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**mariushm** · 03-31-2012, 04:38 PM

Re: Variable Power Supply from ATX

You don't need the 10 watt resistor.

Some crap power supplies, old designs, have problems maintaining the output voltages stable or even start without some load, that most likely won't be the case with what you have.

The instructions in the page you linked are bad however. Basically, it says you can feed that voltage regulator 24 volts by connecting his ground and voltage pins to -12 and 12v.

The problem is the modern power supplies are VERY limited in the current they can deliver on -12v - usually it's less than 1 A. When you do the connections he does, you get 24v but only about 0.6A of it... and the voltage regulator has about 70% efficiency, you're left with very little power.
So as far as I can see, his attempt to get up to 22v or so voltage output is useless.

There's no point in aiming for more than 12v...

If you're still interested I can suggest a simpler circuit... let me draw it up in paint

**mariushm** · 03-31-2012, 05:39 PM

Re: Variable Power Supply from ATX

Here's an example

You can just cut most of the cables from the power supply and take directly the voltages the power supply offers you: 3.3v, 5v, 12v and -12v

Most power supplies, when they don't power a lot of things, deliver voltages that are a bit above the standard values above. For example, instead of 5v, it may output 5.3v and get close to 5v as the load increases (when you have several hard drives connected for example)
For most things, these voltages are "close enough" to work directly with them but when doing breadboard stuff, it's not safe for the electronics on the board.
For example, some Microchip PIC microprocessors support a maximum of 5.6v, so you don't want to power the breadboard directly from the 5v the power supply gives you.

So you use a linear voltage regulator, which takes 12v and generates what you tell it to do.
In the instructables page, they use a chip called LM317 because it's a classic ... but the problem is the specifications are not that great.
All linear regulators have a voltage drop, they can only output a value few volts below the input voltage - in the case of LM317, the value is between 1 and 2 volts depending on the load.
So if you were to use LM317 and feed it 12v from the power supply, you'd get a maximum of about 10v.

The LM2914 I put on the picture has a much lower voltage drop of around 0.5v, but at low loads it's even smaller. So it's safe to say you can get up to around 11.5v regulated output with this chip.

Attached Files

**retiredcaps** · 03-31-2012, 06:19 PM

Re: Variable Power Supply from ATX

I'm looking at doing something like a variable ATX power supply as well. However, I'm not going to use it to power anything. I just want the ability to output varying DC voltage so that I can use it as a "tachometer" so I can compare my multimeters over several DC ranges.

Obviously, I have the fixed ATX DC voltages, but sometimes I want to see the update speeds of the multimeters and the accuracy over a range.

**Evil Lurker** · 03-31-2012, 06:39 PM

Re: Variable Power Supply from ATX

If only a small amount of current is needed an LM317 based voltage regulator is perfect, even if the efficiency is horrid.

If a greater voltage is needed then one should look into a higher voltage supply such as a laptop power brick or step up converter.

And there is absolutely no need to cut the cables on a PSU. That is just plain wasteful. One can order generic "molex" fittings and even motherboard connnectors from Jameco for not much cash... much less than all the quick connect terminals... just bundle up the existing wiring and plug in and go. That way if one ever needs to actually use the PSU as it was originally intended they can plus on the upside it will be possible to actually draw more amperage from the PSU with less risk of overloading any one wire.

**mariushm** · 03-31-2012, 07:03 PM

Re: Variable Power Supply from ATX

I was talking about cutting the cables because I had in mind this videos where the guy uses banana plugs or whatever they're called on a ATX case:

http://www.youtube.com/watch?v=z2oSF...layer_embedded

He's not using any adjustable linear regulator, he just relies on the voltages the power supply gives. But you can add the adjustable part easily if you want to, there's plenty of room in the psu to do it.

These other videos explain how to use a linear voltage regulator to get various voltages. He used a LM317, but as I said this one has some poor specs (big voltage drop):

http://www.youtube.com/watch?feature...&v=2EFnqZUxFZ4
http://www.youtube.com/watch?v=hUGze...layer_embedded

In your case, everything up to the capacitor after the bridge rectifier is your atx power supply 12v line. So you can see the way I connected that LM2941 is pretty much the same as in the video, I just added a default resistor of 3.7k there so that when you're with the potentiometer at 0 ohm, the voltage will be a bit above 5v (the minimum that ic can do)

**Peter9DO** · 03-31-2012, 09:05 PM

Re: Variable Power Supply from ATX

First let me thank you for all the input! I was thinking of incorporating a small multimeter into the mix , use the 3.3v line to power it instead of the 2 AA batteries (obviously with some adjustments). I don't want to be limited to just 12v, 5v and 3.3v. I was hoping to have everything in between as well so thank you for the videos!

I don't want you to think I'm just some cheap ass and won't spend $100 on a decent variable power supply, I just think it would be fun to make one, and then to do it out of stuff that's just laying around here anyways. Next, I would like to try to build one from scratch.

Ideally I would like to be able to work on some car radios I have laying around here and I know this will not produce the AMPS, I will need to test them.

Evil, that's really not a bad idea with the laptop power supplies. I have some higher Amp and voltage chargers, I will have to take a look online to see what the best way would be to make it an adjustable bench top supply. I never even thought of that.... Thank you!

**retiredcaps** · 03-31-2012, 10:12 PM

Re: Variable Power Supply from ATX

Originally posted by Peter9DO View Post

I don't want you to think I'm just some cheap ass and won't spend $100 on a decent variable power supply,

Nothing wrong about saving money because that is how great inventions, ideas, and innovations come about.

**Peter9DO** · 03-31-2012, 10:49 PM

Re: Variable Power Supply from ATX

Question for you! I looked up the data sheet for the LM317 and I noticed a very low current output rating (2.5-100 mA). Do you think the LM317 would be the best choice for a voltage regulator? What about a LD1085V? Is there something I'm missing, but would this be a better choice? Again, I'm just learning so I don't know I'm just throwing this out there.

**budm** · 03-31-2012, 11:02 PM

Re: Variable Power Supply from ATX

You can get up to 1.5A with proper heatsink, may be you were reading different parameters?
http://www.national.com/mpf/LM/LM117.html#Overview
You can also get low drop out regulator.

**Peter9DO** · 03-31-2012, 11:23 PM

Re: Variable Power Supply from ATX

That data sheet that you posted is from the LM117, I was talking about the LM317. Here is the data sheet that I was looking at:

https://cdn.badcaps-static.com/pdfs/...abc8448f8d.pdf

Again, please correct me if I'm wrong. I'm trying to learn this stuff.

**mariushm** · 04-01-2012, 01:44 AM

Re: Variable Power Supply from ATX

You're listing the LM317L, which is in a package without adequate heatsinks, so it's limited to 0.1 A of output.

If you get it in a package more suitable to be heatsinked, you will get the one that's capable of more than 1.5A.
For example, LM317T (t from to-xxx) can do 1.5A+ :

Keep in mind that linear voltage regulators turn the difference of power to HEAT.
You should NOT use something like this to convert the 19v from a laptop adapter to let's say 5 volts. For example, a simple small LCD display with backlight draws about 80mA at 5v - to generate this, the linear voltage regulator would turn to heat (19v - 5v ) * 0.08A = 1.12 watts. You won't be able to insert this into a heated laptop adapter brick, because you won't have a fan in that.

On a psu that has 12v output , the dissipated power for the example above is (12v-5v) * 0.08 = 0.56w so with reasonably cheap heatsinks (the ones rated for 15-20c/w at natural convection) and if you leave the atx psu fan running to get some air moving the heat, the lm317 will probably work with up to 4-5 watts of heat. It won't be safe to push it to the maximum because even when outputting 5v at 1.5A we have (12v-5v) * 1.5 = 10.5 watts which is a lot. But I'll get to this later in the message...

Remember what I told you about the voltage drop these regulators have - the LM317 is particularly sucking at this:

If you power a microcontroller and a small LCD screen with backlight, the output will be a bit less than 0.2A, so at 75c which is normal for linear regulators, the voltage drop will be about 1.6v
However, if you're going to power a row of leds or some fans or something up to 1-1.5A, the voltage drop will be up to 2.5v
So you can't really make an good adjustable power supply because, for example, you can power it from 12v and configure the output to 11v, and it will output 11v at low loads but only 10v when you really demand something from it.

You want to pick a regulator that has less voltage drop, guaranteeing you a wide range of voltages without a huge drop.

If you go on Digikey you can filter by voltage drop (say a maximum of 1v @ 1.5A for example), then filter to get only positive adjustable regulators, then pick through hole components because those are easier to work with.
You will get for example these great regulators:

LT1963AET#PBF IC LDO REG LN 1.5A ADJ TO220-5 - Linear Technology Positive Adjustable 1.21 V ~ 20 V 2.1 V ~ 20 V 0.34V @ 1.5A
http://search.digikey.com/us/en/prod...3PBF-ND/894347 5.5$

MIC2941AWT IC REG LDO 1.25A ADJ TO220-5 - Micrel Inc Positive Adjustable 1.24 V ~ 26 V 2 V ~ 26 V 0.4V @ 1.25A
http://search.digikey.com/us/en/prod...1138-ND/771607 2.54$

MIC29152WT IC REG LDO 1.5A ADJ VOLT TO220-5 - Micrel Inc Positive Adjustable 1.25 V ~ 26 V Up to 26V 0.35V @ 1.5A
http://search.digikey.com/us/en/prod...1111-ND/771580 3.15$

LM2941T/NOPB IC REGULATOR LDO 1A TO-220-5 - National Semiconductor Positive Adjustable 5 V ~ 20 V Up to 26V 0.5V @ 1A
http://search.digikey.com/us/en/prod...NOPB-ND/148144 1.72$

So you can see the one I recommended in the posts above is the cheapest at 1.72$ but it can only do a minimum of 5v - for just a bit more you can get the MIC2941AWT that goes as low as 1.24v at 1.25A for 2.54$.

With this one, the voltage drop at about 0.2A will be as low as 0.2v and at 1A, it will be only 0.4v:

So with an ATX psu, you can go as high as 11.6-11.8v and the circuit is also almost identical to the one I drew:

Now going back to heat output - you don't want to generate less than about 7 volts from 12v from such regulator, because of the heat generated at high loads. You can buy two of these regulators and use one to output voltages between 6.5v and 11.5v.
With the second, you can use a trick to power it with about 7v - use the 5v line from the psu as ground and the 12v line as input, this way the regulator will receive (12v-5v) = 7 volts.
So you'll have the first regulator doing 6.5v-11.5v and the second regulator will do 1.25v-6.5v at any loads.

(Or, you could simply leave the first one to do 4.5v-11.5v and connect the second regulator to normal ground and 5v and have the second regulator do 1.25v-4.5v)

You can simply use a switch to change between the regulators without moving the leads - connect the ground of the first regulator to the ground on the case banana plug (or whatever you use) and flip between the output of the first and the second regulator using a switch like this one, sending whatever you select to the output banana plug:

http://search.digikey.com/us/en/prod...381-ND/2747185

(if you power the second regulator using the 5v as ground trick, don't connect the 5v you use as ground to the ground plug on the case.. it should be obvious you don't want that)

PS. This regulator I chose MIC2941 has also a nice feature.. the on-off switch pin ... if you leave that pin unconnected or tied to ground (real ground, not the 5v i used as "fake ground" for the second regulator in the example above) it's running normally. If you feed it some voltage higher than about 2v, it turns itself off.
So you can easily turn off these adjustable regulators by feeding the 3.3v from the power supply on that pin, for example.

3.3v psu --- flip switch ---- 100-150k ohm resistor ---- on/off pin of regulators.

You can use some simple flip switches like this one on the case to turn on or off the regulators from outside, so that they won't generate heat for nothing inside:

http://search.digikey.com/us/en/prod...SW315-ND/81517

Attached Files

**mariushm** · 04-01-2012, 03:50 AM

Re: Variable Power Supply from ATX

On second thought, for a beginner it might not be such a good idea to use the 5v as ground trick... it gets messy to switch between the regulators later on.

As you don't have 7v or so output from the power supply, so for now you should split the voltage between 1.25v-4.5v (a regulator powered from the 5v line) and 4.5v-11.5v (a regulator powered from 12v)

If you really want to get it right, you can complicate the design by using a switching regulator to convert the 12v to 7-8v and then send the output of the switching regulator to the second linear voltage regulator, and you'd get the whole 1.25-6.5v and have the second regulator do 6.5-11.5v or so, and this way you'd have both regulators doing the same 5v range.

A good switching regulator would be one of these:
http://search.digikey.com/us/en/prod...UDI-ND/1301321
http://search.digikey.com/us/en/prod...3PBF-ND/888671
http://search.digikey.com/us/en/prod...3PBF-ND/807745

You'd need a 47-100uH inductor and a couple of diodes and capacitors I think... you'll have to look up in the datasheet.
The ones that are through hole (easy to solder and heatsink etc) are a bit expensive.

**tom66** · 04-01-2012, 07:46 AM

Re: Variable Power Supply from ATX

I modified the feedback loop of a PSU:

http://www.youtube.com/watch?v=mK_rIs7uFlo

Back when I was dumb and stupid. Now I've modified a 13.8V plug pack instead.

**999999999** · 04-01-2012, 04:18 PM

Re: Variable Power Supply from ATX

I'm not clear on exactly what you want to do. Do you want to merely reproduce the instructable, or build something to meet a specific goal?

If the instructable, just do that. If a specific goal, define the parameters such as what the min and max voltage output is you need, min and max current (many linear regulators need a load drawing at least a few mA and on the other hand, max current determines methods you'll need to get there as well as heatsinking/cooling), and whether you need a gradual (analog) fine increment in voltage adjustment, or rather you want stepped settings like 1V increments (or whatever other...).

An ATX12V PSU will produce enough current on the 12V rail to do rudimentary testing of a car stereo, even if not full load testing on a high power aftermarket type. You can get a fair amount of sound out of one with an amp or two @ 12V, enough to know if the core functionality is working at least.

Even with an LDO regulator, you're still limited to a max regulated voltage (assuming you'd like at least 1A or more current capability) of 11.(n)V from the ATX PSU. Considering the time you'll spend, I suggest building something that will be more versatile for longer. I'd think about abandoning the ATX PSU, and using a dual secondary winding transformer. With the two windings you could have 3 voltage input ranges, high range with the two windings in series, medium with only one winding, and low with the lower of the two windings.

With these three ranges you can input to a linear regulator circuit. If you need more than 1.5A current, throw a transistor into the mix as shown on some of the National Semicon LM317 datasheet examples. By having the three ranges you keep heat down.

Which transformer to choose depends on budget, max current and voltage you might need. There may be a few options at electronics surplus sites (or eBay) that cost quite a bit less than something ordered to spec from an electronics supply house like Digikey or Mouser.

**Peter9DO** · 04-01-2012, 08:40 PM

Re: Variable Power Supply from ATX

mariushm, thank you for taking the time to give such a well rounded and informative comment. I like the idea of just sticking with the ATX power supply for now and doing it with a switching regulator. Now that sounds like some fun!!!! I'm going to do a little reading to put together a parts list and get this thing started hopefully by next weekend. Again, thank you for all the help! I'm sure I will be posting my progress as I go along!

999999999, Thank you for the reply, I'm simply just trying to pull out the soldering iron and put together some kind of project. I was going with an ATX PSU, just because I have some many of them available and figured I could make something useful. I brought up the car radio just because it's something here that I could use a power supply for, so to help, make it feel like I was doing this for a good cause. lol! Honestly, business has just been a little slow and I would like to learn a little more about circuits and how they work and you can't beat doing it hands on from crap laying around your house!

**vinceroger69** · 08-12-2012, 06:42 AM

Re: Variable Power Supply from ATX

http://www.instructables.com/id/Conv...Bench-Top-Lab/
hi i am just about to do this project as i am board would this variable circuit work?
http://www.eleccircuit.com/wp-conten...y-by-lm338.jpg
could i use one of my +12 volts to power it?
i am new to building circuits so bear with me the only part i dont have is a lm107 is there a equivilant thats cheaper?

**noppa** · 08-14-2012, 08:01 PM

Re: Variable Power Supply from ATX

Take a look at this

http://www.chirio.com/switching_power_supply_atx.htm

**LDSisHere** · 08-25-2012, 09:47 PM

Re: Variable Power Supply from ATX

I saw this thread and could not resist posting. I built myself one of these a few months ago for my solderless breadboard projects. I also made a variable voltage regulator, using a LM350T, that plugs directly into the breadboard. I think it was well worth the effort and suits my needs as well or better than anything I could have bought. The most expensive part for me was for the BNC connectors, most everything else I had on hand.

Attached Files

**Peter9DO** · 09-25-2012, 06:59 AM

Re: Variable Power Supply from ATX

Originally posted by noppa View Post

Take a look at this

http://www.chirio.com/switching_power_supply_atx.htm

I wish this was in English, would be a good read....

Originally posted by LDSisHere View Post

I saw this thread and could not resist posting. I built myself one of these a few months ago for my solderless breadboard projects. I also made a variable voltage regulator, using a LM350T, that plugs directly into the breadboard. I think it was well worth the effort and suits my needs as well or better than anything I could have bought. The most expensive part for me was for the BNC connectors, most everything else I had on hand.

Nice looking set up you have going on there! Good job!

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Variable Power Supply from ATX

Variable Power Supply from ATX

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