Transformer output amperage question

17 years ago

I posted this in the electronics thread but thought there might be a guru here too.

Can anyone give an idea on how to determine what the maximum amperage the secondary winding of a step down transformer would be, without over heating it?

The primary winding is (120vac) 126.7 ohms. The secondary winding (12vac) is 5.7 ohms. Any idea how many amps can be loaded on the transformer without causing it to heat excessively?

Here's what I'm up to....I have several strings of 5 halogen light the previous owner installed under the cabinets in the kitchen. 2 of the transformers burned out (open windings). Each string of lights draws 5 amps at 12vac. The cost to replace the transformers is just about the same as to replace the lights. I have a couple of salvaged transformers from other electronic equipment with 12vac outputs. Of course, there are no nomenclatures stamped on those to indicate max. amps output. I'd like to use one of them to run 2 or 3 sets of the lights which will be 10-15 amps 12vac. I plan to put 5 amp fuses on each lead of each string of 5 lights. Again, each string contains 5 10 watt halogen bulbs.

Thanks in advance to any guru's who can show me the math to figure maximum amp capability on the secondary winding.

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Comments (8)

wayne440
17 years ago
Unfortunately, the resistance readings you furnish are useless to determine the current capacity of your transformer. Current capacity is determined by a number of factors, a few of which are core design, size and material plus the wire gauge and type of insulation used for each winding. The winding resistance is virtually unrelated to actual steady state capacity.

An experienced person might (very) roughly estimate a transformer's capability based on observation of the factors I mentioned, but it would be cumbersome to do via this forum.
joed
17 years ago
I agree with the above. The physical size of the transformer would be as good a way as any to guess at amperage. Bigger is better.
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azlighting
17 years ago
If OP can provide the transformer manufacturer and model number, we could look it up by that.

There are a few transformers that are the same dimensions, but are different KVAs
brickeyee
17 years ago
Many transformers are marked with a V-A rating.

If the transformer was pulled from equipment it may not be marked, but should have a manufacturer and part number.

You can still run into a wall though if the part was manufactured to someone specs.
They are not always available even from the transformer manufacturer since they do not own the design.

It takes some loads and accurate volt meters and current meters to measure the capacity after the fact.
windslam
Original Author
17 years ago
Thanks for the info guys, I was hoping there would be a math formula....Murphy's law of not having an easy way out.

Here's what is stamped on the transformer, I hope this can help

5604-5162-900
DN50000-PT
4938 317 Ru (with a backwards "R")

The wire size on the primary side looks to be about 16 or 18 gauge....the secondary wire size appears to be 18-20 gauge.

I have 3 of these so if I have to, I'll take brickeyee's method and keep adding loads on one of them until I get an idea of the max. amps. It would be nice to know ahead of time though, what the rated amps are.

azlighting...
Do you have access to the various manufacturers info? Rather than put you through the effort, if you could point me to where I can do it, that would work.
brickeyee
17 years ago
18 or 20 gauge outputs are likely to be no more than a few amps, if that.

Take some measurements and look around DigiKey or some of the transformer manufacturers web sites.

Small transformers are rarely designed with any margin, so size is a good indicator.
windslam
Original Author
17 years ago
thanks brickeyee...I guess I'm stuck with looking for and buying the transformer I need to do the job. I think I'll still do what you suggested and throw some loads on this one for ha ha's, just to see what I have.
lbpod
17 years ago
Hook up a smaller load and monitor the temperature of
the xfmr., (a car headlight bulb on high beam uses approx.
5 amps). Increase the load until you feel comfortable
to connect your ultimate load. Then 'closely' monitor the
temp. Xfmers are designed to not exceed a certain temperature when fully loaded, but what yours is, is unlknown. A rule of thumb is that if it is too hot to
hold your hand on it, it is overloaded.