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[Solved] Connecting a Transformer: Identify Primary & Secondary Windings, Measure Resistance (1-2, 4-5, 7-8)

nowy.zielony.pczątku 9942 11
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  • #1 17215309
    nowy.zielony.pczątku
    Level 6  
    Posts: 123
    Rate: 43
    Hello, I know there were similar topics but I don't know how to do it. I'm going to do something like in the video:









    The photo is marked with numbers. The connection is there between: 1-2, 4-5, 7-8

    Please tell me between which pins should I measure the resistance (high resistance = primary winding?) To know which winding is primary?
    Where there is the primary winding, I will remove it and there will be a wire, and where the secondary winding is, I will connect it to the electricity. Should it work right?

    Connecting a Transformer: Identify Primary & Secondary Windings, Measure Resistance (1-2, 4-5, 7-8)
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  • #2 17215362
    Justyniunia
    Level 36  
    Posts: 4190
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    The thickness of the wire at the bottom of the photo speaks for the secondary winding 6, 7, 8, 9.
    Don't be surprised, because the primary one can be wound under the secondary one.
  • #3 17215439
    arturdip
    Level 31  
    Posts: 1170
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    Does it have a symbol?
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  • #4 17215489
    mechanix
    Level 14  
    Posts: 88
    Help: 7
    Rate: 40
    Hello
    For such experiences, I would buy a transformer soldering iron. I would take out the transformer and wind the secondary winding if needed. This transformer has flooded windings and there is no room for additional winding.
  • #5 17215506
    Rezystor240
    Level 42  
    Posts: 9240
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    nowy.zielony.pczątku wrote:
    I'm going to do something like in the video:


    Let me put it this way, the pity of our domestic unitra-brand transformer will be lost to such crap.
    How will it apply, will you burn the wires like in a (stupid) movie?
    Waste of time.
    If you care about high current and low voltage, use the transformer soldering transformer.
    It will release about 200A at 0V5.
  • #6 17215535
    nowy.zielony.pczątku
    Level 6  
    Posts: 123
    Rate: 43
    I'm afraid it is a transformer as I write "Justyniunia". I have a second transformer of this construction, but it seems like it would be the opposite, i.e. the primary winding is first and the secondary winding below it (photos). I was wrong to remove the primary winding because you have to remove the secondary one. As the gentlemen from the movies say:
    2:06



    2:37




    Connecting a Transformer: Identify Primary & Secondary Windings, Measure Resistance (1-2, 4-5, 7-8)

    Connecting a Transformer: Identify Primary & Secondary Windings, Measure Resistance (1-2, 4-5, 7-8)

    arturdip wrote:
    Does it have a symbol?

    There are no symbols except numbers.
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  • #7 17215549
    Rezystor240
    Level 42  
    Posts: 9240
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    nowy.zielony.pczątku wrote:
    I'm afraid it is a transformer as I write "Justyniunia".


    @ nowy.zielony.pczątku

    Think if it doesn't hurt you!

    Where do you see the similarity ?!
    Trafo from the last video pasted is a microwave photo.
    I will not write you anything about it because there are tensions of a few KV, so it is deadly.
  • #8 17215570
    vodiczka
    Level 43  
    Posts: 30170
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    Rezystor240 wrote:
    Trafo from the last video pasted is a microwave photo.
    I will not write you anything about it because there are tensions of a few KV, so it is deadly.
    There are no more kV in this video. :)
    What difference does it make if you are rewinding 230V / 230V or 230V / 2400V since in both cases you are removing the secondary winding?
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  • #9 17215576
    nowy.zielony.pczątku
    Level 6  
    Posts: 123
    Rate: 43
    Rezystor240 wrote:


    Think if it doesn't hurt you!

    Where do you see the similarity ?!
    Trafo from the last video pasted is a microwave photo.


    I wrote...

    nowy.zielony.pczątku wrote:
    I have a second transformer of this construction

    As shown in the pictures.

    Rezystor240 wrote:

    Let me put it this way, the pity of our domestic unitra-brand transformer will be lost to such crap.
    How will it apply, will you burn the wires like in a (stupid) movie?
    Waste of time.
    If you care about high current and low voltage, use the transformer soldering transformer.
    It will release about 200A at 0V5.

    I feel sorry for the transformer soldering iron. These transformers were to be dismantled anyway. Okay, I'll try first with a small transformer (see what the results will be).

    Connecting a Transformer: Identify Primary & Secondary Windings, Measure Resistance (1-2, 4-5, 7-8)
  • #10 17215579
    Rezystor240
    Level 42  
    Posts: 9240
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    vodiczka wrote:
    What difference does it make if you are rewinding 230V / 230V or 230V / 2400V since in both cases you are removing the secondary winding?


    It is possible that before rewinding (in this case, rather a modification), he will connect the transformer to the network to check if it works.
    I will not say what may happen next.


    nowy.zielony.pczątku wrote:
    I feel sorry for the transformer soldering iron. These transformers were to be dismantled anyway. Okay, I'll try first with a small transformer (see what the results will be).


    It is a pity to disassemble such a transformer, after all, it is manufactured in Poland, and this can be experienced less and less.

    You don't have to disassemble the transformer soldering iron, you just need to unbend the flat bars and connect the wires, you will always restore them to their original condition.

    This transformer (from the photo) does not even have 30W, so at most you can light a bulb with a piece of rod.
  • Helpful post
    #11 17215661
    vodiczka
    Level 43  
    Posts: 30170
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    Rezystor240 wrote:
    It is possible that before rewinding (in this case, rather a modification), he will connect the transformer to the network to check if it works.
    I will not say what may happen next.
    There are many possibilities, but it is better to make a colleague aware than to complain:
    Rezystor240 wrote:
    I will not write you anything about it because there are tensions of a few KV, so it is deadly.
    The primary winding can be checked with an ohmmeter, and crossing the street on lanes can also be deadly
  • #12 17906624
    nowy.zielony.pczątku
    Level 6  
    Posts: 123
    Rate: 43
    Eventually, I gave up connecting the unitry transformer. I made a welding machine from a transformer from a microwave.
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Topic summary

✨ The discussion revolves around identifying the primary and secondary windings of a transformer and measuring their resistance. Users suggest that the thickness of the wire can indicate the secondary winding, and there is a possibility that the primary winding is located beneath the secondary. The importance of caution is emphasized, particularly regarding the dangers of high voltage transformers, such as those from microwaves. Some participants recommend using a transformer soldering iron for modifications instead of dismantling the unitra-brand transformer, which is noted for its Polish manufacturing. The conversation also touches on the potential risks of connecting the transformer to the network without proper knowledge and the implications of rewinding or modifying the transformer.
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FAQ

TL;DR: Measure resistance across 1–2, 4–5, 7–8; the highest Ω is usually the primary. For welders, "It will release about 200A at 0V5." [Elektroda, Rezystor240, post #17215506]

Why it matters: For hobbyists repurposing mains transformers, this answers how to identify primary/secondary and test safely before rewiring.

Quick Facts

  • Terminals 1–2, 4–5, 7–8 are the likely winding pairs on the referenced unit; measure each pair’s resistance first. [Elektroda, nowy.zielony.pczątku, post #17215309]
  • Primary often shows the highest DC resistance in step‑down designs; confirm with an ohmmeter before applying mains. [“Transformer Basics”]
  • Flooded/potted windings leave no window space for extra turns; pick a different core if you need add‑on windings. [Elektroda, mechanix, post #17215489]
  • Microwave oven transformers have ~2 kV secondaries as built; remove HV winding before any low‑voltage repurpose. [“Microwave oven transformer”]
  • Tiny PCB transformers under ~30 W won’t supply welding currents; expect poor results. [Elektroda, Rezystor240, post #17215579]

How do I identify the primary vs secondary on an unknown transformer?

Use a DMM. Measure resistance across each winding pair. The winding with the highest DC resistance is usually the primary in step-down transformers. Inspect wire gauge: thinner wire suggests primary, thicker wire suggests low-voltage secondary. Do not power any winding until identified. [“Transformer Basics”]

Between which pins should I measure resistance on a unit labeled 1–2, 4–5, 7–8?

Measure only across each numbered pair: 1–2, 4–5, and 7–8. These are likely individual windings or taps. Use the ohmmeter function to confirm continuity. "The primary winding can be checked with an ohmmeter." Record each resistance for comparison before any connection. [Elektroda, vodiczka, post #17215661]

Does thicker wire always mean it’s the secondary?

Thicker enamelled wire often indicates a low‑voltage, high‑current secondary. However, do not rely on layer position. "The primary one can be wound under the secondary one." Confirm by resistance and continuity checks, not by appearance alone. [Elektroda, Justyniunia, post #17215362]

Can the primary be wound under the secondary?

Yes. The primary can sit beneath the secondary in many transformers. Layer order varies, so visual position is not a reliable identifier. Verify using resistance measurements and continuity before connecting anything. [Elektroda, Justyniunia, post #17215362]

Should I remove the primary or the secondary when converting for low‑voltage, high‑current use?

Remove the factory secondary and leave the primary intact for mains. As one expert noted, "in both cases you are removing the secondary winding." [Elektroda, vodiczka, post #17215570]

My transformer is potted/flooded. Can I add more turns like in videos?

No. Flooded or potted windings leave no free window area for extra turns. In such cases, choose a different transformer or approach. [Elektroda, mechanix, post #17215489]

Can a tiny PCB transformer make a spot welder?

No. Small PCB transformers deliver low power. One example discussed "does not even have 30W," so welding is unrealistic. At best, you might glow a small rod. Use a purpose-built core instead. [Elektroda, Rezystor240, post #17215579]

Is a transformer from a soldering iron a good high‑current source?

Yes. For high current and low voltage, a transformer soldering iron core works well. As noted, "It will release about 200A at 0V5." This meets many spot‑welding needs safely and cheaply. [Elektroda, Rezystor240, post #17215506]

Are microwave oven transformers (MOTs) good for DIY welders?

Yes, after modification, many builders report success. The thread author concluded, "I made a welding machine from a transformer from a microwave." [Elektroda, nowy.zielony.pczątku, post #17906624]

How dangerous is a stock MOT?

Very. A stock MOT secondary outputs around 2,000 V AC at significant current. Contact can be fatal. [“Microwave oven transformer”]

How do I safely test a rewound or unknown transformer the first time?

Use a series‑bulb lamp limiter to limit fault current. How‑To:
  1. Wire a 60–100 W incandescent lamp in series with the primary.
  2. Power on; a bright lamp indicates a fault or shorted turns.
  3. If the lamp stays dim, measure secondary voltages. [“The Lamp Limiter”]

Which tools help identify windings quickly?

A digital multimeter with resistance and continuity is enough. As advised, "The primary winding can be checked with an ohmmeter." An LCR meter helps confirm inductance if available. Label each pair before any power test. [Elektroda, vodiczka, post #17215661]

My transformer only has numbers, no symbol. How do I proceed?

That’s normal on many units. When only numbers exist, map pairs by continuity, then compare resistances to identify the primary. Avoid guessing by layer position alone. Document your findings before any wiring. [Elektroda, nowy.zielony.pczątku, post #17215535]

Why did my rewound transformer heat up or trip the breaker immediately?

A shorted turn or miswired primary can draw huge current and overheat fast. Stop and recheck insulation and winding layout. Use a series‑bulb test on the next power‑up to avoid damage. [“The Lamp Limiter”]
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