# transformer help needed

Discussion in 'General Electronics Discussion' started by mahone, Dec 21, 2013.

1. ### mahone

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Dec 21, 2013
hi everyone this is an introduction and a plea for help.

I have 2 transformers that have blown primaries. It has been so long since ive worked on one that im starting from scratch again

I wondered if anyone can give me (or link me to) the equation for calculating the turns required for the primary coil?

ive been googling it for a week to no avail

it is a step down 240 to 12v (uk mains) and I cant unwind and count the turns as the primaries had to be removed with brute force.

many thanks for any help, and merry xmas to all

2. ### duke37

5,364
772
Jan 9, 2011
If you count the secondary turns. you can get the turns per volt.

In theory, the primary will need 20 times the secondary. Due to resistance in the winding, the primary will need less than this to give some excess voltage. I would try 19 times. You cannot reduce the primary turns too much or the core will go into saturation.

3. ### KMoffett

723
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Jan 21, 2009
Unless there is something really special about this transformer, it seems like it would be more cost effective to just buy a new one...after figuring out why you're blowing windings. Since you pulled the primary off, I assume this isn't the type that have a thermal fuse buried in the winding.

Ken

4. ### mahone

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Dec 21, 2013
thanks so much duke once you said it, its glaringly obvious. of course the secondary will give the ratio I need. its funny, sometimes you dont see what's straight in front of you, until someone else points it out

ken, you are absolutely right, but this has become a personal crusade now. I guess I just cant give up.

5,164
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Dec 18, 2013
Hi mahone. I use this if it's of any help N1/N2 = V1/V2 =I2/I1
Thanks

6. ### KMoffett

723
75
Jan 21, 2009
mahone,

You never said what circuit the transformers were being use in. Rewinding the primary without finding out what the cause the failure will only give you lots of practice in rewinding the primaries again...and again...and again.

Ken

7. ### mahone

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Dec 21, 2013
thanks adam, that's the one I was looking for.

ken, one transformer came out of a stereo, the other a microwave. neither of which are a factor because they are for a spot welding machine. basically all I need is a safe primary, the secondary will be set up to give amps.

I posted the same question on instructables, and was told to flip it over and use the secondary as primary. I know how bloody dangerous that is. its frightening that a beginner may do it, ive reported the post but its still up. let this be a warning about using that forum.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
OK, mahone. stop right now.

Whilst instructables is generally the best place to get the worst advice (or at least unreliable advice), what that person said to you may actually have been the right thing.

Tell us exactly what you're doing because some of your reasoning seems to be faulty.

If you connect the high voltage transformer from a microwave the right way, it will be dangerous. If you do it the wrong way it will be absolutely bloody lethal.

You sound like you're planning to do it the wrong way...

9. ### mahone

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Dec 21, 2013
sorry, I just re read this thread and it doesn't make much sense I am typing from a mobile phone, I should have been a bit more specific.

my nephew is building a spot welder for a school project. I personally don't like the idea, but teenagers are teenagers.

I have 3 spare transformers, one is a 240/12v step down, one a 240/30 step down. the secondaries would not be appropriate for use as primaries in my opinion (correct me if I'm wrong) and the microwave transformer probably would have been ok to use secondary as a primary, but that's not an option any more.

I can't do links on this phone, but his idea is very similar to the stuff posted on that site. I will try to upload a schematic asap. whilst I have reservations, the design itself should be safe enough if properly isolated and cooled.

at the end of the day he will do it with his mates with or without my help. at least with my help the only casualty will be the school wiring.

10. ### mursal

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Dec 13, 2013
Might I suggest you get an old microwave from the recycling center and work on the secondary. My mate make one and I'll find out more if its any help?

Use a ELCB/RCD protected plug and that will trip if someone earths the phase wire. Unfortunately it will not protect if someone gets caught between phase and neutral.

11. ### mahone

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Dec 21, 2013
it's funny, every man jack and his wife seems to be scrapping microwaves when you don't want one. but as soon as you need one at short notice...

it was kinda dropped on me with no warning, it's 2 weeks till the schools open back up. till then I will have to make do with what I have and hope that I can hunt out a better part.

an rcd is a must, but thanks for the tip if you have another design I would love to see it. I've been scouring the net looking at various spot welders from some potentially very good designs to the downright scary and potentially lethal.

he found the design on YouTube (I know) which was a grant Thompson? video. I have to admit with a few modifications and a very heavy duty isolation, I think it could be safe. at the end of the day once it has been to the school, I will end up with it to strip down, what use has a 14 year old got for a spot welder?

we are taking pictures of each stage as it happens for the school, I'm new to this forum (and this phone) so I am hoping to be able to upload here and get some feedback.

Alex

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
I would really appreciate it if you can post a link to the video. That way we can give you our opinion on how safe it is as a school project.

13. ### mahone

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Dec 21, 2013
I couldn't copy the YouTube url on my phone but it let me copy this:

m.instructables.com/id/How-to-Make-a-Spot-Welder-for-Cheap/

which links to the video. this was what gave him the original design.

just for fun, if you want to see something incredibly irresponsible look for the yt vid "scariac" I couldn't believe my own eyes when I saw that.