Electrical Question

[David Pascoe]

I have a rather expensive digital timer rated at 120AC, 15A and a Leybold vacuum pump, 220 VAC, 2.0A. Can I wire just one leg of the 220 to the switch and control the pump that way? I realize that leaves one one leg of the pump circuitry energized when the switch shuts off, but it does stop the pump as I once wired a small fan this way without a problem.

Does this idea present any serious problems/dangers? I resort to this because I can't find a 220 digital timer similar to what I already have.

[Jim Becker]

It would likely be better to use a contactor to control the 240v to the pump and use a 120v timer on the control side. IMHO.

[Mike Henderson]

Yep, get a 120V DPST (or DPDT) contactor and switch the 240V.

Mike

[Paul F Franklin]

To answer your question directly, your timer would still be switching 240 volts if connected as you propose and would likely quickly be toast.

So, as Jim and Mike said, use a contactor.

Something like this would work nicely: https://www.functionaldevices.com/products/building-automation/details/RIB01P/

[Wade Lippman]

It was 15 years ago, but I bought a DP contactor on ebay for $10 to replace one on my TS that burned up.

That said... your switch would only see 120v and should work normally. I wouldn't do it though; there are enough dangers in life without creating them needlessly.

[Pat Barry]

I have a rather expensive digital timer rated at 120AC, 15A and a Leybold vacuum pump, 220 VAC, 2.0A. Can I wire just one leg of the 220 to the switch and control the pump that way? I realize that leaves one one leg of the pump circuitry energized when the switch shuts off, but it does stop the pump as I once wired a small fan this way without a problem.

Does this idea present any serious problems/dangers? I resort to this because I can't find a 220 digital timer similar to what I already have.
No - don't do that.

[Dan Friedrichs]

To elaborate on why you shouldn't do this: the switch is still "blocking" 240V, so even though it's only on one leg, the full voltage is applied to the switch. The unswitched leg runs through the pump, forming a circuit which is broken by the switch. So the switch sees the full voltage, and needs to be rated, as such. You could confirm (with a multimeter) that there is 240V across the terminals that you're proposing putting a switch at.

As another way to consider the situation: what if the "other leg" (unswitched) was ground, and the leg you're switching was 600V - what would the switch need to be rated for? 600V.

[Dennis Ford]

Yes it will work (for a short time) and yes it is dangerous.