PC Power Supply - Testing LED and DC Motors Question

[riposte_18]

Hey all,

Like so many others, I'm using a PC to emulate all the various games I want to set up. As I've progressed through the design of my machine, I've got other items that require power, that I was hoping to supply through my PC power supply. In specific, I'm looking at using 3 x 12v LEDs, and two 12 volt DC motors

My question is this - I ripped out the entire power supply from my box, so I could try connecting up these devices in question to the power supply in isolation (I have a probably overly cautious view of electricity after jamming a knife into a power socket as a kid). As per various directions, I used a single yellow (+12V) wire and a black wire from the power cable that usually goes to the fan, and wired it up to an LED.

Switched on the power supply, and got nothing. Not even the smallest flicker that I could see. Tried a motor too, and got nada.

My question is ... well, I guess ... how do the fans in the PC know to turn on? ie, most fans will remain off until the power signal is supplied to the motherboard, and only when the other devices come on will the fans come on? I'm suspecting that's my problem with testing the LED and motor in isolation, and ... that other power load is required for the Fan cable to be supplied power? Its the only thing I can really think of, cos generally PC Fans are only directly connected to the power supply, so the only way they could know to turn on is when there's another load on the power supply?

Can anybody shed any light on this? I'm a bit wary of playing around with wiring up power and stuff to my LEDs while its connected to the rest of my PC, since I don't want to fry anything ... I'd much rather be able to test them in isolation, and then wire it up to the PC at completion.

As a side question - I picked up my LEDs as part of illuminated Push buttons. Any way to tell if I need to put a resistor in series, in order to prevent it from burning out, or if it is one with an inbuilt resistor? Its one where its a bit difficult to tell, since the LED is encased in a plastic housing.

[Donkbaca]

If you got them with the buttons, you probably don't need a resistor.  Do you have a multimeter?  You can use that to test if power is going to the wire. Assuming the power supply is on   one thing:  Are you sure you are connecting the LED right?  THe positive to the anode and ground to the cathode?  If you connect the wrong one to the wrong end, it won't light up.  Where is your power coming from?  I suggest you just cut off a molex connector from the power supply, or if you don't want to do that get a molex extension cable and cut one of the ends off of that.

[riposte_18]

Hey Donkbaca,

Yup, did pretty much what you said, which was cut off a molex connector, and stripped down the wires. Probably should have used an extension connector though, so not to ruin it, but too late now!

No multimeter, I'm afraid, might pick one up if I have to. But yeah, switched terminals on the LED, and go response, and tried a DC motor, which should be able to accept either polarity, but got nothing there as well. And very sure I switched the power on!

But again, it comes back to the same issue as PC fans, which is do I need some sort of load across the power supply for it to provide power to the Molex connector? How do the fans know when to turn on? I've seen some people say you need a load across the 5v wire in order for a stable 12v to be supplied, but I'd expect to see at least some minor activity? maybe even a twinkle?

[Donkbaca]

if you hook the LED's to the 5v wire, they should still glow, but they would be dim.  I think the power supply provides power all the time, the fans that turn on and off are the ones that plug into the motherboard, I think...

Can you post a picture?  I am by no means an expert on any of this stuff, just trying to help you through because I use 12v LEDs and I get my power from my power supply.

[DaOld Man]

If it is an ATX power supply (which it sounds like it is), you will need to short two of the wires in the bundle to turn on the power supply. Fan will start and you should have power.
Cant recall right off which two wires it is.
Search this forum for switching on cab or similar.
About the LED, make sure it has a resistor, if not it will burn very brightly for a few milliseconds.

[Donkbaca]

Oh yeah, the power supply is off!

It doesn't provide power until the motherboard turns it on!

[riposte_18]

Hey Donkbaca,

Cheers for your help, currently I'm at work, so can't provide pictures, but I'm sure if you imagine the simplest setup to link a molex connector to an LED or a motor, that's pretty much what I've got

The reason I ask about the fans is all the big square black fans I've used to assist in exhaust have only ever plugged into the molex connectors of the power supply. I know the CPU fan is powered directly from the motherboard, so that makes sense its power is dependant on the how machine booting up, but the additional exhaust fans only have connectors to the power supply, hence I would expect them to run non-stop if there wasn't some sort of dependancy on another load of the power supply.

Tried the 5v wires too, but no go.

DaOldMan - Thanks for that, that might explain what I need, I'll investigate more and see what I come up with.

Cheers

[BobA]

To turn on an ATX power supply without a motherboard the power on pin in the 20 pin connector has to be connected to a ground or common.  This is typically been said as short the green wire to ground with a paper clip or wire jumper.

http://www.techpowerup.com/articles/other/22

[riposte_18]

Hey Boba,

Yeah, found just that information, and was going to post it here for future searches, but guess you beat me too it!

Thanks all for your help, its probably silly question that appears on this forum on a regular basis, so appreciate all the assistance you guys have provided.

Cheers