110VAC Thermostat danger?

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- tylernt
  
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posted
18 years ago

Sat, Aug 20, 2005 9:59 PM

Over on this thread about remote sensor thermostats for fish tanks (the kind designed to switch on and off a resistive heating device):

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It was reported:

"The problem with these temp controller are they are not designed for appliances with motor or transformer to it.... basically just for heaters and bulbs... if you use a adaptor to control the relay... then the adaptor gets very hot... and may cause fire... I know this because I tried it already...."

Could this be true? I'm a little skeptical, but don't want to burn down the house testing it out.

Here's the thermostat in question:

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Thanks for any insight.

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- Vidar Løkken
  
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posted
18 years ago

Sat, Aug 20, 2005 10:51 PM

snipped-for-privacy@gmail.com wrote this in :

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No, it is not true. Using it to switch on and off a relay should be fine. But a motor has a pfc that is not 1, in other words, it drags more current than you should think when measuring resistance in it. So you need to dimension everything after how much it pulls, so it is a different calculation than with a purely resitive load. So add a relay /designed/ for a inductive load to the termostat, and you should be fine. The relay will be inductive too, but drag so minimal current...

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MVH,
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- CWatters
  
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posted
18 years ago

Sun, Aug 21, 2005 7:57 AM

There are many different types of relay. The above seems to imply the use of something like a 12V mains adaptor to power a relay with a 12V coil. That should work just fine PROVIDED that the power output from the mains adaptor is sufficient to power the relay coil. If it overheated this suggests that the resistance of the relay coil is too low or more likely they hooked it up wrong. If you have a particular relay you want to use tell us about it and we can tell you what the spec of the mains adaptor needs to be.

Note: It is possible to buy relays with coils rated for mains voltages (eliminating the need for a mains adaptor) but that would mean you working with hazardous mains voltages. Best stick to using a mains adaptor and low voltage relay - much safer with water around!