Questions around using a computer power supply

30 amps at 5 volts will be enough to hurt you really badly. My computer can develop 1300 watts at 12 volts and god knows how many dozens of amps...

I am assuming you are working with a non modular power supply.

It would be much safer to get a Power brick and wiring your buses to that instead.

Some computer power supplies are so sensitive that a spark or momentary short will simply kill it. They have to be in the computer because everything is so finely balanced between brute power and .. finesse.

yes, you can make a general purpose power supply from an AT, ATX, etc computer power supply.  just Google something to that effect and you will find a few how-to's.  you'll have to make sure a few of the control inputs are terminated properly.

5vsb is a 5volt standby voltage that stays on during sleep and standby modes of operation.

what use is -12V?  run a load between +5 and -12 and you'll get 17volts.

when it comes time to use the supply, realize that this is a switching supply and to get anything out of it, you must have a load attached and draw a minimal amount of current.

I've used a number of PC power supplies for various projects, they're very cheap power as far as dollar per watt.

Basically, you have to have a minimal load on the 12V rails to insure proper regulation of the outputs, normally an amp is sufficient. 

Note that 5VSB is a very low power output, so that's not an output that you will be using for any kind of load.

Make sure you provide proper ventilation for the supply, if the fan is blocked or stopped, the supply can cook in short order!

Lee, my original plan was to use a K-Line PowerChief 120F.  I bought two and they are already allocated.  One runs my switch machines.  The other is running AC accessories and some lights.  I may look for one again.

I will check out Google.  I couldn't remember where I saw that article.  Thanks!

So would recommend a 1 amp load, which equals 1 ohm resistor permanently attached between +12VDC and Ground?

My power supplies will be sitting on the transformer cart out in the open - lots of air.

Thanks,

George

Actually, a 12 ohm resistor would be a 1A load, unless Ohm's Law has been repealed.

You can do it with a resistor or just a load that's always on the supply when you use it.

John,

I get it.  I tried to do the calculation, but the calculator kept returning a 1.  I guess that's what I get for trusting a web-app.  It worked converting in one direction but not the other!

Thanks!

George

Joe,

Thanks very much!  I will use the 12VDC tap mostly as well, although the 3V might be useful for a Evans Design welding light.

George

That's a good project to re-use an old computer power supply, as long as you do your homework to figure them out.

I've salvaged lots of good useful parts out of various old power supplies including transformers, bridge rectifiers, power resistors, fuse holders, misc IC's and hardware and etc. I got some heavy-duty multi-tap transformers out of old C-band satellite receivers which I've used for O-gauge power on some of my small layout projects.

Just for info; the +5 VDC power output is ideal for Miller signs, to eliminate the battery packs.

Rod

Put one in a building that looks like a power pant and use it to light the miller signs! It could light a whole city now if you could only get little scale electric meters. LOL

Ron

And you can run about 5,000 Miller signs from one computer P/S!

Hey, I have some Miller signs. Thanks for the idea!

Really?  Hmmm.  That's an interesting idea.  So it is -12V and +5V?  What happens if I combine +12 and +5?

Thanks,

George

I'd be careful, those supplies have a common ground, so I'm not sure that's such a keen idea.

I think you'd get 7 volts, not 17.

you can't think of it as 'combining' voltages.  what you are doing is measuring potential between two points.  if one voltage is sitting +5 positive to ground and another is at a potential of -12 negative with respect to ground, the difference between the two potentials is 17 volts.

you're actually always measuring the difference of potential between two points when you measure voltage, it's just that you usually assume ground is zero.

Actually, I think it is you that is confused.  ALL the negative connections are common on a PC power supply.  The exceptions are a very low current -5V and -12V supply.  So, to get any significant current on your 17 volt output is impossible with a PC supply.

Now, if you want .6 amps of 17 volts, then you're in luck, but any more than that, it's not going to happen.  If you measure between the +5 and +12 rails of a PC supply, you get 7 volts.

I hate to burst your bubble, but any significant power out of a PC supply at 17 volts is simply not possible.

I think the older computer power supplies typically have a higher current rating on the +5v and +12v outputs, and lower current ratings on the others, if any. I've seen some older electronics equipment with oddball "+" and "-" voltages going to different parts of the circuitry.

The old C-band satellite dish receivers had some fairly heavy-duty transformers to physically move the large dish antenna. Those old receivers are essentially worthless now, but maybe good for parts salvage.

Right Ace, but the +5 and +12 supplies have a common ground!  They're still the high current outputs.

I pulled a spare supply out of the closet, a 400W one.

+5     - 30A

+3.3  - 18A

+12   - 20A

=5     - .6A

-12    - .6A

+5SB - 2A

To get 17 volts out of that is not going to happen unless you want to use one of the low current negative supplies.

again, the voltage potential difference between -12 volts and +5 volts is 17 volts.  you seem to be the one confused.  i didn't say a word about the load although i can think of many applications where 500ma would be more than sufficient supply current.

Again, the -12 volt supply is 0.6 amps!  If that's all you want, I've already agreed.  However, I'm guessing most folks looking for 17 volts would want a lot more power.

I think the case is closed.  While you didn't say anything about the load, I already mentioned the low current 17 volts, so you're not breaking any new ground here.

Feel free to have the last word.

All:

I finally got enough time to get the power supplies modified and onto a panel which will go on the bottom of my transformer TV cart.  The Eastern (leftmost) power supply is rated at 235 watts.  The Western (rightmost) power supply is rated at 300 watts.

The next photo is a closer view of the Eastern (300 watt) power supply.

George

What are the ratings for your fuses?  It looks like your black ground wires will be the weak link.  I would suggest that you have multiple fuses feeding multiple branches, each fused at the current capacity of your wires to and inside your accessories.

If you have a 30 amp source feeding a dinky wire inside an accessory, you can easily start a fire without the fuse ever budging. 

Hi Dale,

Well, as they say, fire - bad!

Thanks for taking a look.  The fuses are 250 volt, 3 amp fast acting, Radio Shack part number 270-1009.

Please tell me what I'm doing wrong.  I really don't need to set anything on fire.

George

If those fuses are in-line with the accessories, I think you're fine.  Hard to imagine any wire commonly used on the layout that will not carry 3A long enough to blow the fuse.

John - the plan is that a buss will run from say the 12VDC tap (yellow/black on far right of each power supply).  All accessories using 12VDC will have feeders from the buss.  I plan to use suitcase connectors to attach the feeders.  The feeders will go to a small euro-style terminal strip where they will meet the wires from the accessory or lights.  So the fuse is in front of the buss, between the buss (and accessories) and the power supply.  And all wires are 20 gauge.  Does that work?

Thanks,

George

You're saying you're fusing the entire buss at 3A?  That will work, but certainly limits the power you have available for the accessories.  I'd fuse each accessory line.

Yes.  If you look at the photos you'll see the fuses between the two rows of terminal strips.  I have 3 busses - 3 VDC (with its fuse), 5 VDC (with its fuse), and 12 VDC (with its fuse).

So your recommendation would be to remove the 3A fuses from their present location and replace them with a 3A fuse in front of each accessory, correct?

Thanks,

George

As long as you have enough power for all the accessories, your method works fine.  I was reading you had a single fuse, I went back and looked at the picture.  I'm assuming you run several accessories off each fuse, which is fine if they don't require much power.

The bottom line is having the protection, which you seem to have covered.

Well John, I guess the next point of discussion is loading.  Here are my assumptions:

1. I need to calculate the cumulative watts I'm going to need from the accessories on a tap.
2. From the Internet:   Watts = Amps X volts (at a fixed voltage)
3. I'm getting my Amps from the packages of the accessories or lights.
4. My voltage is fixed at 12 VDC from the power supply (theoretically, my voltmeter shows about 10.2 VDC).

So, let's say that I have 3 lights, each of which takes 3 amps.  According to the forumula, we have:

Watts = amps x volts

Watts = (3 amps x 3) x 12 VDC

Watts = 108

Correct?  That assumes all lights are on.  Am I calculating this load correctly?

Thanks,

George

Well, for DC, a simple volts times amps does equal watts, so you have that right.

I'm surprised your power supply is at 10.2, that's way outside the normal +/- 5% that it should deliver.  Do you have any load on the +5V supply?  If not, try about a 2A load on that, it'll help the supply regulate.

John,

I will try a 2A load across the 5 VDC tap.

So now that we know my formula is right, can I calculate the total load on my 12 VDC tap by summing the products of all the lights?  As below:

Total watts = (3 amps x 51) x 12 VDC

Total watts = 1,836

Now, since this supply puts out 300 watts and we should only use 75% of its rated power (225 watts), it seems like I'm going to need a much bigger supply or multiple supplies.  Is that correct?

BTW, thanks for educating me on this.

George

What kind of lights do you have that are 36 watts each?  Are you running something like track lighting on this supply?

You're right, you'll need a MUCH larger supply if you have 51 of those!

Personally, for lights like that, I'd string them in series and run them right from 115VAC, no transformer required.  The only down-side would be you'd want to observe wiring practices for higher voltages.

For 12V lights, you'd probably do well with about 10 of them in series, that gives you 12 volts across each at 120 volts.

No, I'm not running track lighting with this supply.  I have probably misinterpreted something that has caused my calculation to be way off.  Can I send you my list of lights?

Thanks,

George

Post the list here, maybe we can figure out what you really have.  If these are lights on the layout, I doubt they're 36 watts each!

I will do so tonight when I can put together all the pertinent information.

Thanks!

George

OK, I think I know what I did wrong.  Most of my miscalculations came from misplacing the decimal point in the amperage rating.  I used 12VDC for the calculation below.

I'm guessing the items without amp ratings are comparable?

What would be a reasonable assumption for an amperage rating for a small light bulb like those in the table?

George

Many bulbs are in the .1 to .2 amp range for stuff like bayonet bulbs as well as the similar sized screw-in models.  I doubt any of the bulbs in these accessories are going to be more than 1/4 amp.

Given what you have there, I'd say you can probably run most of that stuff with a couple of 3A feeds.  The floodlight tower might have some higher current bulbs as well as the portable spot lights.  Most of the others look to be very low current.

I do think you're still off a notch in the decimal points, I have a hard time with the 1 to 3 milli-amp bulbs.  Are you sure those aren't .03, .01, and .02? respectively?

I checked the bulbs and they are 30 mA (0.03), 10 mA (0.01) and 20 mA (0.02) respectively.

Thanks for all your help!

George

That sounds right.  Given those facts, simple math will allow you to calculate how much power you need on each feed from the P/S.  I'd fuse at about 1.5x to 2x the actual power required for an individual feed.  If there are any motors or other surge loads, you need to take those into consideration when sizing the fuse.  I normally go with the slo-blow fuses for loads with a startup surge current.