Mains transformer voltage lowering

Discussion in 'General Electronics Chat' started by Ctenom, Feb 28, 2011.

  1. Ctenom

    Thread Starter Member

    Nov 1, 2010
    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
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    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
  3. ifixit

    Distinguished Member

    Nov 20, 2008
    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.

  4. SgtWookie


    Jul 17, 2007
    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.
  5. Ctenom

    Thread Starter Member

    Nov 1, 2010
    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
  6. Audioguru


    Dec 20, 2007
    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.
  7. Ctenom

    Thread Starter Member

    Nov 1, 2010
    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.