Convert Power Supply to 12V Lab Power Supply

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Discussion Overview

The discussion revolves around converting a computer power supply into a lab power supply with a focus on achieving a 12V output. Participants explore the feasibility of using multiple 12V rails to increase amperage, the implications of grounding, and the potential for using the power supply with a lithium polymer battery charger.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant inquires about connecting two 12V rails to increase amperage, suggesting a desire for a 32A output.
  • Another participant explains that one of the 12V rails is negative and cannot be used in parallel with the positive rail, citing grounding issues.
  • Concerns are raised about the noise and stability of the output when using the power supply for sensitive applications like audio amplifiers or SSB transmitters.
  • Some participants express uncertainty about the possibility of separating outputs for independent use, suggesting it may be a complex task.
  • Questions arise regarding the necessity of the 5V rail when only using the 12V output, with differing opinions on whether it should be disregarded.
  • One participant mentions that if the two 12V supplies are exactly the same voltage, they could theoretically be paralleled, but warns against this practice due to potential current draw issues.
  • Another participant raises a question about wattage requirements, asking whether a power supply should accommodate the maximum output of a charger or the expected charging wattage.
  • There is a discussion about the efficiency of the power supply and the current requirements based on the wattage of the charger.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of paralleling the 12V rails and the implications of grounding. There is no consensus on whether the 5V rail can be disregarded, and the discussion on wattage requirements reveals varying interpretations of necessary power supply specifications.

Contextual Notes

Participants note that the outputs of the power supply share a common negative rail, which complicates the potential for combining outputs. There are also concerns about the stability of the power supply under varying loads, particularly for sensitive equipment.

Who May Find This Useful

This discussion may be useful for DIY enthusiasts, hobbyists working with power supplies, and individuals interested in powering battery chargers or other electronic devices from computer power supplies.

looklikeaflip
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I want to convert my power supply into a "lab power supply" but a lot less simple. I just want a 12V output on it.

The power supply has 2 12V rails. Is it possible to connect both rails to increase amperage?

Thanks for you input.
 
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One of the 12 volt rails is negative so you can't use that in parallel with the positive one and they are both grounded inside the power supply, so you can't change them.

However, the 12 volt one in my computer is rated at 10 amps, so this would be a very useful output. You can take it from one of the yellow wires that go to the hard drives. The black wires are ground.

It will probably be a bit noisy, so you might not use it for audio amplifiers and it probably wouldn't work too well if the current was changing rapidly (like in a SSB transmitter), but otherwise it should work OK.

Some power supplies won't work unless you connect the green wire on the big power plug to one of the black wires, so you might like to do that.
 
Sorry, I'm not a technician or anything. I'm just doing a "do-it-yourself" project. So please explain things as simple as possible.

I'm trying to power a lithium polymer battery charger to charge at its maximum current of 20 amps.

And i didnt make myself clear enough. It has two positive 12V rails. at 16Amps.

Here's the link to the Power supply.

http://redirectingat.com/?id=42X129...urnigy-charger-hobbyking-too-good-true-3.html

Again, could i group both rails together on one output to produce 32 amps?

Can the power supply handle it?
 
looklikeaflip said:
Sorry, I'm not a technician or anything. I'm just doing a "do-it-yourself" project. So please explain things as simple as possible.
I'm trying to power a lithium polymer battery charger to charge at its maximum current of 20 amps.
And i didnt make myself clear enough. It has two positive 12V rails. at 16Amps.
Here's the link to the Power supply.
Again, could i group both rails together on one output to produce 32 amps?

No, for the reasons that VK6KRO gave in his first sentence. ALL the different rails outputs of the PSU, the 5V, 12V, 3.3V etc all use a common negative (0V) rail


Dave
 
Ah, I see. PLease forgive us less educated people. The language of physics is domineering.
 
looklikeaflip said:
Ah, I see. PLease forgive us less educated people. The language of physics is domineering.

no probs. :) we all started somewhere
I haven't investigated the possibility of separating the various outputs to make them independent of each other. I suspect that this would be a major task. Somewhere in the circuit there will be feedback circuitry between one of the output lines and the input of the power supply, this is to stability (regulation) of the output voltages.


to VK6KRO... quite a few of us amateurs have used puter PSU's for all mode transceiver use they work very well ... just require a little more smoothing and hi freq decoupling on the output.

Dave
VK2TDN
 
looklikeaflip said:
Another question: If I'm only using the 12V output, can I just disregard the 5V rail and cut it off?
I'm using this as a reference on how to convert.
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

interestings little project in that link :)

yes you could ... but I wouldnt, a +5 volt rail capable of a good number of amps is very handy. specially in a lab where you may play with a lot of digital circuits that all run on 5V

Dave
 
Yes, me and my cousin will have fun with this one. Very interesting.:smile:

Is it possible to join the 5v to the 12v rail to increase amp output?
 
  • #10
looklikeaflip said:
Yes, me and my cousin will have fun with this one. Very interesting.:smile:

Is it possible to join the 5v to the 12v rail to increase amp output?

no for the same reason as above, they share the same negative line

Dave
 
  • #11
okay, because some guy in a different forum said i can...
 
  • #12
If your battery is actually 12 volts it may develop 14 volts when charged. This would mean that the battery would not charge from this supply. The supply always has to have a higher voltage than the battery for the battery to charge.

If the two +12 volt supplies were exactly the same voltage, you could parallel them, but this is regarded as poor practice as one could easily be slightly different in voltage to the other and this would cause the lower one to draw current from the higher voltage one.

I tried a computer power supply on a SSB transmitter and the rapid variation in current caused the regulator to go crazy with wide fluctuations in voltage. It didn't seem to be RF interference as it didn't matter if the transceiver was on dummy load or connected to an antenna.
The computer power supply was a box of whistles, too, with lots of unstable carriers in the receiver.
You can be lucky, or not, I guess.
 
  • #13
I see. Very well said!

I have another question about Wattage use.

If a charger has a max output of 300 watts, and I know the max watts I will be charging at is 170, do I need a power supply that accommodates for the 300 or 170 watts?
 
  • #14
I'm trying to power a lithium polymer battery charger to charge at its maximum current of 20 amps.

I assume from the above statement you have an existing charger that uses a 12V source to charge your battery pack (ie a "car charger"), and that the maximum current draw on the 12V source is 20A? If this is the case it is quite likely your charger will operate properly from your computer power supply.

I have several PC PSUs in service for things other than PCs. They are very handy!

Fish
 
  • #15
looklikeaflip said:
I see. Very well said!

I have another question about Wattage use.

If a charger has a max output of 300 watts, and I know the max watts I will be charging at is 170, do I need a power supply that accommodates for the 300 or 170 watts?

Output wattage is always going to be less than input wattage, typically ~ 20%. So, you 12V source would need to provide at least:

(170W/.8)/12V = 17.7A

(300W/.8)/12V = 31.5A

This is a lot of current, more than a single 12V rail in your PSU is designed to handle. You can certainly try paralleling your 12V rails. Worst case you destroy the PSU. But I would assume the PSU is designed to handle parallel connection of the 12V rails, so likely it will work. Efficiency may be pretty bad, but electricity is cheap.

Fish
 
  • #16
looklikeaflip said:
I see. Very well said!

I have another question about Wattage use.

If a charger has a max output of 300 watts, and I know the max watts I will be charging at is 170, do I need a power supply that accommodates for the 300 or 170 watts?

You haven't said what the voltage of your battery is, required to answer this. Is it 3.7V, 7.2V...?
 

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