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 Post subject: unmarked transformer
PostPosted: Fri Dec 01, 2017 5:08 pm 

Joined: Fri May 10, 2013 12:55 pm
Posts: 14
I'm trying to reuse an unmarked transformer.

It comes from an EMCO PSR-7 from 1993. I dug it out of the dust from a pile.

The transformer has on one side black and white wires for primary.

And on the secondary, on top are wires: red-red-black. That measures 24 volts.

On the secondary, on the bottom are blue wires. What are those for?


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 Post subject: Re: unmarked transformer
PostPosted: Sat Dec 02, 2017 5:47 am 
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Joined: Fri Feb 01, 2002 1:01 am
Posts: 1746
Location: Mideast USA
Measure the voltage at the blue wires the same way you did finding 12-0-12 at the reds.

If there is voltage, is another secondary winding for other voltage uses. If no voltage; unplug and measure resistance. If zero ohms, it could be a thermal fuse. If open, may be an open thermal fuse.

A transformer with 2 independent secondary windings can energize two different circuits, instead of using two transformers. Very common to find. Like to produce 24V for amplification and 5V for digital controls.



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 Post subject: Re: unmarked transformer
PostPosted: Sat Dec 02, 2017 5:23 pm 

Joined: Fri May 10, 2013 12:55 pm
Posts: 14
You're right.

The red-black-red are 12-12 v.
The two blue wires measure 4.9 volts.

So you think they are two independent coils?
Will the two coils affect each other?

Will it not be 12-12-5 volts?


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 Post subject: Re: unmarked transformer
PostPosted: Sun Dec 03, 2017 6:22 am 
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Location: Mideast USA
Very unlikely the blues are related to the reds. Prove by probing for continuity between them. They should not.
The blues should provide power for yielding 5VDC circuitry, the center-tapped 12-0-12 for around 24VDC. Any can be used alone, leave the other disconnected if not needed.



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 Post subject: Re: unmarked transformer
PostPosted: Sun Dec 03, 2017 8:24 pm 
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Joined: Thu Oct 26, 2006 7:11 pm
Posts: 1279
Location: Maryland
Solar3000
It seems you are somewhat unfamiliar with transformers, so here is some information that may be of value to you.
There is a great deal of information about transformers but here are some of the more common interest items.

A 'true transformer'.... (see below)
Has both primary and secondary windings.
The applied voltage usually is connected to the primary winding(s), and the load voltage is developed on the secondary winding(s).
Each winding has a voltage and current rating, and they can vary widely within a transformer.
All windings are electrically isolated from each other.
Some windings may be center tapped, so an equal voltage is developed from either end of the winding to the center tap, and the voltage will be twice across the ends of the winding.
Windings have polarity, or phase reference. This means that when the applied voltage wave is going positive, one end of each secondary winding will also be going positive.
Windings can be connected in series, properly phased and the voltages will add, and if they are connected in opposite polarity, they will subtract.
The current of series windings is limited to the current rating of the smallest rated winding
The primary and secondary windings can also be connected in series to obtain higher or lower voltage than applied. This is referred to buck or boost.
The voltage of the secondary is a directly related to the ratio of the primary and secondary windings.
More primary windings than the than secondary, the secondary voltage is lower. The inverse is also true.
The current rating of teh transformer output is a function of the wire size used for the winding.
If you are trying to determine what windings are what in an an unmarked transformer, rather than hooping power up and hoping the smoke doesn't get out, here's a safe method.
Use a ohmmeter to identify and mark all of the windings, and any center taps.
Apply a known low voltage AC to the primary winding, 12 volts is a good choice, and you'll see why later.
Now measure each of the other transformer windings and taps, recording the voltages measured.
Since 12 volts is 1/10 of the normal line voltage, just multiply the measured voltage by 10 to find the expected voltage of the other windings.

With an unknown transformer with 12 volts ac applied to the primary...
Winding #1 measures 3 volts. The expected voltage would be 12 volts x 3 volts = 36 volts.
Winding #2 measures .5 volts so the expected voltage would be 12 volts x 0.5 volts=6 volts
--------------------------
there is another 'transformer-like device' that is called an auto transformer, which is not a 'true transformer'.
It has only one winding. Power is applied across the ends of the winding, and the voltage desired is obtained from one end of the winding and a tap someplace in between.
The position of this tap determines the output voltage.
Often this an adjustable tap, and a Variac is an commercially available, manually adjustable auto transformer. Years ago they found big acceptance for stage light dimming and industrial power control.
The voltage output on a Variac will be between zero and the applied voltage unless the voltage is applied between one winding end and a point someplace before the opposite end of the winding. Then the output voltage will be greater since the winding appears much like two windings connected for boost operation.

Warning! An auto transformer offers NO LINE ISOLATION so the potential for a lethal shock exists on the output voltage circuit.

A transformer is probably the most rugged and reliable component of an electronic circuit, but today because of manufacturing costs and copper wire prices, they have fallen out f favor for use in many power supplies. The switch mode power supply has become king, and rightly so with their many benefits, small size, low weight and surprisingly low cost, but that's another story!

Dig into some transformer theory if you are so inclined. It is amazing what can be accomplished with just a few magnetic fields.



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 Post subject: Re: unmarked transformer
PostPosted: Mon Dec 04, 2017 6:16 pm 

Joined: Fri May 10, 2013 12:55 pm
Posts: 14
OK. Thanks everyone.
I will test them.
I'm likely only going to use the 24/25.2 volts.


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 Post subject: Re: unmarked transformer
PostPosted: Wed Dec 06, 2017 12:55 pm 
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Joined: Sat Nov 08, 2003 1:01 am
Posts: 1579
Location: Phoenixville, PA USA
While you are at it, give this a watch. Quite handy to know.
-CeaSaR
https://youtu.be/gfH79f51JMg



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 Post subject: Unmarked transformer ... Again
PostPosted: Wed Dec 06, 2017 2:36 pm 
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Joined: Thu Oct 26, 2006 7:11 pm
Posts: 1279
Location: Maryland
I forgot to mention in the previous post that if a 120 volt light bulb, like 100W is placed in series with the line power, any connections, even a dead short, will not hurt the transformer or blow a fuse.
The lamp will be bright if the connection is shorted, or made to a low voltage secondary, and dim to out when connected to the primary or a high voltage secondary.
The low resistance of the lamp compared, to the primary impedance of an unloaded transformer, will provide almost the exact voltages as if the transformer were connected directly.
If the test lamp stays on, with the proper connections, it probably indicates a shorted winding in the transformer. Not common, but it can happen with salvaged transformers.
Last word..Even if the windings voltages check out to be good, don't forget to test for leakage from the windings to the metal frame or core. An ohmmeter is OK for a basic test but a high voltage megohmmeter or a Hi Pot tester is really better.

Try this. With the bulb in series,

I use this bulb-in-series technique when repairing line powered inverters for medical ultraviolet treatment equipment. With a 100 watt bulb, a good inverter will cause the series bulb to flash bright for an instant then drop to almost being out as the power supply capacitors charge and the test fluorescent lamp comes on. Without it, one bad switching transformer or diode can let out way too much smoke!

Simple, Safe no muss, no fuss.



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“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
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 Post subject: Re: unmarked transformer
PostPosted: Sat Dec 09, 2017 10:57 pm 

Joined: Fri May 10, 2013 12:55 pm
Posts: 14
Thanks again all.


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