Determining how much output a transformer can give?

Discussion in 'General Electronics Chat' started by Mike33, Dec 7, 2009.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    Great forum, lots of really good suggestions in here!
    Next question on my project: how can one determine just 'how much' a transformer can give? I read somewhere about loading it up with power resistors until the AC voltage sags +/- 10%, and then calling that the level of maximum current output.
    Does this seem accurate/dependable?

    Relating to the tube preamp project I'm working on, I'm just curious if I can push my little transformer to drive another tube (5 more mA...)! Then I could do a CF and add a tone stack...:cool:
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008

    This is a really crude rule-of-thumb we use in ham radio....20 watts per cubic inch of core.

    Your mileage may vary.

  3. SgtWookie


    Jul 17, 2007
    Gee, you already have the simulation built; just reduce the load resistor value!


    Right now, it's 25k Ohms; that gives you a load current of around 186v/25k=7.44mA.
  4. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    Oh, that's not what I was getting at, Wookie...I wanna see if I can get maybe 15mA out of the transformer, but don't want to 'overload' it. As in, cause it to overheat and short or something bad! It was only powering a single 12AU7; I'd like to double that if I can!

    Wondering how the safety of doing that can be determined. It is about .9cu inches of core, if I've measured it correctly. Gotta add in the draw of the heaters, tho...that would be 300mA for 2 tubes in series. 12v*.3A =3.6W.
    Doubling my draw to about 15mA for the tubes, I'd end up with say 180V*.015A=2.7W. Round the result up to 6.5W (I keep rounding up, lol). If the 20W/cu. in. rule is accurate, I think I'm ok :)

    Does my logic make sense here?
  5. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    This particular transformer, I don't know...I hooked up a 22K resistor, measured 137.8VAC RMS (=195VAC), at the expected 6mA. 2 of them in parallel, 11K, resulted in 134VAC RMS (=189VAC), but I only read 6.14 mA flowing thru the circuit. And the heater voltage sagged, too.
    So to answer my own question, the transformer seems to not be able to deliver much more than the current required for 1 tube, eh?
    Guess I'll have to live with the 2 stage circuit rather than something more elaborate!