Transformer identification / finding secondary values

[Johnson777717]

Hey folks! Some more assistance needed, If I may?

I have a box full of power transformers and I need to find the values of these transformers, so that I can use them in upcoming circuits. I've checked on-line for the listed numbers that are indicated on the transformers without any luck. There are a couple transformers with manufacturer logos, I'm still searching those logos to find a manufacturer...

Would you happen to know how I can find the output values of these transformers? Good safe practice please!

I do know that the transformers are all 120VAC primary.
Also, the transformers have different color codes for the output leads. And I know that there is some sort of standard color coding for these transformer wires. Does anyone know the key to these color codes?

Thank you very much for your time!

 

[Russlk]

I would not depend on any color code, however, red usually means high voltage and brown may be low voltage. Black doesn't mean anything. Since you know the primary, measure the secondaries. If you are not sure about the primary, connect it to a low voltage (6 vac) then measure the secondaries.

 

[Dean Huster]

Sorting Transformers

If you KNOW that the primaries are 120v, and you know which leads the primaries are, then just hook the primary up to 120v and use your meter to measure the various secondary voltages.

As to the color codes, first the obvious: same colors are the same winding. Duh. But, I had to start there just in case ....

Transformer color codes were pretty consistent and standardized in the 1940s, 50s and 60s, but have gone a little haywire since then, especially if you have Asian imports. Here's the old color code, or as much of it as I can remember.

Black is the primary; black with a stripe is likely a tap on the primary for an alternate voltage, such as 110v vs. 120v. Typically, any solid color is one end of a winding, while that color with a stripe (usually a yellow stripe) is the center tap.

Red is the high voltage winding, red-yellow the center tap. Be careful of this winding when you're measuring voltage, because if it's a transformer from some older tube equipment, the voltage across the red leads could be as high as 500 or 600 volts!

Green is the 6.3v heater winding on an old tube transformer.

Yellow is the 5.0v rectifier heater winding on an old tube transformer. This winding is well-insulated to withstand hundreds of volts since it will be connected directly to the B+ line.

Brown is another 6.3v heater winding, usually with a lesser current capability.

Orange is sometimes a higher voltage secondary, but not as high as the red leads. That's a rare color to find on the older transformers.

You may have four primary leads, a pair for two independent primaries. This is for a dual voltage transformer. Typically, each primary winding is 120v. Put them in parallel for 120v operation and maximum rated current from the secondaries; put them in series for 240v operation and maximum rated current from the secondaries. Using just one of the 120v primaries for the primary and the other 120v primary as a 120v secondary can be done, but you'll have only half of the original total volt-amps (sorta like watts) available from the secondaries, including the new primary-being-used-as-a-secondary.

Now, most of this goes out the window if you have a load of Asian import transformers. Even the primaries may have weird colors.

In any case, with the transformer connected to nothing, you can use your ohmmeter to determine which windings are connected together, which are center taps, etc.


Dean

 

[Johnson777717]

Hey folks! I really appreciate you taking the time to reply. I definately respect 120VAC and the potential (no pun ) for serious injury. I've been juiced by stupidly placing my hand on an engine distributor while the engine was running. I sure won't forget it! I instantly became respectful of the energy and not becoming a component of circuitry. :shock:

Thanks again!

 

[Johnson777717]

dumb question:
So to measure the secondary wires, connect the common probe to the ground point and then just use the positive probe of the multimeter, and connect with one of the secondary wires??? I'm trying not to fry anything. I want to be absolutely sure. Thanks again!

 

[Johnson777717]

Hey guys, me again. I figured I would post a sketch of the transformer in question. I just need to know how to test the secondary wires with my multimeter. Again, I really don't want to fry the poor thing.

RED and RED have continuity
YELLOW AND YELLOW have continuity

Any suggestions how I would go about finding the voltages and amperage for each secondary wire? Thanks a million!

 

[stevez]

Quite often the windings are independent of one another so there is no common - but not always. The potential you are trying to measure is between at least two of the leads (there could be a center tap). Sometimes the two leads on each winding are the same color - but not always, as Dean indicated.

I can tell you that some of the older amateur radio publications have clues on how to identify this stuff - old TV power transformers were an excellent source of transformers for power supplies. "Radio Amateur's Handbook" by ARRL is one good reference. It is updated/published annually.

 

[Russlk]

The only thing you are apt to fry is your meter if you have it set on a low voltage and try to measure a high voltage, so start with the highest range and work down. There is something not legible between the yellow wires, is that a center tap? Some transformers have a Faraday shield which will not have any continuity to any other wires.

 

[Johnson777717]

The illegible vertical figure states BROWN. This brown wire has continuity with both of the two yellow wires. Center tap?

So the wires from left to right are
1. Yellow
2. Brown
3. Yellow
4. Red
5. Red
Sorry for the small sketch, I thought that it would appear larger on this posting, since my previous posting sketches were huge!

Maybe I'm more afraid of this thing than anything else.

My meter has two settings for VAC. 200 Volts and 600 volts, so I think 'll be okay

Well, I'm going to dive in and measure some volts! Thanks a bunch with all of your help! All of you! :shock:

 

[Johnson777717]

Okay,
Voltage across yellow and yellow is 5.7 volts
Voltage across red and red is 26.7 volts
Voltage from brown to yellow (both first yellow and second yello) is 2.7 volts.

So I have a 120VAC primary, 5.7 VAC and 26.7VAC secondary? Using center tap, 2.7 VAC (3 VAC nominal?) and 26.7 VAC secondary.

Cool stuff!

 

[Dean Huster]

Because of the voltage being measured on the brown wire, we'd call the yellow-yellow winding 5.7vct (5.7v, center-tapped). However, the actual voltages of these secondaries, had the transformer been marked may have been closer to 5.0v and 24v. Unloaded secondaries will show much higher voltages than fully-loaded secondaries.

As to the current you can expect from the secondaries? That's a trial and error deal. If the resistance of the yel/yel secondary is less than 1/5 the resistance of the red/red secondary, it will probably have a higher current capacity. If you can see the actual wire used to wind the two secondaries, it might be an indicator of current. There really isn't a good way to tell how much current they can handle other than by trial-and-error, seeing how hot the transformer gets under load. Still, with two secondaries, you don't know which is causing the heating!

I'd looked for the transformer color code in my 1978 Radio Amateur's Handbook, but it isn't in there. You need an earlier edition from the 1960s. I have the Allied (Radio) Electronics Data Handbook from 1962 or so and I know it has the color code. Unfortunately, it's in storage in a box somewhere I can't get to.



Dean

 

[Johnson777717]

Mr. Hustler,
Thank you again for your reply and I appreciate you taking the time to search through your books and such.

I've learned a great deal, just from you folks, and I now have some use for my box-o-transformers (at least I'm able to find the values).

I suppose the next hurdle is to find the current allowable by the transformer. I probably wont test this, but apply the transformer to a circuit and measure to see if the current is acceptable, per the circuit. Chances are good, that I won't be running over 1 - 2 amps for a while, so I think have a pretty good chance that my transformers will be able to suffice.

Thanks again!

 

[daianum]

Hi man i need a schematic to amplifie my gsm signal for a litel area please help

 

[sam2]

I was going to post a similar question , but I saw this thread, so I will ask here.
I have a medium sized power transformer and I know the primay voltage(120/240) and the secondary voltages,(45,25,7v) but I dont know VA rating.

I need to test it to see how much current it can provide wthout getting to hot. How hot is to hot for the transformer.

It weighs about 10 pounds and the secondaries ar 14 guage wire.

Thanks sam

 

[Leftyretro]

I was going to post a similar question , but I saw this thread, so I will ask here.
I have a medium sized power transformer and I know the primary voltage(120/240) and the secondary voltages,(45,25,7v) but I dont know VA rating.

I need to test it to see how much current it can provide wthout getting to hot. How hot is to hot for the transformer.

It weighs about 10 pounds and the secondaries ar 14 guage wire.

Thanks sam

Not sure of a specific test but power limiting for transformers is limited by two factors:

Core size limits the maximum field strength before magnetic saturation takes place causing a voltage sage. It's effect is immediate and could probably be tested for.

The second limit is the temperature limit of the winding insulation. Failing insulation will lead to shorted turns and or shorts between windings. This is a failure over time and probably not testable in a realistic way.

There was once a graph in an older copy of the ARRL Radio Handbook that showed total wattage rating capacity Vs the area in square inches of the central laminate core, and therefore a good indication of total power capacity (hence amps) of a transformer.

Lefty

 

[sam2]

Not sure of a specific test but power limiting for transformers is limited by two factors:

Core size limits the maximum field strength before magnetic saturation takes place causing a voltage sage. It's effect is immediate and could probably be tested for.

The second limit is the temperature limit of the winding insulation. Failing insulation will lead to shorted turns and or shorts between windings. This is a failure over time and probably not testable in a realistic way.

There was once a graph in an older copy of the ARRL Radio Handbook that showed total wattage rating capacity Vs the area in square inches of the central laminate core, and therefore a good indication of total power capacity (hence amps) of a transformer.

Lefty

Thank you,

By test ,I just meant puttining a load on it in steps and seeing when it got to warm, but you have given me some good ideas.

Can you give me a guess how much voltage sag?

Is "to hot to touch" to hot?

I have seen that formula/graph someplace in the past and I will look for it.

Thanks again, sam

 

[Leftyretro]

Thank you,

By test ,I just meant puttining a load on it in steps and seeing when it got to warm, but you have given me some good ideas.

Can you give me a guess how much voltage sag?

Is "to hot to touch" to hot?

I have seen that formula/graph someplace in the past and I will look for it.

Thanks again, sam

Hi Sam;

Well I've worked on commercial gear where they ran the transformer a whole lot hotter then I would have thought reliable, but they ran reliable none the less.

If I had a bunch of unmarked transformers I would try and come up with some kind of testing protocol and standards based on measurements made on transformers I had that were marked and rated and also tested.

Take a known rated transformer and load it to it's 100% rating and measure it's temp after an hour. Make that your max allowed running temp. Be sure you load all windings to their max ratings if it has more then one secondary winding.

Voltage sag. Again take a known transformer and measure it's unloaded to fully loaded output voltage and make that your percentage of allowed sag.

Don't use those small 'wall wart' transformers to set your standards as I've read that most have pretty poor regulations and are not reflective on normal industrial transformer performance.

Good luck, and if you come up with a good testing protocol consider documenting it and sharing your results with us so we can all benefit.

Lefty

 

[sam2]

Hi Sam;

Well I've worked on commercial gear where they ran the transformer a whole lot hotter then I would have thought reliable, but they ran reliable none the less.

If I had a bunch of unmarked transformers I would try and come up with some kind of testing protocol and standards based on measurements made on transformers I had that were marked and rated and also tested.

Take a known rated transformer and load it to it's 100% rating and measure it's temp after an hour. Make that your max allowed running temp. Be sure you load all windings to their max ratings if it has more then one secondary winding.

Voltage sag. Again take a known transformer and measure it's unloaded to fully loaded output voltage and make that your percentage of allowed sag.

Don't use those small 'wall wart' transformers to set your standards as I've read that most have pretty poor regulations and are not reflective on normal industrial transformer performance.

Good luck, and if you come up with a good testing protocol consider documenting it and sharing your results with us so we can all benefit.

Lefty

Thank you

That sounds like an interesting project, and I have a few other medium sized laminated core transformers( that I know the specs on) to test ,plus many smaller ones.

sam