Using an ATX power supply

[tk26]

Any downsides to this? Seems perfect. Cheap, easy to find, has 12v/5v/3.3v outputs out of the box.

Using it for a project in which i need to power an arduino, a set of speakers and a few motors.

[RuggedCircuits]

It works pretty well. Watch out for power supplies that have minimum load (current) requirements. If you don't draw at least this amount of current from the various supplies, it will not regulate properly and you could end up toasting something with improper voltage (e.g., the "5v" output ends up at 6.5V or something).

--
The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

[tk26]

It works pretty well. Watch out for power supplies that have minimum load (current) requirements. If you don't draw at least this amount of current from the various supplies, it will not regulate properly and you could end up toasting something with improper voltage (e.g., the "5v" output ends up at 6.5V or something).

Thanks for the warning. I had heard about minimum load requirements but I wasn't aware I could toast anything if I didn't meet them.

[retrolefty]

It works pretty well. Watch out for power supplies that have minimum load (current) requirements. If you don't draw at least this amount of current from the various supplies, it will not regulate properly and you could end up toasting something with improper voltage (e.g., the "5v" output ends up at 6.5V or something).

Thanks for the warning. I had heard about minimum load requirements but I wasn't aware I could toast anything if I didn't meet them.

Well it usually is solved simply by wiring a lamp mounted inside or onto the PS, or a power resistor of the correct wattage. There is ton of info on the web about utilizing PC supplies in a standalone mode. This is about the cheapest DC power supply per watt available to a hobbyist, often gotten free.

[tk26]

Well it usually is solved simply by wiring a lamp mounted inside or onto the PS, or a power resistor of the correct wattage. There is ton of info on the web about utilizing PC supplies in a standalone mode. This is about the cheapest DC power supply per watt available to a hobbyist, often gotten free.

Yeah, I was going to hook up a couple of old hard drives just to toy with it.

[cr0sh]

The biggest problem with such a supply is the fact that without modification (not an easy task unless you have a schematic for the supply and are an "expert" in power supply design) you can't set a current limit for a given voltage.

Let's say you have a 5V circuit you are testing out, that you believe (based on your calculations) will only consume 100 mA of current when running, and you don't know there's a short in it. With a good quality bench PSU, you can set a current limit (and a voltage limit, if desired) to say "regulate the output at 5 volts, 100 mA".

If you hooked up your circuit to such a PSU - and turned it on, the over-current indicator would flash (or there would be a tone, or something else), and the current flow would be interrupted; this would happen so quick that there would likely be no damage to your circuit.

Now let's say you hooked the same circuit up to you modified PC power supply: The 5 volt rail of such supplies usually can pump at several 10s of amps. If you are lucky, your circuit fries and perhaps one of the wires goes up in a puff of smoke, a bright flash, and maybe a loud "bang" as it vaporizes (been there, done that - fortunately, it was with a homemade motor, and not anything I really cared about). If you are unlucky - well, you have a small fire on your hands, and you better hope you have a way at hand to put it out quickly...

Now, you could possibly homebrew a small PCB with a few linear regulators and such on it, that you could plug into the output of the power supply that could do current regulation (and be adjustable) - for about an amp or so of output. You could even add voltage regulation to the mix. At that point, though, you are well on your way to building a simple bench power supply...

[ADLC]

I have been working during several weeks to convert an old good PSU I have into something decent.
I don't know how the high-end PSUs are, but, at least for the low/medium-end units, the drawbacks are not only the minimum load requirement and the lack of maximum current setting.

Usually, the switched power units like that only have a main transformers that generates all the voltages, therefore, they only have a feedback circuit to control the commutation on this coil. What that means, basically, is that for instance if you start loading the 5V line, its voltage will decrease meanwhile the voltage of the 12V will start rising. In addition, in my PSU, the 3,3V line is not used in the main feedback circuit, therefore, its voltage decrease quite quickly with the load if I have not something connected to the 5v or 12V lines (which are the ones that control the duty cycle of the PSU). Summarizing, it is cheap, it is good enough for a lot of uses, mainly if you need a lot of ampers, but it is hard to obtain a good regulated voltage from it if you do not modify it deeply. Therefore, maybe it is the best option for driving heavy loads, but if you only need some hundreds of milliampers, maybe you could build something even more cheap using something like:

http://www.ebay.es/itm/DC-Converter-Constant-Current-Voltage-1-25-30V-LED-Driver-Battery-Charger-/251123350374?pt=LH_DefaultDomain_0&hash=item3a781e3b66#ht_2848wt_1271

[jens38]

I was looking for a bench PSU, but was surprised by the prices of these boxes, and also because I'm a Arduino freak (even if beginner) I decided to build my own.

In order to overcome the missing current limitation, I've build a current limiter for the three voltages (3.3V, 5V and 12V) with MAX890L (for 3.3V and 5V) and UCC2915DP (for the 12V) Current Limited High Side Switches. These are Switches which are switching off, if a programmable current is reached or exceeded. All this driven by a ATmega328 programmed with Arduino, what else ?

All this integrated in the PSU, with LCD screen showing the limitation for the 3 voltages, and the actual current.

It's working correctly, only problem is that I have probably un unstable tension in my circuit, so the current measurement is not really acurate, but for what I need (eliminating the risk of smoke) it's fine. One day, when I do not have other projects running, I will refine it, but for the time being it's fine for me.

If somebody is interessted, I can post the circuit, code of the PSU.

I attached a photo of it (the led's are "dimmed" with tape for the photo only)

Cheers

Jens

[Far-seeker]

If somebody is interessted, I can post the circuit, code of the PSU.

I attached a photo of it (the led's are "dimmed" with tape for the photo only)

Cheers

Jens

Very cool Jens, I would be very interestedin the circuit and code.

[jens38]

Very cool Jens, I would be very interestedin the circuit and code.

Thanks.

Here the circuit (hope it's readable).  The code I have to clean before sending it out. Hopefully I will find the time tomorrow, if not you have to wait till this week end.

Cheers

Jens

[Far-seeker]

Here the circuit (hope it's readable).  The code I have to clean before sending it out. Hopefully I will find the time tomorrow, if not you have to wait till this week end.

After a quick look I think it's presented understandably, and I especially like the annotations including the voltage and current ranges for specific areas of the circuit.

[jens38]

Hi,

and here now the code I've used. Nothing special, just the code to put the limits either directly (UCC2915DP) or via digital pot (MAX890L). On top the sensing of the current via the LT6106 on Analog 0-2.

For sure no great art, but as I learnt by doing I have sometimes strange ways to reach the target .

Please free to comment.

Cheers

Jens

[tk26]

That's really cool jens.

One of the main attractions for me was the fact that it'd have 12/5v out of the box, but if that's not reliable then I guess it's not much of a benefit. I was trying to avoid doing as much as I could myself, I know power can get pretty difficult and I'm somewhat of a beginner here.

Also a bit worried about the fact that I can't limit amperage, most of my devices are very low amperage, just a couple are a bit higher.

I'll run some tests on mine when it comes in, it's not a dirt cheap one but not the most expensive either.

Luckily a small ITX motherboard has become part of my project, so even if I can't use the PSU for everything else I need it anyway, so nothing wasted.

Thanks for the feedback!

[jdmac]

Try this.
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
jdmac
Mims, Fl

[jens38]

Try this.
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
jdmac
Mims, Fl

Yeah, but this is giving exactly what you should avoid: no current limitation, and with this you will produce (for sure) a lot of magic blue smoke 

Problem is that the PSU is able to deliver huge amount of current (> 30A on 5V). Imagine you have a short on your test circuit....... Better is to limit the current, or at least integrate fuses (but this was too expensive for me, considering the number of shortages I produce per day in average ] ).