An inexpensive bench-top 12v Power Supply

[Jim 5]

I often have needed a power supply for bench testing 12v auto/motorcycle accessories such as radios, GPS, heated vests etc. I considered using a wall adaptor for cell phones electronics but I found they don't put out much current.

I found these series of plans on the internet for building a bench-top power supply using a discarded computer power supply:

http://www.wikihow.com/Image:Two_642.jpg

http://www.wikihow.com/Create-a-12-Volt-...er-Power-Supply

http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply



Following the JPG that explains the simple wiring connections, I ended up with this:

This was a nice little tuesday night project. All that is needed is:

- Computer power supply (had a few in the basement)
- 12v cigarette lighter outlet
- Radio Shack speaker/binding post
- LED and chrome LED housing
- SPST On/Off switch
- SAE plug
- Some stick-on rubber feet

It results in a handy powerful/clean supply of 12v (I also added a single 5v outlet as many things like cell phones/blackberries run on 5v). Raw wires can be connected at the speaker style outlets (5 or 12v). Cigarette lighter accessories can be plugged directly into the outlet, and things like heated vests etc. can be plugged into the SAE plug.

It took a few hours mostly of drilling holes in the case to accommodate the lighter mount and speaker outlets as well as soldering the appropriate wires and insulating the unused ones.

I think I spent about $20-$25 on this project (the power supply was free) and from what I can see, an decent lab power supply is well over $100. Plus it was an interesting way to spend an evening (once the leg-work of collecting all of the necessary parts was done).

It is not a difficult job for anyone that has ever messed around with electronics/soldering. I think it will be very handy for testing 12v accessories before wiring them up to the bike or car. It's not always convenient to have an old 12v battery or to connect to the car's battery for this purpose.

Here's a shot of the power supply powering up my old radar detector:

For what its worth, I recommend the project to anyone that regularly messes with their automotive or motorcycle electrical system/accessories.

Of course, use at your own risk, check the connections, read and re-read the instructions, and don't try this without knowing what you are doing and taking the appropriate precautions. Line voltage can kill.

 

[Shannow]

Cool, I've oft thought about doing just that.

 

[SecondMonkey]

Are you using any kind of load resistor on the 5v line? Usually they only put out around 10v if the 5v line isn't loaded.

 

[Tosh]

I predict disappointment running any car radios off it: Switch mode power supplies are too noisy for this.

You need something like THIS. I've seen 2 amp cheapies of this type for $12-15.

 

[brianl703]

I doubt a switch mode power supply is any more noisy than a car's electrical system.

 

[Jim 5]

Originally Posted By: Tosh
I predict disappointment running any car radios off it: Switch mode power supplies are too noisy for this.

You need something like THIS. I've seen 2 amp cheapies of this type for $12-15.


I like it! I'm OK with what I've got because I wouldn't actually run a car stereo off this for listening pleasure. Mostly, it would be just a bench-top source of 12v and 5v to simple accessory testing purposes. I'm suspecting I'm getting significantly more current from a 550w power supply as well, although using a simplistic calculation and Ohm's law I get something like 45 amps. However at the 12v side, I'm sure it's dramatically less than that, the wires' gauge inside the box are nowhere near adequate for 45 amps. I'm betting it's bigger than 2 amps though.

I would probably buy something like that (especially at $12-15), but I wanted a a little project to occupy my time, and to justify being a pack-rat and keeping old junk like this power supply. Also, by the time I land it in Canada with shipping, duty and brokerage fees I'm typically paying double the price. I have ordered products from Parts Express before. It's a good company.

 

[Jim 5]

Originally Posted By: SecondMonkey
Are you using any kind of load resistor on the 5v line? Usually they only put out around 10v if the 5v line isn't loaded.


Nope,

It tested it on the multimeter and it showed 4.94v. Using a 12v automotive bulb, it was less than half as bright as the 12v side so I am confident it is putting out 5v with/without a load attached.

I will note however that if click on one of the links attached (there are 2 or three designs on wiki), as you pointed out, one of them does include a load resistor if I'm not mistaken.

I will confess to not understanding the way the power supply works, so your knowledge exceeds mine. I simply followed the simple JPG by wrote and it seems to work.

 

[Quest]

Thank you Jim 5!

You rock!

Also a caution to everyone whose interested in building one of these for general use:

(a) always consider getting a decent quality (used) or new one from a reputable brand/source. Don't go with carrappy ones for they may take down the device you hooked up to along when they decided to die.

(b) these power supply is good for general public use or anything that can take on a rough, slightly "dirty" 12V or 5V DC power supply. Do not use this to power "sensitive" equipments such as amplifiers, RF modulators or things that are sensitive to dirty DC for these computer power supplies are what we called "switchers" and it comes laden with loads of high-order harmonic distortion and a significant amount of noise along in the DC line (can be into the order of tens of mV; old ones can get as much as hundreds of mV) and don't be surprised if you plug in a radio or similar and get some buzz or hash noise.

(c) do not leave any sensitive equipment under charge in one of these units for it will kill the charging/rectifying circuits if left over an extended period of time.

Cheers,

Q.

 

[brianl703]

As a matter of fact I recently used a PC power supply to test an amplifier (a JVC 4x55W RMS unit) and I heard no power supply noise at all. I used the PC power supply because the linear 12V power supply I have is rated for 5amps max and I was getting distortion at higher volume.

Incidentally, how many mV of noise do you expect to find riding on the average 12V electrical system in the average car?

Here's one answer (90mV or less):
http://www.labscopes.com/pg09.htm



ALTERNATOR - Ripple Voltage Test
Using a DMM

The Alternator produces AC voltage and current. The battery requires DC voltage and current to charge properly. Diodes located within the alternator change (rectify) the AC to DC. However, a small amount of AC can still be present and no harm is done. Problems can develop when alternator diode faults permit unacceptable amounts of AC to pass into the vehicle electrical system.

1.

Set the meter to read “AC” Volts (lowest range) .
2.

Connect the black COM lead probe tip to the battery negative post.
3.

Connect the red Volts lead probe tip to the battery positive post.
4.

Run the engine at 1500 RPM.
5.

Turn ON the high blower and high beam lights.
6.

Your meter reading should not exceed .09 volts (90mV) AC.

 

[brianl703]

By the way if we go back far enough in time we find sound cards with ~5 watt amplifiers on them powered directly from the 12V line in the ISA bus.

In fact I am looking at one right now with a TDA1517 amplifier chip (2 x 6 W stereo power amplifier) on it. I do not see any chokes or filters on this card. Nor do I see any capacitors larger than 470uF.

 

[Quest]

brianl,

Points taken.

As a P/T job working in the industrial electrical arena, I've seen, dealt, handled, repaired numerous of swithers related matters due to dirty power supply.

ATX power supply is spec'ed to have no more than a certain degree of ripples:

example: ATX version 1.2 spec. calls for no more than 120mV on +12V DC rail (peak-peak), with +5V DC rail no more than 50mV (peak-peak).

(http://www.enhanceusa.com/designguide/atx12v_v1.2.pdf page 15)

As switchers age, they don't age gracefully for the capacitors, etc. goes on an almost exponential curve in terms of failure. Typical symptoms included elevated ripples and noise.


Because these noise are in the region of 18kHz and upwards, most folks cannot hear/perceive it other than being somewhat of a hash noise.

BTW: switchers are not termed "linear power supplies", it never has. (2 distinctively different types of power supply designs).

Lastly, while automotive alternators have noise also (and varied as the engine revvs up and down), they do not, however, come with high order harmonic distortions(and noise) typical to switchers. Because of that, most automotive-related electronics can confidently cope with these alternator-related noise by some fairly straightforward L-C filtering networks and sometimes with a regulator to make it more effective.

Cheers to you too...

Q.

 

[Quest]

BrianL,

I was initially moved when one of the Taiwanese mobo manufacturers put out a P-4 mobo with a 6DJ8 variety of audio section built in to the audio output section but was extremely turned down when high-freq hash noise (from switching power supply from PC) made it through the analogue portion.

I also deal with a lot of high-end audio designs and I must tell you that while no 2 switchers are the same (in terms of harmonic distortion spectrum and ripples), ATX ones are pretty nasty to say the least.

Cheers,

Q.

btw: have you ever try LM1875? Pretty decent sonics and performance for pennies spent.

 

[SecondMonkey]

Originally Posted By: Jim 5

It tested it on the multimeter and it showed 4.94v. Using a 12v automotive bulb, it was less than half as bright as the 12v side so I am confident it is putting out 5v with/without a load attached.

I will note however that if click on one of the links attached (there are 2 or three designs on wiki), as you pointed out, one of them does include a load resistor if I'm not mistaken.

I will confess to not understanding the way the power supply works, so your knowledge exceeds mine. I simply followed the simple JPG by wrote and it seems to work.


Did you test the voltage on the 12v side? I've used them before for this purpose, the 5v was always ok without a resistor, but the 12v line was never up to 12v...although I was using old AT power supplies.

As for the amperage, it should put out more than the wires will lead you to believe. There's normally a bundle of wires going off to hard drives and everything else in the computer...so each line only carries a few amps, but they all hook into the same place on the board.

My latest power supply is a 5A, 20V supply from a Dell laptop. I then made a simple voltage regulator with an LM388 chip (easy even for a newbie) with a knob that lets me turn up the power from 1v to 19v. I plan on adding a second one in parallel for more amperage.

 

[brianl703]

Originally Posted By: Quest

Lastly, while automotive alternators have noise also (and varied as the engine revvs up and down), they do not, however, come with high order harmonic distortions(and noise) typical to switchers. Because of that, most automotive-related electronics can confidently cope with these alternator-related noise by some fairly straightforward L-C filtering networks and sometimes with a regulator to make it more effective.


A device designed to be powered from a car electrical system should be be designed to cope with more than just alternator noise:

http://www.automotivedesignline.com/howto/205101011

"High-voltage power supply spikes lasting from a few microseconds to hundreds of milliseconds are a common occurrence in the automotive and industrial environments. The most severe transient in automotives is the load dump, caused by a poor battery connection or accidental disconnect while the alternator is charging."

"Noise from the ignition system and various accessories (power windows, door locks) occur frequently."

As far as LC filters go, the inductor found in such a circuit has a higher reactance for higher frequency noise.

 

[brianl703]

Originally Posted By: Quest
but was extremely turned down when high-freq hash noise (from switching power supply from PC) made it through the analogue portion.


Some or all of that noise may be from the digital circuits in the computer. On the occasions I've heard noise from the output of a sound card, it was dependent upon what the computer was doing. For example, accessing the hard drive made the noise change in time with the activity light. You could hear motor noise from the CD-ROM drive spinning up. Etc.



Quote:
btw: have you ever try LM1875? Pretty decent sonics and performance for pennies spent.


I remember seeing that 5-pin amplifer chip before, but I don't remember what it was used in.

 

[brianl703]

I just connected a telephone to the 12V supply on the computer I'm typing this on and heard a fair amount of noise.

I connected the same telephone to the 12V supply I used to power that amp (from an AT power supply that's probably 10 years old) and I heard a low-pitched pulsing noise that sounded a lot like ignition noise.

I connected a hard drive to that power supply with the telephone still connected to the 12V output and I heard noise from the motor spinning up.

Conclusion: The electronic circuits inside a computer contribute their fair share of noise to the power bus.

 

[kc8adu]

use a good power supply and you wont have the noise.
i have an ax533b-tube in my htpc.no noise whatsoever.
got it free in a box of parts.
had bad caps so i rebuilt it.
too many folks buy power supplies by the often inflated big numbers the mfr claims.
lots of junk on the market.

Originally Posted By: Quest
BrianL,

I was initially moved when one of the Taiwanese mobo manufacturers put out a P-4 mobo with a 6DJ8 variety of audio section built in to the audio output section but was extremely turned down when high-freq hash noise (from switching power supply from PC) made it through the analogue portion.

I also deal with a lot of high-end audio designs and I must tell you that while no 2 switchers are the same (in terms of harmonic distortion spectrum and ripples), ATX ones are pretty nasty to say the least.

Cheers,

Q.

btw: have you ever try LM1875? Pretty decent sonics and performance for pennies spent.

 

[kc8adu]

to the o p.
the bestec blow em up special unit you built your project around needs to be recapped.
the design is ok but the beancounters made sure the cheapest parts were used.
in the world of smps and low esr lytics cheap and good are mutually exclusive.
you can ask any emachines owner about that bestec unit.
poof!
since that unit has +5 and +3.3 remote sense it wont suffer as badly as some with no load on +5.
if you can provide a few amps load on+5 it will stay closer to 12v.
but recap it first!
the most important are the 10@50 and 220@25 next to the +5stby transformer.

 

[brianl703]

Originally Posted By: kc8adu
use a good power supply and you wont have the noise.


What I've heard is that unless that power supply costs at least $40, it's probably a piece of trash.

 

[brianl703]

That's for a 300W power supply. If you're buying a 500W power supply for $40, that's probably a piece of trash too.