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  #1  
Old 03-28-2008, 10:47 PM
Howie Howie is offline
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Default +-12 Volt Power Supply

I have built a plus/minus 12 volt power supply using a center-tapped transformer (transforms 120 V AC to approximately +12.6/-12.6 RMS), a bridge rectifier, multiple filter capacitors, and two voltage regulators for the plus and minis ~12 volts. I have a switch and a fuse connected with the transformer and power cord.

What can I do to check before I plug in my power cord to the wall? I'm worried something may be connected wrong and kill the socket and circuit breaker.

Aside from making sure the wires are soldered well and no wires are connected together accidentally to make a short, is there another way to check before I test this on the wall?

Thanks!

Howie

Last edited by Howie; 05-04-2010 at 12:02 AM.
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Old 03-28-2008, 11:03 PM
jpanhalt jpanhalt is online now
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A very old technique is to put a light bulb in series with the black lead. Worst thing that can happen is the light bulb turns on, and your system, depending on the current it is designed for, may or may not survive. At least, you won't take down the grid in your area. John
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Old 03-28-2008, 11:08 PM
Howie Howie is offline
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Quote:
Originally Posted by jpanhalt View Post
A very old technique is to put a light bulb in series with the black lead. Worst thing that can happen is the light bulb turns on, and your system, depending on the current it is designed for, may or may not survive. At least, you won't take down the grid in your area. John
Hmm...not sure...black lead, meaning the bulb in series with with the power cord and primary transformer wire? Or you mean the output?

And I think my transformer is rated 2 amps.

Last edited by Howie; 03-28-2008 at 11:11 PM.
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Old 03-28-2008, 11:27 PM
jpanhalt jpanhalt is online now
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My comment was based on your comment about killing the socket and circuit breaker. A 2-amp transformer shouldn't do that, if properly connected to the mains. If not, then the light bulb will help. Frankly, I wouldn't worry about the circuit breaker given that situation, unless you have concerns about how the primary is wired.
John
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Old 03-28-2008, 11:47 PM
Howie Howie is offline
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Well, the first time I connected it, I put the fuse, switch, and power cord all in parallel. That would...do something bad if plugged in.

Someone suggested to me to use a digital multimeter, and measure all the short circuits and make sure it shows 0 volts...but this means I have to plug in the cord right?
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Old 03-29-2008, 12:24 AM
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At this point, posting a schematic is about the only way for us to be able to see if your stuff is connected properly. But using the light bulb in place of the fuse will limit ant damage from misconnections, and still let the voltages to come up is all is well.
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Old 03-29-2008, 07:26 AM
Cornelius Cornelius is offline
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Quote:
Originally Posted by Howie View Post
Well, the first time I connected it, I put the fuse, switch, and power cord all in parallel. That would...do something bad if plugged in.
Yes, that would do exactly what you are afraid of...

The Switch and the Fuse should be in series in one of the leads on the primary side; the power cord in parallel, as you put it.

Except for obvious shorts, look out for the right polarity of the filter capacitors, and the right connection of the in/out legs on the regulators.

When you say your transformer are rated 2 amps, would that be total on the secondary? or between each secondary winding?
If it's 2x12V, 2x2A on the secondary(which is my guess), then the correct Fuse on the primary side should be about 500mA to protect your transformer.
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Old 03-29-2008, 02:45 PM
Howie Howie is offline
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Originally Posted by Cornelius View Post
Yes, that would do exactly what you are afraid of...

The Switch and the Fuse should be in series in one of the leads on the primary side; the power cord in parallel, as you put it.

Except for obvious shorts, look out for the right polarity of the filter capacitors, and the right connection of the in/out legs on the regulators.

When you say your transformer are rated 2 amps, would that be total on the secondary? or between each secondary winding?
If it's 2x12V, 2x2A on the secondary(which is my guess), then the correct Fuse on the primary side should be about 500mA to protect your transformer.
Alright, I think I am sure of the polarity for the filter capacitors. I used 4 electrolytic 1000 uF caps. 2 for the input and 2 for the output. 1 for the +12 input side, 1 for the -12 input side, 1 for the +12 output side, and one for the -12 output side. For the +12 input/output side, I put the negative side to ground. For the -12 input/output side, I put the positive side to ground. The other two capacitors on the output are ceramic capacitors I added for noise?

I have used a 1.5 amp fuse on the primary side..just over a little 1 amp...

Here's a crude..making of the schematic I used to make my little less than half of a masterpiece:

Comments greatly appreciated! This is my first...soldering try of a real project.

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  #9  
Old 03-29-2008, 03:25 PM
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SgtWookie SgtWookie is offline
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The only things safety related are:
1) The fuse should be the first and only component that is connected to the "hot" wire. As things are now, your fuse is AFTER the switch, which is not good!
2) Reduce the size of your fuse to 1A. You might wish to use a slow-blow type, as charging up the capacitors will put a heavy initial load on the supply.

What are the regulator ICs that you are using? I see "I", "G" and "O", are they fixed 7812/7912 regulators?
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Old 03-29-2008, 03:37 PM
Howie Howie is offline
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Quote:
Originally Posted by SgtWookie View Post
The only things safety related are:
1) The fuse should be the first and only component that is connected to the "hot" wire. As things are now, your fuse is AFTER the switch, which is not good!
2) Reduce the size of your fuse to 1A. You might wish to use a slow-blow type, as charging up the capacitors will put a heavy initial load on the supply.

What are the regulator ICs that you are using? I see "I", "G" and "O", are they fixed 7812/7912 regulators?
1) Hmmm...I don't see why it's not good. Could you explain?

2) I have used a slow blow-type. Why reduce to 1 amp?

3) I've used an LM1320MP-12 for the negative regulator and a LM7812 for the positive regulator. Are these okay? Also..for both...is it: INPUT-GROUND-OUTPUT start from left to right where left is the first letter of writing? Or are the pins different?
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