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240v to 14v Transformer Primary Coil Showing Resistance, Even After Rewinding.

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helion

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Hello All,

I have an old computer UPS which I took to the repairman. He said a transformer is not functioning and it is better to buy a new UPS instead of replacing the transformer.

I identified the transformer, and on checking the leads, I found that the continuity test for Primary coil gives reading of some resistance on the DMM while the secondary coil bleeps like it must.

I decided to rewind it by myself and I bought some thicker gauge wire(of SWG 40, the actual one was measured as SWG 43). After rewinding 2500 turns(by hand of course), even this one showed some resistance. Is this the common behaviour, or is there some small twist or fold in the new wire that can cause this to happen?

Another question: In the said transformer, there are about 4200 turns in primary coil, and about 245 turns in the secondary. Is it okay to reduce the number of turns while taking care to maintain the ratio?

Also, what is the insulating lacquer used in between the E-I laminates? What chemical compound is used for it, so that I can go and buy that too, if necessary? Is it seriously essential if I have to put back the old parts back in place, or should I buy it new and apply?

Thanks. :)
 

Hi Dear helion
Wow ! what did you do ?!!! did you wind that high number of turns with your hands really ???!!!!!!!! how much is it's out put power ? why you don't try to design and make one of them or one better and cheaper than that ?! i think it is more simpler than winding some thousand turns with the hands!!!!
Best Wishes
Goldsmith
 

Hi Dear helion
Wow ! what did you do ?!!! did you wind that high number of turns with your hands really ???!!!!!!!! how much is it's out put power ? why you don't try to design and make one of them or one better and cheaper than that ?! i think it is more simpler than winding some thousand turns with the hands!!!!
Best Wishes
Goldsmith

:D Well yes, by hand watching some cricket match or a movie in two instalments, using some rod/screwdriver as spool axes, so as to avoid any folds/turns of the wire. I wanted to make a small gear machine to operate by hand to complete the job, but it would cost a small bit more for just one transformer. Hence fell back on doing it by hand.

I must try to design a replacement, but for now it is its direct replacement that I am attempting.

It is meant to tolerate 1.4 Amps initial current I guess. One of the half grey areas it is, so I'm trying to redo the wiring exactly as it was. :)
 

better buy a manual coil rewinder than rewinding by hand
Do not reduce the number of turns as this will make
the transformer overheated when energized
The lacquer is called "varnish" and it is available in the shops that sell enamel copper wires, "Nitto" brand made in Japan is of best quality
 
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Is it okay to reduce the number of turns while taking care to maintain the ratio?
For a certain size/material of the core there is a defined number of turnes per Volt, that was calculated by the transformer's manufacturer, so you have to keep it, too ..

I decided to rewind it by myself and I bought some thicker gauge wire(of SWG 40, the actual one was measured as SWG 43). After rewinding 2500 turns(by hand of course), even this one showed some resistance. Is this the common behaviour, or is there some small twist or fold in the new wire that can cause this to happen?

You can calculate the resistance of a copper wire ..
R = [rho] * L / A
rho ≈ 16.7 E-7
L=length[m]
A=cross-section[mm2]

:wink:
IanP
 

"Also, what is the insulating lacquer used in between the E-I laminates? What chemical compound is used for it, so that I can go and buy that too, if necessary? Is it seriously essential if I have to put back the old parts back in place, or should I buy it new and apply?"

If you do not mind a bit of a buzzing you can skip applying the lacquer; it is like paint, it dries solid to hold the E-core components still and not move from applied mag field.

I have rewound a few transformers (secondaries for different voltage output), never used lacquer on the re-assembly.

Jim
 

Hello All,

I have an old computer UPS which I took to the repairman. He said a transformer is not functioning and it is better to buy a new UPS instead of replacing the transformer.

I identified the transformer, and on checking the leads, I found that the continuity test for Primary coil gives reading of some resistance on the DMM while the secondary coil bleeps like it must.

I decided to rewind it by myself and I bought some thicker gauge wire(of SWG 40, the actual one was measured as SWG 43). After rewinding 2500 turns(by hand of course), even this one showed some resistance. Is this the common behaviour, or is there some small twist or fold in the new wire that can cause this to happen?

Another question: In the said transformer, there are about 4200 turns in primary coil, and about 245 turns in the secondary. Is it okay to reduce the number of turns while taking care to maintain the ratio?

Also, what is the insulating lacquer used in between the E-I laminates? What chemical compound is used for it, so that I can go and buy that too, if necessary? Is it seriously essential if I have to put back the old parts back in place, or should I buy it new and apply?

Thanks. :)
Every transformer winding will show some ohmic value when checked with ohm meter if the winding is not short circuited,however the same can be tested with a series lamp(200w),if the lamp is showing quite dim light the transformer is okay otherwise faulty.Moreover you didn't mention the size of yours E-I core,if you provide the core size a complete number of turns will be provided.
secondly you inquired about the lacquer,you may apply some good insulating varnish which can be got from hardware store.
during winding butter paper(insulating paper) should be placed in each layer to avoid eddy current.
you asked about reduction of number of turns,it is quite wrong because the number of of turns depend on the size of the core being used otherwise the transformer will get heated.Regards
 

Every transformer winding will show some ohmic value when checked with ohm meter if the winding is not short circuited,however the same can be tested with a series lamp(200w),if the lamp is showing quite dim light the transformer is okay otherwise faulty.Moreover you didn't mention the size of yours E-I core,if you provide the core size a complete number of turns will be provided.
secondly you inquired about the lacquer,you may apply some good insulating varnish which can be got from hardware store.
during winding butter paper(insulating paper) should be placed in each layer to avoid eddy current.
you asked about reduction of number of turns,it is quite wrong because the number of of turns depend on the size of the core being used otherwise the transformer will get heated.Regards

Well yes, the DMM usually shows well under 50 Ω for the continuity. I skipped the opportunity to test with the bulb or any other device since I took the results of the DMM's continuity test as the final call. The actual Primary coil showed about 1100-1200 Ω and the rewinding of 2500 turns with the new wire showed about 300 Ω.

The spool is of 19.5mm x 13.5mm x 16.25mm, the 16.25mm is the width on which to make the coil's turns, and it has 5mm high flanks for the coil's support.

The dimensions of the E are as follows:

Middle prong - (Stacking Height) x 12mm x 19mm.
Width of the I and the stem of the E, and the rest of the prongs, and the gap between the prongs of the E - ~ 6.25 mm.
Over all, the E is (Stacking Height) x 25.25mm x 38mm.

PS. I might have found it to be okay if I tried it with a light bulb after all, but another transformer from a battery eliminator whose primary coil did not bleep in the continuity test revealed a burn when I opened it and saw. So this was also to be opened then :-D. In any case now, I will have to rewind this one correctly. :smile:
 
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