Hi

Looking to build a diy twin thermostat fan speed controller. I have looked at the diagrams and they have a relay switching both fans to full when temps to high.

This relay is just switched via the thermostatic switch.

Why use an interposing relay when the thermostat switch is rated to switch two fans at full ?

Cheers

because the thermostat only has one set of contacts

Why do you require to when all the relay/thermostat does it put 240v on to both fans, just wire both fans onto the one thermostat contact ?


Or does one fan just goto full speed when overtemp ?

because you can't just put 240v onto the fans with the output of the variacs still connected, otherwise !!BANG!!, so the relay disconnects the outputs from each variac, and put's 240v onto the fans

This is what i thought mate but keep coming across diagrams with the variac variable outside side connected to the 240v for full speed, not sure how this would effect the variac when not at full speed (or near 240v) ?? But people seem to be doing it and it works !!!!!

Here's one pic i just came across -

Click to view attachment


Cheers

that's for a triac fan speed controller, not a variac. do this on a variac and you'll severely fuck the variac and probably set fire to something

Now thats makes sense, cheers Scraglor.

this man knows his stuff, as commented a long time ago by g love........where is he

imagine the triac type controller, just drops voltage across it like an inline resistance. if you put 240v on both sides of it (input and output) you have 240v pushing one way and 240v pushing the other way and they cancel eachother out and no current flows through the controller. on a variac, the output terminal can be at any position along the length of the coil, the coil is only designed to take 240v along it's entire length, if you put 240v on the with the input disconnected with the output at say halfway, then the coil of the transformer will only have half the reactance of the full length of the coil, so more than rated current will flow, because it's now acting as a step up transformer, you will be seeing around 500v on the input terminal, if the transformer is set at 25% make that 750v!! you get the picture. if the input is still connected and you put 240v on the output with the output set at say 120v, then you will have a 120v difference between the output voltage and the 240v you've just applied, and no (well very little) resistance, for examples sake, say the coils resistance is 1 ohm, 120v/1ohm = 120A, that's a lot for your 0.75A maplins variac!!!