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Home brew power supply

N

nav2010

Active Member
Sep 5, 2010
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Just recently converted a 650W computer power supply for use with CB equipment. The supply specs were 2x23amp 12v+ on the 12vdc rail, 30amp on the 5vdc rail and 20amp on the 3.3vdc rail. Desoldered the 3.3 and 5v wires off the board apart from one 5vdc + wire which I needed for a shunt resister.
I used an old Zetagi BV135 case to mount the pc supply inside, fitted a pair of output poles which are fed by 8 ground wires off the board and 8 yellow 12vdc+ and an LED, desoldered the kettle supply and switch at the rear of the pc unit and then used the BV's power cable and on/off switch hard wired to the unit. Added a 10w 10ohm ceramic shunt resister to the 5vdc + rail to provide constant load which is recommended practise with these units. Also placed the shunt resister on an heat sink and placed inside the BV case. The green wire which is used normally used to turn on the unit from a pc motherboard is soldered into the ground to provide current constantly.
Been running it at low load for the first week and used it on a cb at about 5 watts. Voltage loss under this load (mic keyed up) went from 12.56vdc to 12.28vdc which isn't bad. The shunt resister is drawing approx 1.9 amps constant.
This morning have been testing the supply under an higher load. 63 watts on the 12vdc + rail plus the 10 watt shunt resister on the 5vdc + rail obviously.
This shows 5.8 amps and the voltage drop has increased across the dc output as expected. 12.56vdc to 12.15vdc. Still exceptable.
The total 12vdc peak is 46 amps but i've read that the actual figure on this model is about 37 amps. Continous load across the 12vdc output at a safe level is hard to say at this stage but I will not be drawing more than about half the peak load so 18amps is my limit. This would run a Zetagi b300 or a KL400 without a problem I should imagine.
 

Cool project. What would it require to up the voltage to 14.4vdc? I wonder if you would net less voltage drop if you replaced the small yellow wires with a larger single 4ga?
Could a pair of these supplies be paralleled? I'd like to build one strong enough to run an Ameritron AL-500.
Hey, what does your signature mean?
 
Needle Bender said:
Cool project. What would it require to up the voltage to 14.4vdc? I wonder if you would net less voltage drop if you replaced the small yellow wires with a larger single 4ga?
Could a pair of these supplies be paralleled? I'd like to build one strong enough to run an Ameritron AL-500.
Hey, what does your signature mean?
Click to expand...
You can't put em in parallel because the current draw nearly always pulls on one supply much more than the other. Yes there is a mod to change to 13.8vdc, google it. Yes the 8 smaller wires would be better replaced with a 30 amp single.
Just slapped a 100 watt kicker on this and there are problems. RF is feeding back on the VDC output back into the supply and interfering with the switching stage. This is causing an unacceptable voltage drop under load of 1.5vdc with 80 watts. Time to get the ferrite clamps out and maybe a cap across the dc output.
My signature is a couple of quotes rolled into one from Smokey and the bandit.
 
nav2010 said:
You can't put em in parallel because the current draw nearly always pulls on one supply much more than the other.
Click to expand...

Give this a try. Put a 25 Amp-50prv diode in the positive output line of each supply.This isolates each supply from the other, and prevents one supply from carrying too much of the load. You'll have to bump the supply voltage up slightly because of the 0.7v drop across the diodes. Bypass each supply with a 20000uf/50V electrolytic cap (for voltage stability) and a 100pf npo to shunt any rf on the supply lines to ground.

-399
 
Going to try and get rid of the RF from the dc output in the next couple of days. I've sourced some caps and i'm also replacing the 8 single positive and negative 12vdc leads with 40 amp single. Looking at ferrite thats effective at these frequencies also. The stock psu I have uses a 47uf cap across the terminals to ground the RF so that will be my first port of call. Will be interesting to see if I can remove this annoying RF easily.
 
Time for an update after 5 months of use with this unit. Cured the RF problem with ferrite clamps and several caps across the DC output voltage. The main issue with these units is they tend to drop voltage when under heavy load because they are not designed to be under heavy load in an instantaneous fashion such as keying the mic. With a current draw of 4 amps the unit drops from 12.6dcv to 12.4dcv which isn't bad for a normal rig kicking out about 10 watts. Under a load of 12 amps the unit drops from 12.6dcv to 11.8dcv which is getting too much. Under a load of 22 amps the unit drops to 10.5dcv which is no use at all really for using on amps etc.
If these units are to be used seriously in radio there is a definate need for some sort of regulation on the voltage and perhaps additional cooling to boot. They are not heavy enough duty to be pulling large currents in bursts and something tells me if you continue to use them on a regular basis with high current draw something will go pop.
However, the unit has been used for 5 months on low current draw with no problems so they can be used as a bench test supply or as a supply for anything on low draw around the 12dcv range. Now I know why when I've opened up a 30amp linear power supply everything is heavy duty with massive caps and a big tasty transformer.
So in conclusion PC power supplies are for PC's - period.
edit:-
I replaced the 8 wires in the positive and negative dcv output with single 40 amp cable, I also tried different caps across the dcv output to try and stabilize the voltage to no avail. In my opinion the problem lies in circuitry of the board because there is absolutely no point in having 40 amp cable from the board to the poles if the tracks on the board supplying that voltage are not heavy duty and could be subject to high resistive loss. One tends to think that the tracks on the board may be slightly glowing if a 30amp draw was used. I reckon by the look of them they cannot be possibly rated more than 10 amps and therefore we have a board mounted light bulb lol.
 
Last edited:
i tried to connect 3 pc power supplies together in series using the 5 volt red wires ...got 15 volts to run amp but one supply always kicked out...so prject got dumped
 

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