I'm shopping for some AC relays to hook up a thermostat to a bar fridge for my homebrew setup (I like beer).

I've come across some suspiciously cheap items on eBay, like this this one. Normally when it comes to cheap electronics I'm willing to give it a crack but since I'm going to be passing 240v through it I'm a little bit more skeptical.

Having said that, I know nothing about the internals of relays or their failure modes. What is the risk here? Could it,

  1. Just stop working, or
  2. Burn the house down?
  • 1
    \$\begingroup\$ it would be hard for it to burn down your house; it will just stop switching if the tapper arcs away... check the info on the part itself (don't trust ebay listings) \$\endgroup\$
    – dandavis
    May 29, 2017 at 21:44
  • \$\begingroup\$ As a general rule, I won't use electronics from cheap, dubious sources on mains voltage. I've seen too many underspecced, counterfeit, or simply crappy Ebay thingamabobs that I simply don't want to think about the risks of putting mains near them. To get an idea, google for cheap power supply / charger teardowns (example). \$\endgroup\$
    – marcelm
    Jun 1, 2017 at 9:57

2 Answers 2


What is the risk here?

This can be generalized into risk and cost. I find that in all my years of being a design engineer that the most cost effective and safe practice to adopt is this: -

  • Source parts from reputable dealers
  • Source parts that have reasonable data sheets
  • Use those parts within the constraints imposed by those data sheets
  • And more recently, only use parts that can be adequately modeled i.e. ensure there is a decent spice model available and use it (relays are usually exempt from this of course).

Following all the rules above ticks all the quality boxes for me but doesn't prevent me (or anyone) screwing up by being an idiot because that is human nature. However, it is about reducing risk.

Doing anything outside those rules usually bites you either by wasting your time (cost) or adding some form of risk to you or other people.

I've never sourced a component from ebay and I've never used a component that hasn't got a reliable source or a decent data sheet. In fact I've gone out of my way to use only components that do have decent data sheets. I'm involved with a project at the moment that is attempting to use a design made about 20 years ago and I just cannot use it because it doesn't tick all the boxes. I have to stand my ground because, at the end of the day, if we did use it, we would likely end up pissing customers off and damaging our reputation.


hook up a thermostat to a bar fridge

That is not a very good description of what you are trying to do, but it likely involves starting and stopping the compressor motor in the fridge. If you look at the side of the relay, you can see that there is an "AC1" rating marked. There does not seem to be any other AC duty marked. An AC1 rating indicates that the relay is suitable for use at the voltage and current marked with a non-inductive or slightly inductive load. A refrigerator compressor is more than slightly inductive. A refrigeration compressor requires a relay with an AC8a or AC8b rating depending on the overload protection scheme. Even if you found better data on the relay than just reading the side of the relay, it is unlikely that the relay has the required rating.

With eBay parts, you can often find complete data using information that can be seen in the photo even if the seller doesn't otherwise provide the data. You may even be able to determine if the item is a reputable brand. However the item may have been manufactured by someone under contract to the manufacturer and is being sold on eBay because the manufacturer rejected the item due to inspection failure. There is also the possibility that the item is counterfeit.

What is the risk here?

What do you think?


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