Sunday, 9 December 2018

Driving Inductive loads with the Sonoff Basic

The Sonoff Basic Smart Home power switch is very handy and has a claimed output rating of 10A at 230Vac. The basis for this is the output relay rating, a SONGLE SRD-05VDC-SL-C.

SONGLE SRD-05VDC-SL-C Miniature Relay


Now in tropical Malaysia, one of the most handy home appliances is a fan. There is usually quietly ruthless jostling for fans so there is usually at least one in use.

Poppy the dog usually wins pole position


One of my favourites is a Pensonic PFF-20B, rated at a modest 200W or  0.87A.

Pensonic PFF-20B 230V 200W

Trouble is the Sonoff Basic does not last long switching the PFF-20B, even at a modest 1A load. The contacts often get stuck, ie the relay might turn on but fail to turn off. A sharp knock on the Sonoff Basic will often jar the contacts back into position, but it will invariably fail again. Inductive loads cause arcing of the contacts resulting in damaging micro-welds.

On the other end of the spectrum is a magnetic contactor, like the LSIS MC-9b. Modestly rated at 9A for inductive loads (AC3), it is a veritable King Kong next to the SONGLE SRD-05VDC-SL-C.

LSIS MC-9b 230V 9A (AC3)

The obvious answer is to use the Sonoff Basic to drive the MC-9b. The MC-9b has a 230Vac input relay coil. I picked one up at RM36 from my local store, but you can find them online for as low as RM28. The wiring is straightforward, so I'll dispense with the schematics.


The contactor jumps about quite a bit, so it is necessary to mount it securely and to ensure the jolting does not transfer too much to the Sonoff Basic.

Here's a youtube video of a voice controlled Pensonic PFF-20B. The MC-9b did not faze Poppy at all. In Malaysia, it takes a lot to give up one's spot in front of the fan.

Happy Trails.

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