I want to use a reed switch to control a 12VDC motor, infrequent use for maybe 10 seconds at a time, 30 seconds max, steady state current draw of maybe 500 milliamps, peak 2 amps. It's not a mission-critical part, failure is at worse a minor inconvenience. The goal is to use parts I have on hand to automate a bothersome task. It'll be a float switch for operating a pump.

I have here a reed relay which tests show will function just fine as a magnetic proximity switch. It's rated 0.5 amps at 120V, or 10W continuous use.

I figure I can always switch a transistor with it, but I'm thinking about just powering the motor directly.

I've done some research on switch design, AC vs. DC applications, arcing, heat, etc. Just wondering, what's the probability of getting a few years' good service in an application like this? A snubber ciruit will improve my chances?

One last question, if anyone cares to comment, I'd appreciate some advice on simple solution to avoid a lot of on/off cycling.

Thanks!

 

I assume the motor is AC. Is that correct?

 

sorry, it's a 12V DC motor, added to OP, thx

 

It may work ok with just the reed switch but motors have a large inrush current when first turned on which is tough on switch contacts. Measure the DC resistance of the motor if you can. You can calculate the initial inrush current from that.

I would suggest adding a transistor. It will greatly increase the switch life.

In either case place a snubber diode in parallel with the motor leads to absorb any inductive kick when you turn the motor off. It can be any diode rated at 20V, 1A or more. Connect the cathode to the positive motor lead and the anode to the negative lead.

Reducing the on/off cycling requires adding hystersis. Commercial sump pumps have a float that slides up and down through the switch toggle with the turn-on contact point several inches above the turn-off point. Thus it always pumps several inches of water each time it turns on.

One possible way to do this with a magnetic switch is to have a long magnet at an angle to the switch movement. The reed switch will stay latched at a lower magnetic field than it requires to energize. Perhaps with proper orientation of the magnet it can be made to not energinze until near the top of the magnet, but release near the bottom.

 

You could use a second relay thats capable of handling the motor current/voltage, controlled by the reed switch contacts.

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Eric
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