Understanding/decoding transformer markings

[Skyfox]

Hi all,

I have a lot of salvaged transformers with various markings that resemble serial numbers, and no other markings to indicate input and output voltages, etc.  With no manufacturer indicated I don't know what to look up for a data sheet.  Is there a way to decode these markings and understand some specs of the transformer?

For example, here are two small transformers sitting on my bench right now.  One has 4 pins, two on each side opposite the other, with the markings:
30–408
486–8442
[plus a polarity dot above one pin that goes to the inner winding, which has 13Ω; the outer winding pins on the blank side have no dot and are 26Ω]

The other has 4 wires all on the same side with the markings:
30–398
486–8437
[no polarity dot; wires are blue & red at about 56Ω, and white & black at about 8Ω]

So can these numbers be decoded?  And is there an industry standard for this, or does it vary from one manufacturer to the next?  Some do have more markings that make them easier to understand or are directly labeled for what's what, but others like these are rather vague.

[Richard Crowley]

There is a slim chance that you MIGHT make some sense of those markings if BOTH of these are true:
1) You know the brand (and perhaps also the model) of the gear that it came out of.
2) Somebody has posted a reference document online that correlates corporate OEM "house-numbers" into commodity part numbers.

However the chances of BOTH of those things being true seem extraordinarily slim.

And even if you knew the make and model of the original gear, transformers were very often custom-made and never available as a commodity part.
An even slimmer chance would be that somebody has posted the specifications of custom parts like that.
Frankly, IME, unless 10 minutes on Google doesn't reveal anything solid, you probably have a hopeless cause.

If you want to re-use a bunch of old transformers, you will need to figure out for yourself what they are (power?, signal? RF? etc.) and run your own tests to characterize them.  Figure out primary and secondary connections, which windings are which, power levels and/or frequency response, etc. etc.

There are probably videos on YouTube about characterizing random unknown transformers.  Of course, it wouldn't hurt to take some good photos and post them in forums like this.  Along with pertinent details like weight, size, materials (laminated iron?  pressed ferrite?  etc.)  How many windings, wire colors, winding resistance, etc. etc.

Presumably, if there had been a manufacturer's name, you would have mentioned it?  That would put you well along the way, but many OEM transformers were not labeled with the manufacturer's name.

YOU can see the transformers, but WE can only guess that maybe these are more recent PC-mount transformers (with "pins"?).  But even that is a shot in the dark, absent any other information.  Without further reference like photo, size, weight, etc., not much we can help you with.  Assuming you have already done the Google thing with all the visible numbers.

[vk6zgo]

Hi all,

I have a lot of salvaged transformers with various markings that resemble serial numbers, and no other markings to indicate input and output voltages, etc.  With no manufacturer indicated I don't know what to look up for a data sheet.  Is there a way to decode these markings and understand some specs of the transformer?

For example, here are two small transformers sitting on my bench right now.  One has 4 pins, two on each side opposite the other, with the markings:
30–408
486–8442
[plus a polarity dot above one pin that goes to the inner winding, which has 13Ω; the outer winding pins on the blank side have no dot and are 26Ω]

The other has 4 wires all on the same side with the markings:
30–398
486–8437
[no polarity dot; wires are blue & red at about 56Ω, and white & black at about 8Ω]

So can these numbers be decoded?  And is there an industry standard for this, or does it vary from one manufacturer to the next?  Some do have more markings that make them easier to understand or are directly labeled for what's what, but others like these are rather vague.

All the resistance values point to these transformers not being intended for use with the AC Mains supply.

As an example, in conjunction with another thread, I recently measured the primary winding of my clock radio's Mains transformer at 334 ohms.
This is a 230v system so you could expect about half that for a 120v system.

Your highest resistance reading is 56 ohms which falls a long way short of that.

[james_s]

It's usually not too hard to identify the primary winding(s) and then you can power it up and work out what other windings it has. You can estimate the power by the physical size and verify by checking how much it sags and how much the temperature rises under load. Occasionally a transformer is an off the shelf part with a standard part number but most of the time it will be custom made for the equipment it was used in.

[tautech]

Probably SMPS transformers due to low R values.
Only suitable for switched primary supply.

Pics required.

[cdev]

If you have a signal generator that is often useful in characterizing transformers. Also a simple continuity and resistance test will tell you a lot, you can identify the different windings and get a rough feel for the ratio of turns primary and secondary by the size of the wire and the resistance.

Most power transformers in my experience have some marking indicating that.

[station240]

In theory there is an corresponding relationship between resistance and voltage.
Lower resistance = less turns of thicker copper = lower voltage winding
Higher resistance = more turns of thinner copper = higher voltage winding
Where it gets confusing is when you have multiple outputs, aux winding etc.

Find whichever winding has the higher resistance, and feed it a suitable frequency at 50% duty cycle at a safe voltage (12V AC).
Then measure the output voltages to get the turns ratio.

The tricky part, figuring out what the maximum current of the output/input windings, can be worked out by measuring the wire gauge of the enameled copper wire.
Also the optimal frequency, you might get results with 100kHz but the actual circuit uses 65kHz. 

You could make things easier on us by posting some pictures.

[Skyfox]

Thanks for the info and replies, all.  True, the two example transformers I described have no manufacturer marked on them.  I didn't post photos of these two because I was looking for tips on general information on typing a transformer rather than getting specifics on only these two transformers.  I'll attach a photo of these two, however.  Yes, I did google searches and turned up nothing.

For most of the transformers I have, I have no idea what equipment they came out of.  Some came out of UPS units and switching power supplies, some from industrial control boards, some from various electronics and appliances, and so on.  I've been collecting salvage parts for years and really never documented the sources of the parts.  As far as resistance goes, one big transformer I have that converts 120VAC to 33VAC is only 0.8Ω on the primary, and another 120VAC transformer with various lower outputs is 3.4Ω on the primary.


And I don't have a working signal generator.  My sig-gen is still on the blink and I've been unable to figure out what's wrong with it, and money this year has been far too tight to buy another one.

[vk6zgo]

I was referring to the small transformers you quoted the resistance of, & small transformers in general.
Obviously, large transformers can utilise heavier gauge wire for their windings with less resistance for a given number of turns.
If the secondary winding has less turns than the primary, but uses thinner wire, the difference in resistance will not be so distinct.

[Richard Crowley]

...I didn't post photos of these two because I was looking for tips on general information on typing a transformer rather than getting specifics on only these two transformers. 

The situation isn't nearly as simple as that.  As you have discovered, internal OEM "house numbers" are very rarely decipherable except within the manufacturer's own environment, typically secret or at least not externally published. You must use all the clues available including the physical characteristics and whatever electrical measurements you can make. 

And transformers like those are almost certainly custom-made for the particular product they came out of.  Knowledge of what that product was is a major clue to discovering what the transformer is.  Of course transformers can be used for applications other than what they were originally designed for. As long as the power capacity of the windings and core are not exceeded.  And, of course, electrical parameters like impedances, winding ratios, frequency response, etc. are compatible with your proposed re-purposing.

Both of your transformers are small, iron-core transformers. Just looking at them, they could be audio transformers, or they could be mains-frequency power transformers.  From your description, it may be more likely that they are power transformers.  And more likely that they are mains-frequency (50 Hz or 60 Hz) rather than high-frequency (10 KHz to 100 KHz) switch-mode power-supply (SMPS) transformers.  High-frequency transformers tend more to be ferrite core rather than laminated iron-core like your examples.d

There are several decent videos on YouTube about transformer identification.  You could probably learn a few good techniques by spending some time there.

[Richard Crowley]

A "signal generator" doesn't have to be an expensive piece of test equipment.  It could be a 99-cent board from Ebay.

[cdev]

One of the two pictured is an inductor - You just need to know its inductance.

You know what is super useful and cheap for measuring unknown parts, although it won't do your job in this specific instance with the transformer (apart from measuring the windings inductance) however it will identify a great many two and three terminal unknown parts accurately, if thats what you're setting yourself out to do -

and its cheap enough so that it wont break the budget ($15 at most for a basic one)

the AVR Transistor Tester devices.

They will measure almost anything with two or three leads and do an excellent job of it.

You can measure capacitor ESR, identify types of transistors, values of inductors, diodes, resistors, caps and so on. The fact that they will even measure inductance, although not so accurate with really small values, makes them an extraordinary value.

Just stick the leads in and press the button.

I have one (mine is an older model, from around 2012) thats now housed in the box my raspberry pi came in. Just to keep the dust out, I made a little lid and everything. I keep it at my desk.

The firmware keeps getting better.   


I would look for opinions in the thread of the current best buy. One tip, you don't need the fanciest model, but make sure to buy one with a socketed AVR chip in case you burn it out by mistake. Also so you can update the firmware if you want to, with a new chip (keeping the original FW on the working old chip if the new FW isnt what you want)



there is also a thread on cases for them.

[cdev]

The numbers on your two transformers are so similar the odds are good that they both came from the same manufacturer, probably no longer in business.

I bet you if you keep the numbers handy and check for them on Google, perhaps making a list of possibilities as you go on, eventually you will find out what they are from/for.

The volume of a transformer can give you a good idea of its relative power-handling capability.

[Skyfox]

While I have seen those super cheap frequency generators (and I have a small supply of function generator chips that I could probably build something out of), I'd prefer to have something that has a display, or at least a well calibrated adjustment knob, showing me exactly what frequency it's putting out.  I was hoping to find one in kit form that I could solder together but have been unable to find one that has such a screen.  I do have an L/C tester for capacitors and inductors, and a transistor/component tester that also has both a PWM generator and a square wave frequency generator built in.

Thanks for the additional info.

[james_s]

Put one of these on it.
https://www.ebay.com/itm/8LED-Display-0-1-60MHz-20MHz-2-4GHz-RF-Signal-Frequency-Counter-Cymometer-Tester/253108045587

I have one and it works fine.

[Richard Crowley]

While I have seen those super cheap frequency generators (and I have a small supply of function generator chips that I could probably build something out of), I'd prefer to have something that has a display, or at least a well calibrated adjustment knob, showing me exactly what frequency it's putting out.  I was hoping to find one in kit form that I could solder together but have been unable to find one that has such a screen.  I do have an L/C tester for capacitors and inductors, and a transistor/component tester that also has both a PWM generator and a square wave frequency generator built in.

Thanks for the additional info.

The super-cheap 555 generators would be handy for doing basic sleuthing of unknown transformers.  The exact frequency (or amplitude)  is not really terribly important for that kind of work.  That is why I suggested that you didn't need an expensive signal generator just to play with old transformers.

OTOH, there are some modestly-priced Function/Signal Generator board kits with frequency readout on Ebay. Here is the first one I saw...

https://www.ebay.com/itm/Mini-DDS-Function-Signal-Generator-Sine-Triangle-Square-Waveform-DIY-Kit-Panel/351495788170?

[ArthurDent]

My "guess" is that the larger transformer is an audio output transformer with the red/blue being the primary and the black/white being the output to the speaker, probably 8 ohms. That was generally the color scheme that was used in the tube era (red for B+ and blue for plate) and it may have carried into the solid-state stuff as well. Some transformers were mounted on the speakers and rather than use screws and nuts just had bend-over metal tabs to hold the transformer on the speaker frame. This is a guess and here is a photo of some output transformers.

http://billmaudio.com/wp/wp-content/uploads/2009/09/PRRI_TO20_BJR.jpg

Here is the color code on a push-pull output transformer with red being B+ and brown and blue being the two plates. The 16 ohm output would be black and white. Note the impedance will be higher than the D.C. resistance of the windings by a factor of perhaps 10 to 20.

[ishmaiel]

https://www.ecianow.org/eia-source-codes

You have to look through the various years , but when I look in the first catalog at this link, I discover I have a transformer with 126839 on it, which is think according to the 1946 EIA source book means:

126 for Bentley Harris MFG co, 8 for the 8th week and 39, for 1939, or 126839.

Then it is further stamped 606-0-37.  I am looking for confirmation on the 0, but based on what I've read, 606 means a 12 volt primary with a center tap (6 volts either side of the center).  The transformer is dead, so I can't hook it up with out a fireworks display, but there is no center tap on the output, so I'm guessing that we have 37 volts on the output.  I read that the middle number might be a current rating, but I only see 5 listed elsewhere. Zero is what, an amp reading?  How can it be zero amps?  Does that designate that it is for signal only and not for serious current?