Calculating current rating of transformers

Hiya,

I have 2 transformer I plan on connecting in series to form part of a
bench top power supply. These have both been pulled from old kit that I
had laying about in my shack

I was woundering how I would go about calculating the power rating/
current rating of these transformers so that I can do more calculations
to aid my design.

Yours gratefully

David

 

[John Fields]

On 27 Sep 2005 07:28:20 -0700, googlinggoogler@hotmail.com wrote:

Hiya,

I have 2 transformer I plan on connecting in series to form part of a
bench top power supply. These have both been pulled from old kit that I
had laying about in my shack

I was woundering how I would go about calculating the power rating/
current rating of these transformers so that I can do more calculations
to aid my design.

---
The way I'd do it would be to:

1. Get a ball-park idea of the power it could handle by weighing it,
then visiting some transformer manufacurers' web sites and
looking at the data sheets for their transformers. Get a few
samples from each web site and you'll probably find that for the
same rating they probably all weigh pretty much the same.

2. Fire it up with no load on the secondary, measure the
secondary voltage, then divide that into the rating that you got
in step 1. That'll give you the current rating of the secondary.
Well, not really, because the secondary voltage will be higher
when it's unloaded, but you'll be on the safe side.

3. Fire it up again and load down the secondary with the current you
got in step 2 while monitoring the temperature rise of the
transformer. If it gets too hot to hold for more than a few
seconds then it would probably be wise to lower the current until
it gets cool enough to handle.

Another way would be to load the secondary down until its voltage
fell to a "standard" value, (24V, 12.6V, 6.3V...) and then
monitor the temperature rise. This method is based on the
assumption that the transformer was wound to get a "standard"
voltage out of the loaded secondary, and that unloaded it'll be
higher than that. It's just an assumption, though, and the real
criterion to determine what the transformer can handle will be
how hot it gets.

--
John Fields
Professional Circuit Designer