Transformer Help

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M

metroplex

Golden Member
Jul 24, 2001
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Transformer Diagram

THat's a diagram of a transformer I'm trying to build.

I got the toroid wound:
5 22AWG wires for the primary windings (4 windings per primary) and single 15.5AWG wires for the secondary windings.

How would I test this using a 120VAC/60Hz wall power input??

I don't have access to an oscilloscope or a function generator until school starts - so all I have is a digital voltmeter that can read up to 500V AC and the 120VAC wall power.

Do I hook up two wires to the primary leads, leave the center tap alone... then hook up my DMM to the secondary leads and leave the center tap alone?

Thanks!
 
Scarpozzi

Scarpozzi

Lifer
Jun 13, 2000
26,389
1,778
126
I *hate* toroids (well, winding them at least).
Click to expand...
Agreed... I always wondered what the first guy that did that was thinking when he wound the first one. :p
 
M

metroplex

Golden Member
Jul 24, 2001
1,064
0
71
Hmmm

I did that but I got zero volts on the primary end, and yeah winding toroids are a PITA.

I wound it this way (i'll try to describe as BEST as I can)

wrapped the toroid with electrical tape
started off #1, 4 turns of five 22AWG magnet wire.
started off #3, 4 turns of five 22AWG right over #1 in the same direction.

For #4, I started at the opposite end and worked my way in the opposite direction of how I wound #1 and #3. For #6, I wound it over #4 (btw 4 and 6 are both 12 turns).

I kept the center taps isolated from one another so that It wouldn't short out the primaries.

but i still get no voltage at the secondary.
 
Colt45

Colt45

Lifer
Apr 18, 2001
19,721
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what do you need a tapped primary for anyways?

the primary needs to be shorted if you want to pass any voltage...




is this for your car amp thing? I take it you'll be running it off some kind of chopper then?
 
Colt45

Colt45

Lifer
Apr 18, 2001
19,721
1
0
jeez, only 4 turns?

dont plug that into 120v unless you want to pop the breaker.
 
M

metroplex

Golden Member
Jul 24, 2001
1,064
0
71
Hi Colt45:
Yeah its for my car amp - I have a FWB + filtering capacitors, but I just need to get the xfmr made first.

This xfmr is for a SMPS that uses 12-14V RMS square wave (at 100 KHz or so), so I think the primary center tap is for the +12V DC to run through the switching MOSFETS off of the PWM controller output.

So if I wanted to test it using a sine wave, I'd short the primary center tap?
How about for a square/rect wave?
 
Analog

Analog

Lifer
Jan 7, 2002
12,755
3
0
OK, I'm not sure what this transformer is for, so I'm making some assumptions. First, it looks like this is some type of switching power supply transformer, used to boost 12VDC systems to higher voltages to power amplifiers for cars or something of that nature. If this is true, then BY NO MEANS plug this thing into the wall power. Why? Because the impedances are all wrong. The red flags went up as soon as you mentioned the number of turns on the primary. 60Hz is just too low a frequency to make this thing work without melting the primary wires, or just popping a breaker, or just causing the end piece of the wire to vaporize.

A switching power supply switches at a MUCH higher frequency, so the inductive reactance of the transformer kicks in and limits the current on such few turns.

Another possibility is that you are building an impedance matching transformer for a speaker / amplifier system. Again, I'm working in the dark. You may want to give a better description of the application and what you are trying to accomplish here.
 
Colt45

Colt45

Lifer
Apr 18, 2001
19,721
1
0
I got a toroid from a car amp here somewhere.

but it steps the voltage up, yours doesnt seem to..

4 turns primary or so (each side of the CT), driven by some kind of mosfet chopper deal (not too sure.. solid state is my weak point.. ;) ).

I think about 10 turns each side of the CT on the secondary, which after filtering gives ya ±35v or so.

you need ±12v correct?
 
Analog

Analog

Lifer
Jan 7, 2002
12,755
3
0
Originally posted by: Colt45
I got a toroid from a car amp here somewhere.

but it steps the voltage up, yours doesnt seem to..

4 turns primary or so (each side of the CT), driven by some kind of mosfet chopper deal (not too sure.. solid state is my weak point.. ;) ).

I think about 10 turns each side of the CT on the secondary, which after filtering gives ya ±35v or so.

you need ±12v correct?
Click to expand...

I don't believe its stepping down. It must be stepping up. Most likely, the primary is being driven by a push-pull mosfet output stage with a bootstrap on the N-channel to get the proper gate bias. That's why the ct on the primary is shown to ground. You need to step this voltage up so that your amp can drive a decent impedance speaker and give good power with stability.
 
M

metroplex

Golden Member
Jul 24, 2001
1,064
0
71
Yellow fiero and colt45:

It's a transformer that's used for a SMPS that is going to power a car amplifier.

The 1:3 ratio indicates a step up transformer.

12-14VRMS primary, 36-42VRMS secondary w/ a chopper at the secondary.

With diode losses, I should get 35-38V DC rail voltages (hopefully)!

So how should I test the transformer?

Use a function generator + oscilloscope - with the input being somewhat close to what I'll be using:
12-14VRMS, square wave, roughly 50% duty cycle?

I just have to short the CT on the primary side right? What should I do with the CT on the secondary side (during testing)?
 
Colt45

Colt45

Lifer
Apr 18, 2001
19,721
1
0
to test it (dont really see why you need to ;) )

put a few volts and a decent freq' on the primary (full or half, whatever) and check on the secondary (full or half..)

this is what your going for right? or are you going for something fancier?
 
M

metroplex

Golden Member
Jul 24, 2001
1,064
0
71
that's what it is more or less!

So I don't really need to test the xfmr? I'm not over-confident or anything but this is the first xfmr I've ever wound. I dunno if I got it right on the 2nd try? :)
 
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