Shop OBEX P1 Docs P2 Docs Learn Events
Transformer VA? — Parallax Forums

Transformer VA?

idbruceidbruce Posts: 6,197
edited 2012-05-17 15:50 in General Discussion
Hello Everyone

I recently salvaged an unmarked multi-tap transformer from a Harmon Kardon FM tuner. The primary is 120V and there are three secondaries. One of the secondaries is 7.5V, while the other two are 29.5V being center tapped. The 7.5V secondary has a 3A fuse, so I am assuming that it can handle at least that amount of current. As for both of the 29.5V leads, neither one of them has a fuse to indicate what kind of current they can handle.

Is there any possible way to determine the VA ratings of this transformer using Ohm's Law?

Bruce

Comments

  • LeonLeon Posts: 7,620
    edited 2012-05-17 06:25
    The 3A fuse means that the 7.5V winding can only handle a lot less than 3A.

    The only way to determine the rating is to put variable loads on it and see when it starts getting hot. It can be estimated from the core size.
  • RDL2004RDL2004 Posts: 2,554
    edited 2012-05-17 06:41
    The 7.5 volts is probably feeding a 5 volt regulated supply for any microcontroller/logic/support circuitry. The other two are probably for whatever kind of stereo amplifier the unit has and may be fused on the outputs. Is it really just a tuner or is it a receiver?
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2012-05-17 07:58
    idbruce wrote: »
    Hello Everyone

    I recently salvaged an unmarked multi-tap transformer from a Harmon Kardon FM tuner. The primary is 120V and there are three secondaries. One of the secondaries is 7.5V, while the other two are 29.5V being center tapped. The 7.5V secondary has a 3A fuse, so I am assuming that it can handle at least that amount of current. As for both of the 29.5V leads, neither one of them has a fuse to indicate what kind of current they can handle.

    Is there any possible way to determine the VA ratings of this transformer using Ohm's Law?

    I know of some tools to check inductors for bad windings and values but I haven't seen anything similar for transformers. If you can find any Ham Radio guys they may be able help or perhaps there may be something like that in their handbook:

    http://www.arrl.org/arrl-handbook-2012

    If you know what the model # of the old FM Tuner was then perhaps you can track down a Service Manual for it that may contain the specs for that transformer. That is probably the best way to get the specs for that if they are listed.

    Robert
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-05-17 10:45
    The rule of thumb I've used in the past is this: measure the thickness of the core in inches, then multiply by 100. That will give you an approximate wattage rating.

    -Phil
  • stamptrolstamptrol Posts: 1,731
    edited 2012-05-17 11:18
    I'd say Phil's pretty close with his rule of thumb.

    Being a generous sort, suppose the designer used a fuse for 150% current. Max current would be 2A. At 7.5 volts that gives you a 15VA capability in the iron. Don't forget, that VA rating is the combined capacity of all windings.

    Cheers,
  • idbruceidbruce Posts: 6,197
    edited 2012-05-17 12:34
    Thanks for all the input guys.

    Actually, I believe it was a reciever instead of a tuner as RDL2004 questioned.

    If I had one more leg of the 29.5V, it would have probably been a nice transformer for a small XYZ machine controlled by a Propeller Proto Board. Either way, I have small XY machine in mind in which it will probably do the trick.

    Thanks again for the input.

    Bruce
  • Mark_TMark_T Posts: 1,981
    edited 2012-05-17 13:20
    stamptrol wrote: »
    I'd say Phil's pretty close with his rule of thumb.

    Being a generous sort, suppose the designer used a fuse for 150% current. Max current would be 2A. At 7.5 volts that gives you a 15VA capability in the iron. Don't forget, that VA rating is the combined capacity of all windings.

    Cheers,

    You definitely don't want a fuse rated at 100% of load current! Remember a fuse is designed to blow on a fault condition, a short-circuit, and to avoid the fuse getting hot or dropping too many volts its important not to run it very close to its rated current on a normal load. A 3A fuse really only tells you the transformer winding can exceed 3A substantially if shorted out.

    You also can't tell much from the total VA rating if there are several windings - each winding has its own VA rating that depends on how much copper is in it. Measuring the winding resistance will tell you something - you wouldn't expect the winding to dissipate more than about 5% to 10% of the load power at full current for instance.

    If a set of secondaries are designed to dissipate the same power then you would expect their winding resistances to be proportional to the square of their individual voltages, so the ratio of V*V/R between windings should give a rough measure of relative power rating. So if you have an estimate of the total VA from the size of the transformer it should be possible to estimate the ratings per winding.
  • idbruceidbruce Posts: 6,197
    edited 2012-05-17 13:35
    I can't say much about general electronics, because it is not my forte, but in a residential setting, the overcurrent protection device (circuit breaker) is normally rated at 80% of what the current carrying conductors can handle.
    The 3A fuse means that the 7.5V winding can only handle a lot less than 3A.
    Being a generous sort, suppose the designer used a fuse for 150% current. Max current would be 2A.
    A 3A fuse really only tells you the transformer winding can exceed 3A substantially if shorted out.
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2012-05-17 13:51
    The transformer may be setup to easily handle more than 3A. As I recall the fuse is sized for the expected load and to protect the electronics that it is powering rather than sized for the transformer itself. That just gives a ballpark estimate on what the transformer should be able to handle. It all depends on how close they were to the margins on the parts they used.

    Robert
  • idbruceidbruce Posts: 6,197
    edited 2012-05-17 13:52
    the transformer may be setup to easily handle more than 3a. As i recall the fuse is sized for the expected load and to protect the electronics that it is powering rather than sized for the transformer itself.

    exactly!!!!
  • idbruceidbruce Posts: 6,197
    edited 2012-05-17 13:54
    Actually I would say the fuse is there just in case there is a load greater than 3A on the electronics side, such as a short. In which case, the fuse would blow, ceasing the flow of current.
  • LeonLeon Posts: 7,620
    edited 2012-05-17 14:12
    The fuse is intended to protect the transformer and the circuit it is powering.
  • skylightskylight Posts: 1,915
    edited 2012-05-17 15:50
    It depends on the type of fuse, certain types of encapsulated fuses are no better than fuse wire which can hold an overcurrent of three times it's rating for up to 4 hours!
    A breaker can take up to 1.5 times it's rating in an overcurrent situation.
    The IEE produce disconnection time graphs for different types of fuse protection which manufacturers can consult when designing circuits.
Sign In or Register to comment.