View Full Version : question about relays

22-01-2012, 10:55 AM
Hi all can someone give me an idea how to enable a bonmet servo drive (12v dc) by using the 5v enable output from my breakout board.I was thinking of using a small 5v relay to switch the 12v supply but i wondered how much current this would pull or should i use a solid state relay.

Robin Hewitt
22-01-2012, 08:47 PM
A fat Darlinton transistor perhaps?

If you must have a relay why not a 12 volt relay and drive it through an NPN?

Make sure you have a diode from the collector to the power rail.

If this doesn't make a lick of sense I can explain...

23-01-2012, 12:18 AM
Another option.....an N power mosfet.

The mosfet source connects to ground, the mosfet gate goes to your 5V switching input ....and the mosfet drain goes to one side of your 12V relay coil ...on the other side the coil is 12V supply. With no 5V switching waveform, no current flows through the moseft & your 12V relay is denergised. Once 5V switching waveform is appliedto the moseft gate, MOSFET is turned on, 12V flows through your relay coil to ground through the mosfet.

Have a look at this...

http://www.falstad.com/circuit/e-nmosfet.html (but the top of that circuit would be your coil to 12V)

If you want isolation, you can source these with opto isolators built in.

Web Goblin
23-01-2012, 06:47 AM
I would go for SSRs. Used them for part of my build.