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Making one's own transformer

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IamSmooth

Sun Jan 14 2007, 10:20PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I need a 10vac and 100vac/CT secondary winding off of a 120ac line. I have these voltages with two separate transformers, but I was hoping to find/make one with both. I have read some articles on making one's own transformer, but I think it will probably be too time-consuming when I can just use what I have on hand.

That being said, if I don't need more than a few ma for the 100vac and I need about 4 amps for the 10vac, could I just buy some laminations, wind one bobbin with say 240 turns and the second with 200 turns (tapped at center) to get my 100vac/CT? I would choose wire capable of handling the right current. Would I then wind a second bobbin for my 10vac or could I just tap into my 100 turn bobbin at about 12 turns from the center on each side? I know transformer design is more complicated than this, but I was hoping this would get me close.

Sam

Sun Jan 14 2007, 10:48PM

Registered Member #227 Joined: Mon Feb 20 2006, 10:47PM
Location: Cambridge Ontario, Canada!!
Posts: 127

get a MO transformer, cut it open, rewind and weld. there was a post I think I made on this before... its not to hard

IamSmooth

Sun Jan 14 2007, 10:54PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Thanks. I'm reading some of the older posts. Cutting the laminations seems messy. These things are welded and varnished. How do you get them apart without making a mess?

ragnar

Sun Jan 14 2007, 10:54PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

I suggest:

Buy your 4A, 10VAC transformer, then separate it so you can get at the windings (e.g. carefully saw off the I from the E, if necessary)... take some nice thin wire, wind ten turns on, calculate the volts/turn.

Now you can wind as many turns of thin wire as you need to get your 100VAC... you might want to put some electrical tape over where you're winding it, first.

I did this for my quadraphonic plasma speaker system -- I had two toroidal transformers, each with two 60V primaries. I needed separate low-current 12V supplies for the drivers, so wound my own secondaries on the toroids. It wasn't too hard.

Sulaiman

Sun Jan 14 2007, 10:56PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

First, the core must be large enough - for your needs a 50VA seems about right.
Then you need to wind a proper primary
(enough turns for 120 Vac and thick enough wire for 50VA)
some suppliers sell laminations with a ready-wound primary
in which case you will be told how many turns-per-volt
e.g. RS Stock no. 182-9919 OR similar
Or get an old 50VA (or thereabouts) transformer
remove the secondary counting turns
wind new secondaries.

Too few primary turns = core saturation & overheating
Too many primary turns = less VA for a given amount of steel.

Overall TWO separate transformers is better (more options)
unless for commercial/volume use.

IamSmooth

Sun Jan 14 2007, 11:06PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Your comments are all very helpful. If I saw off the I part of the core will this affect performance?

ragnar

Sun Jan 14 2007, 11:16PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

If you can find a mains toroidal transformer near to the voltage and current you need, you won't need to cut anything. You can add turns to the secondary as needed to get the exact voltage you want, and since you only need a few mA at 100V, then you don't need to worry about much other than the turncount. Why reinvent the wheel?

IamSmooth

Sun Jan 14 2007, 11:57PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I found a transformer I had scrapped in my basement. I cut off the I and monkey'd with the secondary to get 10v. I didn't touch the primary. Unfortunately, this transformer was meant for a few ma and there aren't enough turns on my step-down. I reconnect and place the I on top and when I power it up with a variac it hums really loudly. I get about 4.5-5 volts on the secondary with about 30 turns. How low on the turns can I go? If I reweld the I-lamination will the hum stop?

ragnar

Mon Jan 15 2007, 12:25AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

The hum can come from the windings, the laminations, everywhere... Re-welding, or clamping with a F/G-clamp should show you whether it's worth welding or not (for noise). As you draw more current, it'll buzz, too.

I really recommend a toroidal transformer -- so easy to modify. :P

IamSmooth

Mon Jan 15 2007, 01:28AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I might try the toroid, but I like challenges. I found out that the hum is the vibration of the laminations as they are being shaken by the field. This is easy to fix.

What I am noticing is that the primary for an open secondary is drawing a lot of current? I am using the core from a 120 to 12v ac adapter. I have about 150 turns on the primary with 22g wire. I guess I need a bigger core to hold more turns? Am I saturating the core with too few turns?

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