I have a 9v 450ma mains transformer which i was going to use on a thermostat circuit (relay output) i built which run on a 9v battery. However the transformer outputs 13-14v when outputting the 50-150ma my circuit draws. Would adding a resister to the mains coil lower the voltage? I can modify my circuit to the voltage as long as it is stable which my 9v battery wasnt and kept changing the switching temperature. If a risister can be used would a 0.5w rated one be OK? Or would a 9v regulator be the best way forward?

PS i live in the UK so its 240 50Hz

Thank you for taking the time to read this

 

If you are sure of the voltage level, being above 9 volts, then the regulator would be a good solution. Look for one that is 'low dropout'. Meaning it can function with only a volt or so more than the regulated output

 

Since your 9V load varies from 50 to 150 mA, a resistor would not work very well as the 9V voltage would vary with the load change. This is not what you want. A linear regulator is a better choice. Google: LM317.

Regards,
Ifixit

 

You could also use a 7809 fixed regulator. Note that these regulators require an 0.33uF (330nF) capacitor from IN to GND, and an 0.1uF (100nF) capacitor from OUT to GND, as close to the regulator as possible. Caps can be ceramic or metalized poly film.

 

Thank you all very much for your responses. I found an old electric oil radiator with a variable thermal switch on it that dose the job. Its a little less accurate but still works well. I am still going to build my circuit and have just ordered a 7809 as Sgtwookie suggested.

Thank you all for your time it is very much appreciated

 

You have a 9V transformer. It has an AC output which is not DC. It needs a rectifier bridge and filter capacitor to make 11.2VDC at 318mA. Since your load uses less current then the voltage will be a little higher.

 

yes that is correct i am using a db102 rectifier and am getting the 13-14vdc after it. I think the problem lies in the fact the transformer was one i got out of 9V lamp charger, and like most chargers it produces a higer than stated voltage.

Thank you all once again for your time.