### Author Topic: Transformers  (Read 2332 times)

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#### jsmoker

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##### Transformers
« on: October 15, 2008, 08:55:39 AM »
I want to use a transformer to build my own power supply, but I feel like there's something I'm missing (concept wise).  My tranformer is a 120VAC to 25VAC center tapped rated for a 2A output (think it said 750mA at the supply side).  I measured the resistance of the primary wire and it showed 11ohm...if I hooked up a 120VAC to it that would be 120/11=~11Amps....which is above the limit.  I been looking at some peoples diy power supplies and not one uses a resister in series.  I know missing some concept, can anyone help?  I already burnt out one transformer trying to put this together and also the fuse in my multimeter trying to measure the current.

-Jason

(PS I'm trying to make a +/- 12.5 unregulated supply to drive a motor amp using 2 transistors)

#### ArcMan

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##### Re: Transformers
« Reply #1 on: October 15, 2008, 09:19:08 AM »
You are measuring the DC resistance of the transformer coil.  What really matters is the impedance of the coil at 60 Hz AC.  That coil is an inductor which has a different impedance at 60 Hz than at DC.

Just wire it through a 1A slo blo fuse to 120VAC.  Make sure your fuse is a slo blo type.  The transformer inrush current will blow a fast blo fuse every time.  Then you can (carefully) measure the AC current if you want.  Note that the measured current will be very low until you start loading up the secondary.

#### jsmoker

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##### Re: Transformers
« Reply #2 on: October 15, 2008, 09:39:41 AM »
Hmm, I actually did hook up a 2A fuse (the first time around I bought the fuse mount but forgot to pick up the fuse itself so I just shorted it...I know stupid).  I think perhaps I have something wrong with my circuit after the transformer to pull so much current.  I'm posting a link to the schematic incase someone might pick up what I did wrong.

http://www.jasonjsmoker.com/pictures/diagram.jpg

#### jsmoker

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##### Re: Transformers
« Reply #3 on: October 15, 2008, 10:26:07 AM »
I cut off everything from the transistors on and I found that the voltage over the bottom half including the fan was 14V and the top half is 22V.  What's up with that?  How am I getting a higher total difference (36V) than the specified 25VAC.  If anything should I be getting something lower since I'm converting from AC to DC?

#### ArcMan

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##### Re: Transformers
« Reply #4 on: October 15, 2008, 11:09:10 AM »
Are you measuring the AC voltage right at the transformer?

If so, then it sounds like your transformer doesn't meet its specs.

#### jsmoker

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##### Re: Transformers
« Reply #5 on: October 15, 2008, 01:11:39 PM »
No, this is after the rectifier.  I measured at the transformer and it does have a 25V peak to peak.  What I'm saying is I'm confused that the DC voltage after the rectifier is larger than the AC voltage before the rectifier when there's no voltage multiplier involved.

#### ArcMan

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##### Re: Transformers
« Reply #6 on: October 15, 2008, 01:45:50 PM »
The AC voltage measurement you get from your meter is called the Root Mean Square (RMS) voltage.  The actual peak-to-peak voltage is greater by a factor of 1.414 (1 / squrt(2)).  If you rectify an AC voltage and feed it into a capacitor to smooth it out, you will measure a DC voltage nearly equal to the peak-to-peak voltage.  As soon as you start loading the capacitor, the DC voltage will go down from there.

As an important side note, your capacitors must be rated to handle that greater peak-to-peak voltage, not just the RMS AC voltage.  When I was a young pup (before my engineering degree), I learned that the hard way.  A nice big juicy 1000 mic cap blew up in my face when I over-voltaged it.

• Robot Overlord
• Posts: 158
##### Re: Transformers
« Reply #7 on: October 15, 2008, 04:42:21 PM »
By useing the center tap as ground your are creating a +12.5 volt supply and a -12.5 volt supply. In that setup ground is negative for the +12.5 volt side of the supply but it is positive for the -12.5 volt side of the supply. If you are powering both transistors at the same time that would create a short between the +12.5 and the -12.5 which would be the full 25 volts. Now if you powered them one at a time one would cause the motor to spin one way and the other would cause the motor to spin the opposite direction.

#### jsmoker

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##### Re: Transformers
« Reply #8 on: October 16, 2008, 01:41:45 PM »
I finally got it to work.  I think I was just in too much of a hurry the first time I hooked it up and probably had a miss connection, I just hooked it up testng each section progressively and it works great.  The only addition to the diagram that I had to add was a voltage amplifier to the input.  Putting a 5Volt input only gave me ~3.5  on the output which wasn't going to cut it cause the DAC only went to 10V and I needed about that much on the output.  But now with the amp I've got milliamps coming in from a NI DAC board and half an amp coming out the other end.