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#60087 - 12/21/05 11:03 AM Transformers: what determines the amp rating?
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
What determines the amp rating of a transformer? Many aren't marked. Is it simply the wire size of one side or the other?

I notice that some transformers have thin wire on one side and thick wire on the other one. Why thick and thin? Thin seems to have the benefit of more wraps, but seems that it would have a lower amp rating and more resistance. The thick wire seems to be limited to fewer wraps, seems that it would have more amp capacity and it obviously has lower resistance.

Is there a chart where I can determine the amp rating of a transformer based on wire size alone?

[This message has been edited by Spark Master Flash (edited 12-21-2005).]
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#60088 - 12/21/05 11:10 AM Re: Transformers: what determines the amp rating?
ghost307 Offline
Member

Registered: 05/09/05
Posts: 884
Loc: Chicago Illinois USA
Since the only constant is the watts, transformers are rated in kVA. Let's assume a power factor of 1.0 to make the math easier.
50kVA transformer will handle 50,000 watts.
That's 50,000 watts (416.67Amps) on the 120V side and 50,000 watts (4166.67Amps on the 12V side. The wire is sized to handle the Amps. When you look at any transformer, the Amps on the high voltage side will be less than the Amps on the low voltage side. Thus, the difference in wire sizes.
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#60089 - 12/21/05 11:37 AM Re: Transformers: what determines the amp rating?
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
Most of the transformers I'm looking at right now don't show any ratings on them. Is there a way I can determine the amp rating or kVA of a small transformer when there's nothing on the transformer telling me the ratings?

Thanks

SMF
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#60090 - 12/21/05 11:41 AM Re: Transformers: what determines the amp rating?
Ron Offline
Member

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
Spark Master Flash,
If there is no label, and the transformer is energized, you could figure out the primary voltage and secondary voltage with a meter, but I don't know of a field test that would determine the kVA rating. If it isn't energized, then I don't know of a field test that would determine the exact voltage configuration either.
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#60091 - 12/21/05 11:50 AM Re: Transformers: what determines the amp rating?
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
It seems that I should be able to put my dial caliper on the wire, measure its thickness in thousandths of an inch, then determine its amp rating that way.

I guess I can check the resistance of the different legs of the transformer to determine the secondary voltage before I even turn it on - does somebody know if that's right?
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#60092 - 12/21/05 02:56 PM Re: Transformers: what determines the amp rating?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Sparkmaster.

You can not determine the rating of a transformer by measuring the wire size. 310.16 in the NEC means absolutely nothing to the manufacturers of transformers.


I have to guess these are fairly small transformers if they have leads on them.

It is most unusual to not have a name tag of some sorts so I am also going to guess that these are some sort of used / old transformers.

What do they look like?

Where are the leads?

Where did they come from?

Basically if they are not marked you can not install them.
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#60093 - 12/21/05 04:00 PM Re: Transformers: what determines the amp rating?
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
Hey iwire, the transformer I'm using right now came out of a Malibu light timer. I plugged it into 120, it put out 12. It says "Coils Inc. 378C1B 119LV102".
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#60094 - 12/21/05 05:56 PM Re: Transformers: what determines the amp rating?
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
Spark,
So I gather that you scavenged this TX out of the power supply portion of a timer. You can usually assume that they don't put more oomph in the TX than they need. For instance, if your TX leads to 4, 1N4001 diodes, a few hundred microfarads of capacitance and a LM7805/LM340T-5 regulator, sticking up in the air, you can derive a few things. 1.) The 1N4001 is a 1A diode. The TX probably isn't going to be more than 1A, and probably less. 2.) Small caps---> light load. 3.) The regulator, on its best day with a huge heat sink, is working hard at 1.5A. We already know that we aren't drawing 1.5A because they use 1A diodes and small caps. If the transformer is only feeding a 5 volt regulator, it probably isn't more than about an 8 volt TX.
Transformers smaller than your fist are usually 4 amps or less. Many small ones are in the 300mA-450mA range.

So we circuit nerds are going to get a good ballpark hunch on our TX from its size and what we see around it. Then, we might tend to load it down a bit and see how the output drops while keeping tabs on the case temperature. But you MUST put your ohmmeter away if you ever hope to figure out transformers.(unless you think you have an open winding)

Now, all you need is 3 pens and a pocket protector and you're good to go.
Joe

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#60095 - 12/21/05 09:13 PM Re: Transformers: what determines the amp rating?
RODALCO Offline
Member

Registered: 12/08/05
Posts: 863
Loc: Titirangi, Akld, New Zealand
I have to agree completely with JoeTestingEngr. the components downstream and the physical size of TX give you usually a guide to the rating of the TX. if you take it out of a defective apparatus.

If not sure at all the thicker windings are usually the secondary windings.

Use a 60, 100 or 200 Watt lamp in series ( safest )depending on TX size or a use variac and ramp it up slowly and monitor I, U and watch out for smoke. if the latter happens on the variac set up the voltage was probably too high and the TX fried.
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#60096 - 12/21/05 10:27 PM Re: Transformers: what determines the amp rating?
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
Just a little warning about your scavenged transformers. Please remember that just because it might look like a power transformer, it might not be. Many phase control ckts will have pulse coupling or trigger transformers to drive the gates of SCRs, referenced to their cathodes. There are also cases where small transformers are used in "chopper" ckts to couple linear signals across large voltage differences.
An example would be current or voltage transducers where you might be measuring 50mV but it's across a shunt elevated 600 volts. These small transformers will have excellent dielectric strength but are not built to be power transformers.

In short, if it doesn't obviously come out of the power supply area, don't use it as a power transformer.
Joe

[This message has been edited by JoeTestingEngr (edited 12-22-2005).]

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