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This should be a simple question for someone who knows what he's doing. Unfortunately, I don't, so I've been researching like crazy and am stuck on where to go from here.

I am trying to modify a mini fridgerator for as cheap as possible to allow me to control the temperature from 55 to 90 degrees. I stumbled across several simple bimetal thermostat switches that have a pretty good range in them, but they are heating thermostats. I need to reverse these to cooling so that the fridge will activate at a certain warm temperature and turn off once it reaches set cooling.

Thus, when the thermostat is open, I need the fridge running. When the thermostat is closed, I need to fridge to stop running.

I think I've narrowed it down to either a SPST NC relay or a SPDT relay, but I'm stumbling heavily on the exact values I'm looking for. The normal fridge would accept 110 volt AC from the wall and output 110 volt AC into the unit. Normal operating amperage of these I beliefe is 3 - 5 amps, and most suggest the start up should provide 1.5 times that ammount. So my first question is, I guess I am looking for a 110v AC relay that can handle 5 - 7 AMPs?

If I'm on the right track, then my next concern is cost. I can find these in expensive electronics catalogs. I've looked on ebay, and they have some fairly simple and cost effective relays, but they are made for China outputting 240V capacity. Does this affect me, or because it's more than the 110 I should be ok?

Does anyone have better suggestions? Again, only looking to reverse this thermostat from heating to cooling. Shouldn't be this complicated! :)

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Some thoughts:

  • Yes, your calculations for relay power are correct: you probably want some head room for the compressor startup. 2x the power rating is probably even better.
  • Whether 240V is OK depends on the relay. Some types of solid state relays (SSRs) require a particular type of AC. Most likely, however, the relays you are looking at are 240V maximum, in which case it is fine. I would ask the Ebay seller for specifics, when in doublt (choosing for the "Capt'n Obvios" badge here). If the relays are mechanical then it is definitely fine: those would be the maximum ratings.

Coupld of more points:

  • If this is for temperature control, you want to think about your relay lifetime, if you are using a mechanical relay. If, for example, it is rated for 10,000 cycles, and you turn it on and off once a minute, the relay will last for only six days. So you'd want to figure out how often the relay cycles: but this is going to be partly dependent on the hysterisis of your thermostat. Note that solid state relays are much more longer lasting than mechanical relays because there are no moving parts.
  • On the other hand, the cheap Chinese Ebay SSR have been claimed to be of inferior quality. If you search the internets you will find claims where such relays have self-destructed: this is, of course, a fire hazard, so you want to be careful with those.

Anindo Ghosh recommends Omron SSRs from Ebay, something like this or this. Note that the output on these is 24-240VAC, so 110/120VAC is going to be perfectly fine. Anindo Ghosh says that he had units like this tested by Omron and they said they were genuine. And if you buy it and your house burns down, you can come and live with him :)

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  • \$\begingroup\$ Actually its for a reptile brumator and incubator so 55-90 (although a wide range) is what I need + or - a couple degrees. I appreciate your response being so quick and all your points taken, especially the life. Something I will investigate. Could you point me to a product suitable for this job by chance? \$\endgroup\$ – user22190 Apr 6 '13 at 0:13
  • \$\begingroup\$ :-) All I'll add is, when you buy any SSR from eBay, test it at rated normal operating power for at least a couple of hours immediately after receipt, and invoke eBay buyer protection in case anything goes bad. If the device survives infant death it's good to go. The "bad" or "fake" SSRs are typically low power rated parts with a reprint stating increased ratings. Also check the part number and the certification logos before placing the order, and after receipt of the part. \$\endgroup\$ – Anindo Ghosh Apr 6 '13 at 5:05

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