Testing transformer amperage

[ALBATROS1234]

Is there a practical way to determine amperage of a transformer with no markings? I know just measuring it doesn't cut it. For instance I have a large transformer for a camera  power supply the unit claims to provide 10a but when I test output it's around 725ma.  On my amp build I am using an industrial control transformer to get 290vdc and I was using 2 wall warts to provide heater supply. I used 2 because each one was rated at 300 to 500 ma. I have a slightly larger one which is unmarked stripped to just a xfrmr it putts out 6.4vac and measures 330ma. But when I hook it up to the 2 tubes and 7v light bulb it works fine and even after being on an played for a couple hours it only gets a bit warm. According to tube data and using ohms law on the bulb i should be drawing 1.4 a and before I mount this thing  I wondered if there was some way to test it. Also I was thinking it should be ok to wire the main xfmr and the filament one above in parallel from my feed voltage,? Thanks in advance for replies

[kagliostro]

Measure the voltage, thed add resistive load till the voltage drop 5%, leave the transformer to cook for 1/2 or better 1 hour

if it didn't become too hot but only a bit or nothing, measure the current that flows on the load and you have your transformer current rating

if you want you can try also with a load that gives a drop of 10% and see what will happen

Franco

[PRR]

> when I test output it's around 725ma.

How are you measuring that??

Into a Dead Short, most transformers will deliver, for some seconds, about 10X their safe current. So a true "current test" is both dangerous and inconclusive. But clearly you are not getting full dead-short current in your "test".

[92Volts]

So a true "current test" is both dangerous and inconclusive. But clearly you are not getting full dead-short current in your "test".

The current test setting of most multimeters is supposed to present a short circuit, to be used in series with the existing circuit under test... not directly across a power supply, in other words not in parallel with the load.

With that said, cheap multimeters and their probes present a non-zero resistance depending on their quality and the extent to which they've been abused previously!

It wouldn't surprise me if a cheap multimeter, connected across the power supply, either allowed only 725ma to flow (depending on voltage) or measured current so inaccurately the result is near meaningless.



Without special equipment you can test with known resistances and math. Say it's a 12v transformer. Connect a 12 ohm resistor and you know 1 amp must flow, if it's maintaining proper voltage. Test voltage, is it near spec or sagging? Connect a 3 ohm resistor and 4 amps must flow, provided it's maintaining proper voltage... etc.

[ALBATROS1234]

well i have an expensive industrial spec multimeter but i was just putting 120v on the primary and testing amps at secondary not in the circuit and thats what im getting. i know about having to put an ammeter in the circuit to test true amperage. but when a xfrmr says 2.5a and you get .04a when you a measuring like i did i know thats not right ,which is what i did as reference because i have a few older transformers that have no markings and its easy to test voltage and i just would love to know how many amps they can handle so i can determine how useful they are and what i can use them on. like i said i want to use this transformer i have, i does get pretty warm after time. i did find another on which is marked at 2a and puts out 7volts . but, when i run it in parallel to the 2 heaters and a 7 volt bulb after 20 to 30 min it gets somewhat warm. i guess what im asking now is how warm is a red flag. even the one i was using previously was working but after 15 min got fairly warm/hottish , i can hold my hand on it and not get burned. i think im just gonna go with the 2a one i found, i am supposedly drawing 1.4 to 1.5 according to my calculations. but it still gets a little warm. my high voltage xfrmr barely gets lukewarm though its a 50va industrial control type, and i am running it as 2 separate 290volt feeds with a dual bridge/filter stages. i know its unconventional but free is cheaper than $100 it would have cost to get a "proper" multi wind made for a tube amp xfrmr sooooo here i sit.

[sluckey]

It's still unclear to me what kind of load you are using to test the transformers.

[Ed_Chambley]

This is the best method I know of.  Build a load box and use it as explained here:


http://www.angelfire.com/electronic/funwithtubes/LoadBox-1.html

[kagliostro]

From my archive





Unfortunately I never remember for a long thime this stuff also if I've read more than one time 

Franco

[shooter]

I never remember for a long thime this stuff

There's an old saying "If you don't use it, you lose it"
the key is knowing how to find IT, again, and my friend, you do well

[ALBATROS1234]

@sluckey i was using my multimeter no load, i was posting because off the top of my head i couldnt think of a legit way to figure out current capacity of an unmarked xfrmr

@ed thanks will read

@kagliostro very nice will read when i get a few min.