# Determining the amperage of an unknown transformer

• posted

I have a toroidal power transformer that I'm considering using for a power supply project. It has two secondaries -- 18v-0-18v and 0-48v. Part number is 7-40-0011 (already tried googling it, no luck). I'm pretty sure I purchased it from one of the online surplus places, maybe All Electronics, probably about a decade ago.

What's the best way to determine how many amps the secondary is rated for? Can I just throw a resistive load on it an measure the current? or does this risk damage?

• posted

On a sunny day (Tue, 10 Aug 2010 08:32:06 -0700 (PDT)) it happened Scott wrote in :

Wire diameter?

• posted

Weigh it, then look in the Digi-Key or Mouser catalog for toroidal transformers of similar weight -- that'll come close to telling you the VAC rating of the thing overall.

For output current, measure the resistance of the windings, then find the currents for the two secondaries that seems to make them equally efficient from an open-circuit vs. loaded condition.

```--
Tim Wescott
Wescott Design Services```
• posted

This will get you started.

ftp://jjlarkin.lmi.net/XfmrScatter.JPG

Or just load it progressively and see how hot it gets.

John

• posted

Watching temp is good.

I load down a transformer until the voltage drops 15%. This a bit tricky with multiple windings. A great transformer is rated for 10% drop A poor transformer is rated for a 30% drop.

I still have a batch of old Signal Transformer Co. trans. that seem to go by the 10% system.

greg

• posted

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Hmmm...

Since when are pounds SI units?```
• posted

multiple windings.

I always found the Signal transformers to be a bit soft. I think they used higher-temperature varnish and such to reduce the amount of copper.

John

• posted

Idiot. I post something useful, and you follow up with something useless. It's gotten to be a pattern.

John

• posted

Measure DCR. Assume 5% regulation per winding (including the primary, that's 10% total, i.e. ~90% efficiency). Current is then DCR / (0.05 * Vout).

Tim

```--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms```
• posted

with multiple windings.

You're a goddamned idiot. They use single or double strength, standard off-the-shelf mag wire, and ANY 'varnishing' takes place AFTER the wind.

Got any more absolutely glaring non-sense?

Nobody gets to cheat on copper. It comes down to core media, and winding particulars. Take an xformer made that way, and try to "cheat on the copper" and you'll find that you no longer have the transformer that you originally designed.

All the varnish does is lock everything down mechanically and provide environmental segregation.

Got any other segments of the scientific realm that you wish to insult today? Maybe a sport that you are to thick to grasp?

• posted

Not only that, but using weight as the rule is not going to be very accurate either.

• posted

with multiple windings.

Obviously. They seem to design for fairly high operating temperature, which saves copper. That requires better insulation and varnish. Every transformer designer has to make that tradeoff.

John

• posted

I posted a graph, based on a lot of real transformers, with real numbers and a useful trend. You and JF do nothing but talk fuzz and bitch; no facts, no theory, no numbers, just whining.

John

• posted

```--
I'd be willing to bet that you worked at Burger King for a while
because you sure do know how to cook up some Whoppers!```
• posted

The points on the graph are real. And all you are doing is clucking. You have said nothing about transformers.

John

• posted

Oh, my first paying work was fixing radios and TV sets for friends and family. And I used to build Heathkits for people. My first official job was working in research in microwave spectroscopy at LSUNO, which included some design. I think I was about 15, summer job. I've never worked in anything but electronics.

How about you? Ever worked in food service?

John

• posted

a
.

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?

cky with multiple windings.

Obviously and seem in the same reference. Hmmmm.

Designing for high op temp does NOT "save copper".

Insulation on mag wire is 1500 V/mil. They do NOT require "better insulation" They use standard single strength or double strength mag wire in ALL of their COTS designs. Nothing 'special' at all. As far as the varnish goes, they use ONE variety, and it is the same across the industry, and the only time it would be specialized is for a specialized contract.

That would most likely be Dolph's AC-43.

Sorry, John, but folks are not cutting corners the way you claim they are.

• posted

d

The point was that your graph proposes weight as a main factor, and it is NOT, and that CANNOT be a primary determining factor.

You posted a graph of the FEW transformers you characterized one day when you knew less than you know now.

Your problem is that you intersperse bullshit in with your learning, and you have skewed the truth.

You are what is known as 'hard wired stupid'.

• posted

"No numbers" ??? Ever see a Larkin-posted schematic with values on it? Not a chance. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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