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I'd like to use a Beaglebone Black(3.3v Logic) plus a 3.3V to 5V logic level shifter(74HC125) to control an opto-isloated Songle Relay that has the following specs: enter image description here

Rated Resistive load: 10A 125VAC; Rated Inductive Load: 3A 120VAC

The relay would be switching on/off the mains plug for the following window AC unit:

link to Ac Unit

It's 4.8 Amps, 115 VAC (with a GFCI plug)


Is a compressor a pure inductive load? If so it appears the 4.8 amps is over the rated 3A for inductive load.

If so, what does that mean? What is the failure mode? Will it blow up? will it just not last as long? Will it maybe be ok? What would be a suitable replacement that works with my controller?

This is a proof of concept, so if it lasts a few months, I'm ok with that.

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    \$\begingroup\$ Expect the contacts to weld shut, running the air conditioner full blast until you replace the relay. Air con is likely to be a worse-than-inductive load. Its main component is a large motor, which (if it takes 4.8A running) probably takes 25-30A while starting. \$\endgroup\$ – Brian Drummond Apr 19 '16 at 23:08
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Switching contacts don't like inductors- anything wound like a motor or transformer. The more inductance the worse it is, and an air conditioner will be very inductive. The problem is that in an inductor, current keeps flowing when you turn off the voltage (that's what inductance is, basically) as energy is stored in the magnetic field, so as switch contacts break you get arcing which rapidly ruins them unless they are adequately designed.

Short answer: no, this isn't the right relay. You should find one rated for the inductive load and if you want it to last, you should over-specify as generously as possible.

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You should use a relay designed for a compressor motor load- with an appropriate HP rating.

Omron has some appropriate relays, but expect to pay maybe 20 times as much as for those little relays.

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