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mjohnson 10-09-2012 01:07 PM

Possible inexpensive way to use STC-1000 with larger appliances - will it work?
I was dubious about using my stc-1000 with my large older freezer (5amp running rating - no max amp listed)

I posted some investigation in the Electric Brewing forum. It seems like the relays in my unit are probably rated at 15amps


But I got to looking around to see what I could do if I wanted to be sure. I found this relay:

Seems like it might do the trick. Is this how you'd wire it: STC-1000 cooling output and neutral should go into the coil. Line Power goes into the common. Outlet power comes out of the NO. NC is not used.

Thoughts? If it works, I thought it might help others who want to use a 10A controller with larger amp appliances.

Huff360 10-09-2012 04:28 PM

1 Attachment(s)
Yeah, that'd work no problem. Quick and dirty schematic...

Attachment 78876

porcupine73 10-09-2012 05:02 PM

Sure you can use an interposing type relay, though for the load you listed the STC-1000 will probably switch it just fine.

Only thing in that drawing above would be to label the neutral and hot. I'm assuming the far left wire is neutral. Then to the 120V coil relay, maybe break the hot instead of the neutral. Breaking the neutral usually isn't done even in control circuitry, with the exception of motor overload heater contacts.

The other thing you could do there is get a 12VDC STC-1000 and a 12VDC interposing relay; main benefit there being then you have only low voltage at your STC-1000 though it really doesn't make much difference.

mjohnson 10-09-2012 05:16 PM

Yeah, the relays in my unit seem to be 15A ones. I've read that its good to budget 3x operating load for startup on induction loads. That would be 15A on my freezer.

I found this and thought it might help folks. The 10A rating seems to be one of the complaints of this controller. Thought this might be an economical fix for that.

As a side note, if I wanted to test the startup load on my freezer, what would be the preferred method?

porcupine73 10-09-2012 05:24 PM

You can certainly incorporate the external relay. I've noticed the STC-1000 relays might be rated to 15 amps, but inside the STC-1000, the solder traces running to the relays and to the terminals, those I wonder if can really handle 15 amps.

Usually on squirrel cage induction motors you're talking like 5 or 6 times the full load current on startup. Usually they have a NEMA code for the blocked rotor current multiplier. But refrigeration compressors are a little different. To measure your startup current I think you'd need an analog ac ammeter. You could try using something like the killawatt but the startup current's short duration probably won't be visible on that device.

mjohnson 10-09-2012 05:44 PM

Good point on the traces. I'm not one to overbuild things - but at the same time, I prefer to do things right. For a couple bucks, I think I'll just add the relay along with a 10A fuse protecting the controller. Then I don't have to worry about it.

I looked at the Kill A Watt briefly - it didn't seem up to the task.

porcupine73 10-09-2012 06:16 PM

Sounds good, there's certainly nothing wrong with using another relay. That's a pretty nice price on that relay. If you want to use a fuse to protect the STC-1000 you could get away even just a 1 amp fuse there most likely.

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