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Old 19th February 2007, 08:50 PM   #1
pchw is offline pchw  United States
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Default Possible to estimate transformer specs?

Hi all,

I was wandering around in my parent's neighborhood yesterday, and the wallet bited when I saw a garage sale. I came away with some power transformers and a choke. By using a DMM and a 12V AC source, I was able to figure out the secondary voltages; however, I am wondering there is any method to estimate the current rating of each of the secondary outputs?

Thanks,
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Old 20th February 2007, 12:51 AM   #2
cerrem is offline cerrem  United States
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There are a bunch of ways to determine the power ratting of the transformer....once you know the voltage ratios..ect...
You take a peek between the layers and see the wire gauges.. I can tell gauge # on sight from experience...and each gauge has an associated Circular Mills...Most wires in powers can be about 800 CM/ Ampere up to 1000 CM/Ampere....
Heater winding that are on the outside of the build can be as low as 500 CM/Ampere....

The other way is to load the secondaries and vary the load and let it sit for a few hours with a thermo-couple on the core, such as Fluke has one that plugs into meter...wait for the temp to stabilize ..you are loooking for 105 C, since this is most likely Class A insulation.... Then check the primary current and applied voltage for wattage ratting...

CHris
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Old 20th February 2007, 01:31 AM   #3
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Measure the winding resistance. If you have a DVM and 1A power supply you can measure the heater windings (1 mV drop at 1A is 1 milliOhm. Figure one watt to three watts as the limit for copper loss per winding. Using 2 watts for example, I= sqrt(2/R). This a BALLPARK figure, but matches pretty well to transformers I've seen.
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Old 20th February 2007, 01:32 AM   #4
pchw is offline pchw  United States
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Thanks , Chris.
One of them looks like an organ power supply.
The PT is pretty beefy, a lot bigger than a Dynaco PA774A (for ST35).
It is about the same size as the ST70's PT!! See the picture. It has a
5V secondary which implies rectification by 5AR4, 5U4G etc. Since 5AR4
uses the least current, I can assume it won't get me in trouble if I drop in a 5AR4 ....
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Old 20th February 2007, 01:34 AM   #5
pchw is offline pchw  United States
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Thanks, Tom. Here is the bottom, both top and bottom have end bells, I will have to open it up to see the wire gauge. Try measuring the winding resistance is definitely easier. When you said applying 1A, did you mean applying 6.3V/1A to the 6.3 windings?

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Old 20th February 2007, 03:39 AM   #6
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No, a current limited DC power supply is what I had in mind. And you need a meter that reads millivolts...

The high voltage windings and primary are easily measured with an ohmmeter.
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Old 11th April 2008, 02:22 PM   #7
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352= Essex (Transformer Division). The numbering looks similar to a Silvertone organ I parted out. They typically have large silver numbers printed on them.
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Old 11th April 2008, 04:50 PM   #8
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Hi all

PCHW also mentioned he bought a choke, which I believe he wants to test as well... so my question is not off topic

I bought some HV chokes that are rated at 25H 'around' 100mA. How can I check for a more exact figure? I thought about using a LCR bridge across the choke and pumping in DC current with an adjustable CCS (PS with adjustable current) up to the point where the inductance starts to fall?!

Many thanks, Erik
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