Need help with help homemade bench power supply problem

[James k]

My psu will power thing from the 3v port but when I try to power stuff in the 5v and 12v it shuts down? I'm running a 10w 10ohm resistor on the 5v rail and a power indicator light and a cooling fan for the resistor on the 12v rail everything runs and volts check good in all the ports but when I try to power anything in the 5v and the 12v port it turns off? What is the problem? Can anybody help please?

[Nusa]

Did it ever work?

What "anything" are you trying to power?

[tpowell1830]

My psu will power thing from the 3v port but when I try to power stuff in the 5v and 12v it shuts down? I'm running a 10w 10ohm resistor on the 5v rail and a power indicator light and a cooling fan for the resistor on the 12v rail everything runs and volts check good in all the ports but when I try to power anything in the 5v and the 12v port it turns off? What is the problem? Can anybody help please?

Hi James k, welcome to the forum.

Since this supply is homemade, do you have a schematic? A picture of the board may help as well.

[Kleinstein]

This sound like a more or less modified PC (ATX?) power supply. This is usually not a good bench power supply.

The usual ATX supplies have limited capability on how the load is shared on the different outputs.

[James k]

Yes this is a modified ATX power supply. And the things I would like to run with it are battery powered drills electric motors lights things of that nature. And it has never been working I had just got it put together in the last few days. Do you think I need to put more load on the 12v rail ? More load on the 5v? It is made by bestec power electronics corp. model :atx100-5 input rating 100-120v-3a 200v-240v-1.5a  output rating : 100w(max)  wire color and volt goes. Red +5 10A yellow +12v 2A blue -12v 0.3A  orange +3.3v 6A purple +5vsb 0.8A black GND  gray power good I have a brown wire tied in with the orang. I am not using the purple blue and gray. Have the green wire tied to black for start up ? Any thoughts ? And thank you all so much !!

[TheUnnamedNewbie]

I don't have any references on this, but I seem to remember many battery-powerd drilles drawing a *lot* of current, a lot more than the 2A your 12V rail can provide.

[James k]

Would it be safe to tie into the 5v that has 10amp? Add a red to the yellow wires?and use the amps from the red mixed with the yellow 12v

[TheUnnamedNewbie]

Would it be safe to tie into the 5v that has 10amp? Add a red to the yellow wires?and use the amps from the red mixed with the yellow 12v

No, that's not the way powersupply ratinga work.
The fact that you ask this question leads me to believe you should probably pick up a basic introductory book on electronics before you deal with things that can supply 10A.

[James k]

And what would cause it to shut down like it does when I try to use the 5v and 12v ports? Tried to do some looking around determined I needed to load the red 5v wires that's when I added the 10watt 10ohm resistor. But still the same problem? Oddly enough the 3.3 volt port will power the drill without shutting down just at a slow speed .

[janoc]

And what would cause it to shut down like it does when I try to use the 5v and 12v ports? Tried to do some looking around determined I needed to load the red 5v wires that's when I added the 10watt 10ohm resistor. But still the same problem? Oddly enough the 3.3 volt port will power the drill without shutting down just at a slow speed .

Likely overload/short circuit and the protection in the ATX supply kicks in. I would strongly recommend not tinkering with these until you know a bit more about electronics. An average ATX supply is capable of supplying 20A of current or more, happily melting whatever you connect to it into a pile of slag. Or setting your house on fire.

If you really do need the high power, ask someone more experienced around you to help you and check your work.

[Nusa]

And what would cause it to shut down like it does when I try to use the 5v and 12v ports? Tried to do some looking around determined I needed to load the red 5v wires that's when I added the 10watt 10ohm resistor. But still the same problem? Oddly enough the 3.3 volt port will power the drill without shutting down just at a slow speed .

I doubt that load resistor you put on 5V is doing anything useful for you. That advice was for people using only the 12V on supplies with large 12V capability. Your supply does not have large 12V capability.

Look up Ohm's Law. One version of it is: Current in Amps = Voltage in Volts / Resistance in Ohms.
Let's suppose your simple motor has a resistance of 0.5 Ohms.
At 3.3 Volts, the motor wants to draw 3.3 Volts / 0.5 Ohms = 6.6 Amps. Slightly more than your supply is rated for. If it works at all, you're pushing the limits.
At 5 Volts, the motor wants to draw 5 Volts / 0.5 Ohms =  10 Amps. Which is exactly what your supply is rated for, so a borderline situation.
At 12 Volts, the motor wants to draw 12 Volts / 0.5 Ohms =  24 Amps. Clearly 24 Amps is more than the 2 Amps your supply is rated for. Not a chance!

I don't know the actual resistance of your drill motor, but I do know THIS supply will never run that motor in a useful way. As ATX supplies go, it may be the wimpiest one I've ever seen.

[James k]

Thank you NUSA ! You have been very helpful! Although dream crushing lol I will take your advice and be on the lookout for a better supply. Not so wimpy!

[janoc]

100W ATX supply? That's pretty weird - I think the smallest is 250W. The CPU alone can take 100W of power.

[alm]

Probably for one of those old small form factor computers like the Via Eden (EPIA) stuff.

[Brumby]

The other thing to be aware of is that the current required to get a motor going will be quite significantly higher than to keep it running.

It may only take a fraction of a second, but these supplies are not designed to provide this and the protection will kick in far too quickly.  You'd be lucky to see the shaft twitch.

[bktemp]

100W ATX supply? That's pretty weird - I think the smallest is 250W. The CPU alone can take 100W of power.

ATX has been used for about 20 years. The first generation Pentium CPUs hat a TDP of around 10W and the CPU was powered from the 5V or 3.3V rail. The 12V rail was only needed for HDDs and CDROM drives.
That's why old ATX power supplies can supply large currents on 5V and 3.3V rails (often >2/3 of the total output power) but only rather small currents on +12V rail.

But almost no power supply monitors the individual output rails. So you can typically draw much more current from the +12V rail if you do not have much load on the +5V rail (but enough to keep it in regulation to prevent the power supply from shutting down). If you draw too much current you will eventually break the rectifier diodes.

[janoc]

100W ATX supply? That's pretty weird - I think the smallest is 250W. The CPU alone can take 100W of power.

ATX has been used for about 20 years. The first generation Pentium CPUs hat a TDP of around 10W and the CPU was powered from the 5V or 3.3V rail. The 12V rail was only needed for HDDs and CDROM drives.
That's why old ATX power supplies can supply large currents on 5V and 3.3V rails (often >2/3 of the total output power) but only rather small currents on +12V rail.

Yes, I am aware of that. However, even at those times the 12V rails were used for things like motors in disk drives. 2A sounds rather woefully underpowered for that. An old harddrive spinning up can draw an amp or more easily. Even the old AT (not ATX) supplies could usually deliver more current from the 12V rail than this. Seems like a really oddball supply, probably intended for some special application.

[James k]

So no ATX supply could power a drill? I found one that said it is 600w puts out 20A on almost all rails would this one do the job? And would it need a load put on the 5v or 12v to keep that one from shutting down

[James k]

Would like to make one with selectable voltage and a display with only two ports +&- and maybe a USB port as well

[Nusa]

So no ATX supply could power a drill? I found one that said it is 600w puts out 20A on almost all rails would this one do the job? And would it need a load put on the 5v or 12v to keep that one from shutting down

Not enough information about the drill in question to answer that. Drills are potentially very high current devices, especially when you stall the motor on a difficult task.

If the real goal here is to power a DC drill, I'd suggest just putting battery clips on it and attaching it to a car battery (still in the car, if you want), emergency start pack, or something beefy like that. Those batteries are designed for high surge currents. You can charge the battery when you aren't using the drill.

If you want a drill that you don't have to recharge, I'd suggest buying a corded drill. They still make them.

[James k]

Was wanting to make a sort of all purpose supply cheep for use in the shop. Small motors lights gadgets. Battery powered drills are almost discarded by people because when the battery packs that are more $ than the drill no longer take a charge they are practically given away. I have about 4 that I have found for less than 5$. And have gotten one to run great off of a laptop power supply. But have it permanently wired into the gutted battery pack. And it's output is to high for smaller jobs. So the ATX with switchable volts is kinda ideal for the task I do. If I could just get them to work on all the different volt settings. The "wimpy one" I built would be ok for other things if the 5v and 12v wouldn't shut the device off? Even a small 12v light will kill it? A 12v RC car motor shuts it off. So would a 600w ATX perform those task ?

[James k]

Brumby mentioned the start up taking more than the actual running of the drill. So would a capacitor mounted in the empty battery pack act as the start up ? Have seen large power tools motors have a capacitor mounted on them is that what I need to do?

[James k]

I have even gotten a wall wort to power the drill. Mind you all I am no engineer and do not have a great understanding of the tricks of the trade. Much respect! How is it that the wall wort could do it and the laptop power supply ?

[James k]

How about this. ? Could I build a box with a bunch of wall warts wired in a parallel on the input side and run all the outputs to a 4 to 1 pin rotary switch thus just selecting a different wall wort? Is it ok to hook them in parallel on the input side  ?

[Brumby]

Brumby mentioned the start up taking more than the actual running of the drill. So would a capacitor mounted in the empty battery pack act as the start up ? Have seen large power tools motors have a capacitor mounted on them is that what I need to do?

First, let's get this sorted out: "Have seen large power tools motors have a capacitor mounted on them".  Those capacitors are doing a different job in a very different way to what you are trying to do.  They are often seen on single phase AC motors and provide a phase-shifted current for the purpose of starting such a motor and/or running it.

As for using a capacitor as a "start up reservoir", you might have success - but you are also likely to find that the capacitance required to provide anything useful might be just a bit too bulky.  Even if you did find a setup that got the motor going, the odds are that as soon as you put a decent load on it, the "capacitor bank" would run out of puff so quickly that the drill would stop in a very short time.  The peak current for a motor starting and one under high load might be similar - but at start-up this peak will only last for a fraction of a second whereas under load it will be required continuously.


The other thing you need to understand is that it is very difficult to give accurate suggestions or comment on your exact circumstances, since we don't have enough information to do that.  The gaps in our knowledge have been filled in by our typical experience with the sorts of gear you are wrangling.