a job.  If the motor is lower power and draws less than about 500 mA, then I highly doubt a relay would be a problem--but if you're toasting transistors, then you probably are doing a couple amps or more and need to consider a solenoid or H bridge driver.

[niftydog]

couldn't you either replace or add to the existing transistor driver circuitry to beef up it's current handling?

Perhaps a bit of bias tweaking and some new high current transistors will do the job? Failing that, use the existing circuitry to drive some high power darlingtons or something like that...

just some ideas!

Heavy duty relays are kinda expensive, and you'll definately require snubber circuitry to surpress the inductive kick from the motor coils.