ATX power supply (noob question)

[edgari]

The problem is that my lab power supply died on me last night while working on some circuits. I found an ATX power supply from one of my old computers and decided to use it as a power supply, i got the pin outs from the internet.

The question is: If I want to use 12V for my circuit, can I use one of the 12V molex connectors to my breadboard?? (is it that simple?) What kind of amperage am i going to get??

I found a way to convert the ATX power supply to a lab power supply by hooking up all the yellow wires (12V), red wires (5V), etc, etc, but instead of doing that why isn't is possible to just run a wire from one of the molex connectors to the circuit? does it have to do with amperage??

I'm a complete noob with basic knowledge of electronics so please be nice

 

[vne147]

The power supply provides a voltage. The current rating is there so you know how much current can be supplied before the voltage output drops. For instance, a 12V 5A supply will provde 12V to a circuit and depending on the resistance of the circuit attached to it will supply anywhere between 0 to 5A before the output voltage drops. Here's an example. If you have a 12V supply and you place a 100Ω ohm resistor as the load,

V = IR -> I = V/R -> I = 12/100 = .12 A

So .12 amps will be drawn from the power supply. If you change the resistor to 200Ω then:

V = IR -> I = V/R -> I = 12/200 = .06 A

So, .06 amps will be drawn from the power supply.

Voltage only creates a potential for current to flow. The amount of current that actually flows depends on the resistance of the load. It doesn't matter if the power supply is 12V at 5A or 12V at 5000A, it's only going to draw as much as the circuit attached to it allows.

 

[edgari]

ok

The power supply provides a voltage. The current rating is there so you know how much current can be supplied before the voltage output drops. For instance, a 12V 5A supply will provde 12V to a circuit and depending on the resistance of the circuit attached to it will supply anywhere between 0 to 5A before the output voltage drops. Here's an example. If you have a 12V supply and you place a 100Ω ohm resistor as the load,

Ok so then why do so many people go into a big trouble of modding their ATX power supply when they can use the molex connectors as their power supply?? for example: YouTube - Converted Computer ATX Power Supply to Lab Power Supply Unit
That's assuming that all you have to do is short out the one of the pins so that the ATX thinks it is still connected to a computer...

I really don't want to fry my electronic components so i'm just asking if it's safe to hook up a 12V line from one of the molex connectors to my breadboard and use it as a power supply.

 

[vne147]

Ok so then why do so many people go into a big trouble of modding their ATX power supply when they can use the molex connectors as their power supply?? for example: YouTube - Converted Computer ATX Power Supply to Lab Power Supply Unit

I don't really understand your question. The volatge will always come from the ATX power supply. The connectors do not become the power supply if you hook up circuits through them as opposed to clipping them off an just using the loose wires. I don't know why that particular person in the Youtube video decided to modify his power supply that way. He didn't change or "convert" it from how it would normally operate by simply connecting all the like outputs. Maybe he just thought it would be easier to hook things up to it if the outputs were tied together as opposed to having to pin out from the connectors. Whether or not you clip off the connectors as shown in the video or keep them on and stick pins into the appropriate sockets has no effect on the voltage that is being supplied.

That's assuming that all you have to do is short out the one of the pins so that the ATX thinks it is still connected to a computer...

I'm not sure if it's OK to just short out one of the pins to place a load on the power supply and keep it from turning off. Maybe someone else can chime in about that. I modded an ATX power supply a few years ago that I used to power a pump and I placed the load on it by soldering a 10Ω power resistor between one of the 5V outputs and ground.

I really don't want to fry my electronic components so i'm just asking if it's safe to hook up a 12V line from one of the molex connectors to my breadboard and use it as a power supply.

I thought I answered this in my last post but I'll try again.

The power supply provides a voltage. The current that is allowed to flow at any given voltage depends on the resistance of the load attached to it. If you are having trouble understanding that concept go read this.

I cannot tell you if it is safe to hook up a circuit or a component to a 12V supply without knowing what that circuit or component is. No one here can. Some circuits will do fine if you apply 12V to them. Some will fry. Some will not have enough voltage and won't work. Your question is not specific enough.

Imagine if you went to the store to buy a tire for your car and you told the guy there "I want a tire". He then asks, "what kind of car do you have", to which you reply "I don't know". The guy will not be able to tell you if a particular tire will work or be safe with your car because he has no idea what kind of car you will be attaching it to.

What your asking is kind of like the tire analogy. Whether or not it's safe to hook up 12V to your breadborad or not totally depends on what circuit is in the breadboard.

If you know that it is OK to apply 12V to a given circuit, it doesn't matter if that voltage comes out of the connector or, if you remove the connector and hook up the loose wires. The electrons don't care.

 

[smanches]

Edgari, yes it is because of the amperage. Not sure what max current on the 12V line is, but the 5V line can easily be 20+ amps available. This is way too much current to pull through a single connector, which is the reason for the mods. If you look, you'll see there are multiple 12V and 5V (even GND) wires into the ATX board itself to handle this.

And you are correct, to turn on the power supply you need to short the PS_ON pin to ground.

 

[edgari]

thanks

If you know that it is OK to apply 12V to a given circuit, it doesn't matter if that voltage comes out of the connector or, if you remove the connector and hook up the loose wires. The electrons don't care.

Yes the circuit I have runs on 12V. Thanks for your patience and taking time to explain it to me.

 

[edgari]

...the 5V line can easily be 20+ amps available. This is way too much current to pull through a single connector, which is the reason for the mods. If you look, you'll see there are multiple 12V and 5V (even GND) wires into the ATX board itself to handle this...

From the specs on the ATX power supply the DC output is +12V/25A, +5V/36A, +3.3V/30A, -12V/0.8A, +5Vsb/2A and a total of 580W.

So what you're saying is that people only mod their atx supply into one line in case one wants to use the 20+ amps, right? If i'm not using that much current then I should be ok just using one of the molex connectors for my circuit, correct?

Sounds like the only thing i should be concerned in this case is the wire gauge...

(sorry for the noobish questions! it's been a while since i took electronics and forgot how much fun this is...)

 

[superfrog]

Many atx power supply won't work with load on only one of their output rails.

Do not trust the rating of the power supply unless it is an expensive server/high quality brand one.
Anyway, the regulation is usually fairly poor at low load.

Many power supply have multiple 12V rails, I would not connect those together if I could avoid it.

Hope this helps

 

[MrAl]

Hi,


You can use an ATX as power supply by simply grounding the green wire on most supplies. That means connecting the green wire to one of the black wires.
You can do this on the molex connectors or you can cut some wires off and connect them together, or even wire in a switch to turn it on with.

You wont get super perfect regulation, but still most ATX supplies work this way. It's also good to connect a small load to the 5v line and maybe the 12v line too to get the voltage down in the right range. Even without that though you should get something near 12v. You can take the output off of any of the molex connectors and get the ground there too.

It wont be as good as a lab supply but it will be good for many general purpose testing.
Some of the better supplies regulate a little better too.

You still have to observe the max current rating for each line.