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ATX power supply -> lab bench power supply

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Cobalt60

New Member
At work I have access to a nice bench power supply. I can set the voltage anywhere i want up to 30V in increments of 0.1V. However, when I get home, I rely on a collection of "wallwart" power supplies. But, I have a few ATX power supplies as well, and would like to convert one into a useful bench power supply. If possible, I would like a smooth, variable output.

The 2 things I cant figure out are; How can I make this a variable power supply? And how can I get more than 12V out of it?

One (small) power supply I have in front of me has the following outputs:

5V/18A
12V/17A
-12V/0.5A
3.3V/5A

I'll start with the over 12V problem. If I use 12V and -12V as ground, that should be 24V. But then what kind of amperage could I expect? If thats not the solution for greater than 12V, then what is?

OK and now for the variable part. Im not too familiar with methods used for varying voltages. Im familiar with PWM, but I am assuming their square wave would be suitable for little more than lights and motors? I am aware of voltage regulators, but I do not know how exactly they work or how expensive one would be to provide some decent amperage.

I'll keep it short for now. If a true variable (and smooth) option proves too costly, I will settle for a few select voltages.

Thanks

-Chris P
 

colin55

Well-Known Member
It's the current that determines the capability of the output. You can connect any of the windings in any order and get all sorts of output voltages.
You can connect them around one way to add the voltages or around the other way to get a subtraction and then add a third winding.
The final output current will be the winding with the lowest current capability.

I am assuming each winding has 2 leads, so they can be connected together.
 
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Cobalt60

New Member
So, in the example I gave, if I connected the 12V/17A and the -12V/0.5A, the final rating would be 24V/0.5A?
 

colin55

Well-Known Member
If both those outputs had two wires and you connected them in one direction, it would give 24v/0.5A In the other direction it would be 0v.
 

Menticol

Active Member
Adding a LM317 variable regulator would be great, don't forget a nice analog or digital display. An amperemeter in series with GND will show you the consumption on any of the +12, +5 rails, very useful too
 
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Cobalt60

New Member
Does the LM137 provide a smooth output? I see it is rated at (only) 1.5A. Not too bad, especially considering my nice bench supply at work doesnt even hit 4A. But I have to ask, is there a similar adj. regulator out there rated for more amps? If I really wanted to, could I combine several LM137s to multiply the amp draw (for example 2 LM137s for 3A). If the LM137 really does provide a smooth output, does this mean I would have no motivation to use the 5V or 3.3V lines of the ATX PS?

And I have to ask. I have sometimes connected 12V fans in computers to the 5V line because 12V would be to loud. However, with some of the fans I have, I feel 5V would not be enough. Could I wire the positive of a 12V fan to the 12v line, and the negative to the 5V line, for 7V, with no adverse effects on the rest of the system?

As far as adding meters, thatd be great, but Im trying to keep this cheap. I know one place to get a digital multimeter for $8, so thats one idea. Other than that I havent yet found a standalone meter for less than $12, which is $24 for a pair (DC and A), which is a bit more than Id like to spend.
 

colin55

Well-Known Member
See these on the web:

LM138/LM338 5-Amp Adjustable Regulators

LM138K STEEL, 5 Amp Adjustable Regulator

Regulators have a smooth output.

Remember this:

If you have an input voltage of 12v and an output voltage of 5v @ 3amp, the losses of 7x3 = 21 watts will be produced by the regulator and it will get very hot if it is not on a big heatsink.


You can try connecting the fan to +12v and +5v line. It will not affect anything else.

Use cheap digital or analogue meters. That's what I do. They are $8.00 from the "junk shops."
 
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Willbe

New Member
the positive of a 12V fan to the 12v line, and the negative to the 5V line, for 7V, with no adverse effects on the rest of the system?
You're asking the +5v to sink current; it probably won't. You may have to shunt this supply with a resistor to ground so that the net effect is that the +5v always sources current when connected to your 7v load.

For these supplies
5V/18A
12V/17A
-12V/0.5A
3.3V/5A
Do you have or can you get a separate return for each, or is there only one return?
 
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colin55

Well-Known Member
Yes, the Chinese Junk Shops. They have EVERYTHING. I mean a $8.00 multimeter. You can set it to a range and leave it connected.


"You're asking the +5v to sink current; it probably won't."

Of course it will work. Just reverse the connection to the 5v line and you have -5v.

Providing all outputs have 2 leads.
 
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Menticol

Active Member
Yes, the Chinese Junk Shops. They have EVERYTHING. I mean a $8.00 multimeter. You can set it to a range and leave it connected.
Man, we have a phrase here: "Barato sale caro", cheaper gets expensive. Don't buy that chinese crap. The multimeter also needs batteries, batteries cost. Is preferable to invest a bit more and make a decent thing, it's a very useful equipment.
 

colin55

Well-Known Member
Where do you think everything comes from?

We buy everything from China and never had a problem.

Our next cars will be Chinese (Asian). All the other manufacturers will be defunct.

You could not buy the display and components etc for $8.00 anywhere else and the fact is: the display would be made in China or Thailand ANYWAY.
 
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Cobalt60

New Member
Also, if I did use a multimeter as a display, it wouldnt be run off batteries, it would run off the power supply. id also gut the multimeter and only use the parts I need.
 

Menticol

Active Member
Where do you think everything comes from?

We buy everything from China and never had a problem.

Our next cars will be Chinese (Asian). All the other manufacturers will be defunct.
It's obvious that Intel, AMD, Sony, and other decent brands make their products in China and other parts of Asia. Another example are Korean cars, great in price vs. quality.

But please!! saying that dollar store products are great it's an ofense.
Saying that chinese cars are great, is a crime.
Saying that chinese cars will make the other brands dissapear, is ridiculous.
Even the japanese Toyotas, excellent indestructible cars, are not enough to make Mercedes, BMW, VW or Peugeot to vanish.

And the statement "never had a problem" it's an indicator of a serious problem!

See the attachments, and this video

YouTube - Chinese car crash test failure
 

Attachments

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colin55

Well-Known Member
Where do you think all our TV manufacturers have gone?

Where do you think ALL the displays are made?

Where do think all the components are made?

What about the chips?

All manufacturers of meters and multimeters etc get their parts from the same sources.

So, if you say FLUKE is good, then you are saying all Chinese things are good.

Who do you think perfected the transistor? The Japanese.

Who do you think perfected the car (reliability). The Japanese.
I am talking about overall improvement.

I can go so far back that I can prove it to you.

Which cars are the most unreliable: Mercedes, BMW, Peugeot etc etc etc. For the number sold, they have a very high incidence of breakdown. Possibly because they are driven hard.
 
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colin55

Well-Known Member
You will notice I have corrected one of my replies from above.

The correct answer is:
"You can try connecting the fan to +12v and +5v line. It will not affect anything else."

When you connect the fan to these outputs, the resulting voltage will be the difference between the two, namely 7v.
In other words, connect one lead of the fan to the +12v output and the other lead of the 12v output goes to ground. Connect the other lead of the fan to the +5v output and the other lead of the 5v goes to ground.
 

Cobalt60

New Member
I dont know if you are familiar with ATX power supplies. But a standard power connector for the various components is a 4 pin molex connector. One pin is 12V, one is 5V, and two are ground. So if Im understanding correctly, there is only one lead for each output.

So, what Im asking is, could I simply connect the pos of the fan to 12V, and the neg of the fan to 5V, without any adverse effects, to run the fan at 7V?

But as for the original topic:

Using an LM317 type device is a great idea. Its, cheap, easy, and provides a great signal. However, I have to ask, should I consider a switching regulator with accompanying circuit for signal smoothing?

Is the output voltage of an LM317 based on its input voltage? For example, If I wanted to set the output voltage to 3V, could I switch between using a 5V input and a 12V input?

Also, you mentioned heat dissipation of the LM317. So to minimize heat dissipation, I could, for example, use a 5V input for voltages less than 5V (assuming the LM317 has no voltage drop). But would the trick of using a positive voltage as ground work to also make an optional input of 8.7V (12V-3.3V) for the LM317? This would mean that, assuming I use multiple input voltages of 3.3, 5, 8.7, and 12, the maximum voltage difference the LM317 would have to provide would be 3.7V, reducing its heat dissipation.
 

colin55

Well-Known Member
1. Can you separate the two ground pins.
If so, you have two separate windings.

2. The LM317 is adjustable from 1.25 to more than 30v.
3. It smoothes the output to almost perfect DC output (only about 10mV ripple). You don’t need any switching regulators or other components. Just a few capacitors (electrolytics) and some resistors to “jack-up” the voltage from 1.25v to any voltage you require.
4. The 3-terminal (adjustable) LM317 regulator requires about 2-3v across it to provide good regulation.
5. It will deliver up to 1.5 amps but it will need a heatsink for almost any application.
The heat generated is determined by the voltage across it, say: 14v in – 5v out = 9v multiplied by the current (say 1 amp) = 9 watts.
6. The 5v line will give about 6v after rectification and you will just be able to get 3v from the regulator.
 
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mneary

New Member
1. Can you separate the two ground pins.
If so, you have two separate windings.
You cannot do this on a standard ATX power supply. All black wires are connected together and to the case, and are connected to the Earth pin of the Mains plug.
 
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