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Old 19th November 2013, 09:25 PM   #1
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Default Old Computer Power Supply Mod

Hello

I have Started modding an old pc power supply to use it with car amplifiers and for cb, ham radio usage.

Modding it for 13,8 Volts output and need some help from someone more experienced about this mod.

I used two resistors for voltage divider between the 5v and 12v rails and managed to get 13,8v at output.I remouved the coil and capacitor between the 5v rail output but a sound comes from the transformer if i am not wrong and one resistor is getting little hot.
Did not test it with any load yet
Resistors i used was 18 Ohm 5w and 8 Ohm 5w.

Could anybody suggest any idea what else can i do or what causes the sound ?
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Old 19th November 2013, 11:13 PM   #2
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On these power supplies is a 0v rail. It is not possible to separate the 5v high current rail from the low current 12v rail, effectively adding it to make 13.8v.
I would sport about a fiver on a dedicated psu.
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Old 20th November 2013, 06:31 AM   #3
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Started this mod after reading this page:

Converting Computer Power Supplies to stabilized 13.8 V DC 20 A

The author claims he can draw 20A from it
Why could i not mod it ?

I would be happy with 13.8V 10A (the psu i use is 200W)
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Old 20th November 2013, 09:56 AM   #4
AndrewT is offline AndrewT  Scotland
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post1 pic1 shows the mains isolation transformer. It's the big rectangular one between the two heatsinks.

That transformer and all to it's right runs direct off the mains !!!!!!!!!

While you are experimenting I strongly suggest you use a separate mains isolating transformer and to keep one hand permanently in your pocket while this thing is powered up.
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Old 20th November 2013, 10:34 AM   #5
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I know how to protect myself from mains power and i know where the mains power is located on pcb

Thanks for your concern but i didn't ask for safety advice
If you have any helpful ideas for the mod please post them or just ignore it
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Old 20th November 2013, 11:19 AM   #6
sesebe is offline sesebe  Romania
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Were it is connected this resistor?

BTW do you disconnect the build in over and under voltage protection?
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Old 20th November 2013, 11:47 AM   #7
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Have a look at the links pic:

big-pc-power-supply-modification.gif Photo by sm5zbs | Photobucket

I did not disconnect anything except the two parts mentioned above(coil and capacitor pic 4 first topic)
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Old 20th November 2013, 11:51 AM   #8
Art M is offline Art M  United States
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Quote:
Originally Posted by enter View Post
I know how to protect myself from mains power and i know where the mains power is located on pcb

Thanks for your concern but i didn't ask for safety advice
If you have any helpful ideas for the mod please post them or just ignore it
There is a Prohibition about discussing Line Operated switching power supply electronics on this site. It is obvious that you are modifying a circuit that you do not fully understand. Better just move on.

Is this a helpful Idea ?
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Old 20th November 2013, 11:54 AM   #9
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The local ham club in Athens probably has a copy of one of the ARRL handbooks from the 2000's -- they have an article describing exactly what you seek to do including details on winding new inductors. Would suggest that this is a better venue.
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Old 20th November 2013, 12:06 PM   #10
marce is offline marce  United Kingdom
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You are modding a different power supply, do you have a circuit diagram?
It looks like the idea is to change the voltage fed back to the controller to fool the supply into thinking the voltage is to low thus boosting it a bit to get the 13.8V.
Personally I would get a circuit diagram as a minimum and the data sheet for the controller, then at least you can work out what you are doing.
Though I also think it is not the safest or well thought out mod I have seen.
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