Need help with help homemade bench power supply problem - Page 1
Forum section: electronics / beginners

author: James k
author_rank: Contributor
timestamp: March 25, 2017, 02:47:16 am
content: 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?

author: Nusa
author_rank: Super Contributor
timestamp: March 25, 2017, 05:50:38 am
content: Did it ever work?What "anything" are you trying to power?

author: tpowell1830
author_rank: Frequent Contributor
timestamp: March 25, 2017, 05:53:57 am
content: Quote from: James k on March 25, 2017, 02:47:16 amHi 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2017, 10:11:33 am
content: 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.

author: James k
author_rank: Contributor
timestamp: March 25, 2017, 04:30:46 pm
content: 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 !!

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 25, 2017, 04:40:36 pm
content: 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.

author: James k
author_rank: Contributor
timestamp: March 25, 2017, 04:49:39 pm
content: 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

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 25, 2017, 05:11:07 pm
content: Quote from: James k on March 25, 2017, 04:49:39 pmNo, 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.

author: James k
author_rank: Contributor
timestamp: March 25, 2017, 05:20:08 pm
content: 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 .

author: janoc
author_rank: Super Contributor
timestamp: March 25, 2017, 05:56:58 pm
content: Quote from: James k on March 25, 2017, 05:20:08 pmLikely 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.

author: Nusa
author_rank: Super Contributor
timestamp: March 25, 2017, 06:25:24 pm
content: Quote from: James k on March 25, 2017, 05:20:08 pmI 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.

author: James k
author_rank: Contributor
timestamp: March 25, 2017, 07:23:15 pm
content: 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!

author: janoc
author_rank: Super Contributor
timestamp: March 25, 2017, 09:24:23 pm
content: 100W ATX supply? That's pretty weird - I think the smallest is 250W. The CPU alone can take 100W of power.

author: alm
author_rank: Super Contributor
timestamp: March 25, 2017, 10:08:11 pm
content: Probably for one of those old small form factor computers like the Via Eden (EPIA) stuff.

author: Brumby
author_rank: Unknown
timestamp: March 26, 2017, 06:07:17 am
content: 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.

author: bktemp
author_rank: Super Contributor
timestamp: March 26, 2017, 06:26:23 am
content: Quote from: janoc on March 25, 2017, 09:24:23 pmATX 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.

author: janoc
author_rank: Super Contributor
timestamp: March 26, 2017, 10:14:13 am
content: Quote from: bktemp on March 26, 2017, 06:26:23 amYes, 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.

author: James k
author_rank: Contributor
timestamp: March 27, 2017, 12:30:55 am
content: 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

author: James k
author_rank: Contributor
timestamp: March 27, 2017, 12:39:08 am
content: Would like to make one with selectable voltage and a display with only two ports +&- and maybe a USB port as well

author: Nusa
author_rank: Super Contributor
timestamp: March 27, 2017, 01:39:54 am
content: Quote from: James k on March 27, 2017, 12:30:55 amNot 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.

author: James k
author_rank: Contributor
timestamp: March 27, 2017, 02:06:00 am
content: 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 ?

author: James k
author_rank: Contributor
timestamp: March 27, 2017, 02:27:43 am
content: 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?

author: James k
author_rank: Contributor
timestamp: March 27, 2017, 03:01:26 am
content: 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 ?

author: James k
author_rank: Contributor
timestamp: March 27, 2017, 03:34:21 am
content: 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 ?

author: Brumby
author_rank: Unknown
timestamp: March 27, 2017, 04:54:44 am
content: Quote from: James k on March 27, 2017, 02:27:43 amFirst, 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 averydifferent 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 youdidfind 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 byourtypical experience with the sorts of gear you are wrangling.

author: Brumby
author_rank: Unknown
timestamp: March 27, 2017, 05:03:17 am
content: Quote from: James k on March 27, 2017, 03:34:21 amI assume you are talking about plugging 4 wall warts into a power board (or an equivalent arrangement.)Not a pretty idea ... but it should work. Just make sure you don't connect anyoutputsof the wall warts in parallel.Seriously though, please consider getting a real lab-style supply. You'll be over the moon.

author: janoc
author_rank: Super Contributor
timestamp: March 27, 2017, 08:58:59 am
content: Quote from: James k on March 27, 2017, 02:06:00 amIf this is what you want, then amodernATX supply with a sufficiently powerful 12V rail (15-20A or more rated) could do the job. However, most battery powered drills need more than 12V (14-19V being typical) and there is not really a good way to get that out of an ATX supply cheaply and safely, especially with your level of experience. The drill may run at 12V, but it will very likely lack power to the point of being unusable. Only the smallest cordless screwdrivers run at voltages lower than 10V or so.If you post the ratings of your drills, then some more specific advice could be given - e.g. an industrial power supply with a fixed voltage and sufficient power rating could be an option. Then have a second adjustable supply for your low power things.However, without knowing more about the type of drills or motors you want to power it is shooting in the dark. "Battery drill" or an "RC motor" is too vague - RC motor could be a small DC servo taking milliamps of current but also a large brushless motor drawing 100+ amperes ...QuoteA wall wart isextremely unlikelyto have sufficient power to run a drill. You will most likely fry the wall wart if you try it, especially if you have bought some cheap junk ones without much of an output protection.

author: James k
author_rank: Contributor
timestamp: March 27, 2017, 07:48:06 pm
content: Ok so for the selectable wall wort idea. Is it ok to have the negative output wires of 4to5 wall warts connected to one banana plug? And the positives connected to a 5 to 1 pole rotary switch and from that to a banana plug ? Or will there be any kinda flow back from the negative wires all being joined ?

author: Brumby
author_rank: Unknown
timestamp: March 28, 2017, 01:32:34 pm
content: There shouldn't be any problems doing that.

author: James k
author_rank: Contributor
timestamp: March 28, 2017, 02:04:36 pm
content: So in the selectable wall wort box idea, could all the black - output wires of 4-5 be joined together at one banana plug and all the red + wires go to a 5 to one pole rotary switch only powering one at a time to get different volts. Or would it create flow back into other wall worts ?

author: James k
author_rank: Contributor
timestamp: March 28, 2017, 04:44:30 pm
content: Brumby that goes for laptop supply's as well? Because I do have one that powers a drill very well even under load like drilling thick wood and metal. It would be ok to combine that one into the box with the other wall worts?