13.8v Power Supply from Junk

Alan, G4FZP

Bury Radio Society

March 2021

I haven’t seen anything half-decent for under 30 quid,most are a fair bit more

How about onefor FREE?

• By converting an ATX Power Supply Unit from an old dead PC

SAFETY• Switched-Mode PSUs are dangerous

and can kill you!

• Not Isolated from the mains! Assume everything inside the box is live at mains voltage and treat it as such

• The input-side DC caps can be charged to over 300v EVEN WHEN IT’S OFF!

• The heatsinks may be live!

• ALWAYS unplug the mains lead and let it sit. Every single time!

• NEVER run it with the board out of the case

• Attach measurement wires with it off, then test. NEVER probe about with it on

• ONE HAND ONLY!

• G4FZP and Bury and Warrington Radio Societies are not responsible for what you do with the information presented in this talk

CAVEATS• You might get RFI on HF unless you filter the output and your unit comes

with a decent mains input filter (or you add one). Not found it to be an issue on VHF/UHF

• You’ll be pushing component tolerances – the smoothing caps on the 12v rail are typically fat 16v electrolytics and we’ll be taking them up to nearly 14v

• The fan on yours might be noisy but you can always swap it

UPSIDES• It’s free! Or at least, very cheap

• You’ll get all the built-in ATX supply features such as power-factor correction, undervoltage and overvoltage protection, short-circuit protection and (usually) overload protection and/or thermal shutdown

Test your supply• The supply won’t run

outside the PC as it stands

• Find the big plug

• Connect the GREEN wire to any BLACK wire to start it up

• Check the output voltagesYellow – 12vRed – 5vOrange – 3.3v

• Would make a nice bench supply as-is

Take the Lid Off

Check the caps for bloated ends!

DC Side

CONTROL IC – This one is a 2005B

ICs you might find: KA7500B, TL494, MB3759, IR3MO2, 2005B, SG6105

• Find the Control IC, the biggest one on the board

• Identify it. This unit has a 2005B

• Google the Datasheet and Application Notes for the IC that you have

Control IC

• 2005B in this case

• The datasheet shows the control input on this IC is Pin 1

• Starting at Pin 1 and working outward, follow the tracks and links and physically identify all the associated components on the board

• Control IC 2005B circuit

• Measure voltage on Pin 1, usually 2.5v but sometimes 5v

• Lift Pin 1 end of R1 and measure its value. Then reconnect

• Lift R3 (and R6 if present)

• Lift the existing R2

• Calculate new R2 to give the right voltage at Pin 1 when the output is 13.8v

• Install new R2, make up with fixed resistor(s) and a smaller value trimpot in series for fine adjustment

• Control IC SG6105 Circuit

• Measure voltage on Pin 17, should be 2.5v

• Lift top end of R97 and measure value of R97 + VR3 together

• Don’t disconnect components on Pin 16!

• Lift R91

• Lift the existing R94

• Calculate new R94 to give 2.5v at Pin 17 when the output is 13.8v

• Install new R94 made up of resistors in series as required

• Nice of them to already provide VR3 for fine adjustment!

Finishing the Job• Put it back together and see if it starts!

• If it doesn’t, check your physical work and your calculation for the value of R2 - If the voltage on the control pin is wrong, it won’t start

• Fault-finding tip – with the unit OFF, put 13.8v from an external supply or string of batteries into the 12v output terminals and check the control pin voltage

• You can always reconnect the existing resistors to get it running again while you figure out what you did

• Once it’s running with your new resistors, verify the output voltage and adjust as necessary

• Check it regulates. A car bulb makes a good test load

• Permanently connect the Green wire to the 0v rail

• Chop off all the output cables except the Yellow (now 13.8v) cables and a matching number of Black (0v) cables

• Fit some sort of output terminals/connector and an LED

• Consider winding Yellow and Black round/through a ferrite toroid just before the output terminals and perhaps adding an .01 uF disc ceramic across the terminals if concerned about RFI

Here’s mine• We’re now

going to attempt live camera so you can take a look at the one I made

Need More Info?• This has been done before by others. There’s info out there

• The July 1992 edition of Radio Communication has an article on converting a computer switch mode power supply to 13.8v

• Radio and Communications November 1998 and December 1998 contain a two part article on building a 13.5 Volt 20 Amp Switch-mode Power Supply written by Phil Harman VK6APH. This explains in great detail how the power supply works

• I have an abstract of those articles, some other bits of info I’ve found and some (but not all) of the control IC data sheets. Contact me via Bury Radio Society if anyone’s interested

That’s all folks!• 13.8v regulated

at 10A for the price of a few resistors

• Hope you found this useful

• Any questions?