Modify 8V/5A to 5V

Thread Starter

Bking2587

Joined Apr 13, 2021
5
Hi everyone, great site here.
I’m looking for suggestions to modify this 8v 5 amp supply into a 5v. I don’t want to add any DC to DC circuits because I think it can be modified. I just got this basic diagram, that’s all. I’m just wondering if anyone has done something similar. The voltage pot goes from 7v to 9v. I haven’t tried anything yet. Note the extra transformer secondary. Looks like several ways to accomplish this but hoping to find the right way. I attached two versions of the picture so at least one will work with phone screens.
 

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Papabravo

Joined Feb 24, 2006
16,773
This design does not appear to be straight forward or easy to modify. Where is this voltage pot you mentioned in the text? So what would be the first thing you might like to try?
 

Thread Starter

Bking2587

Joined Apr 13, 2021
5
R7 on the - sense line is the voltage adj. It also has an over volt pot and current pot not shown. I will try some changes in the - sense line.
I think getting rid of 3 volts will end up causing heat dissipation somewhere.

This design does not appear to be straight forward or easy to modify. Where is this voltage pot you mentioned in the text? So what would be the first thing you might like to try?
 

crutschow

Joined Mar 14, 2008
27,697
Reducing the value of R17 will likely reduce the minimum voltage output.
But you should reduce the maximum output current by a factor of about 5/7 when operating a 5V to avoid overheating the transistors.
 

Thread Starter

Bking2587

Joined Apr 13, 2021
5
I’ll try all that when I can.
I was an electronic tech from 1976 to 2005 and finally setting up a permanent home bench. I built some supplies back in the early 80’s so have everything I want except a higher current 5v. Thanks to all for the help.
 

DickCappels

Joined Aug 21, 2008
7,690
If nothing else, dropping the output voltage without dropping the voltage across the transformer secondary will increase the temperature rise per amp of a few transistors, so it would be a good idea to reduce the current limit while you are at it.
 

Thread Starter

Bking2587

Joined Apr 13, 2021
5
The general schematic ended up being quite a bit different. I put a 2.2k resistor in parallel with the one in the -sense line and it varies from 4 to 6 volts with the voltage adj pot. I mistakenly said this supply had other pots for current and over voltage, it was the other one I just worked on. I put a 2.25 amp resistive load on it and it works fine except the two output transistors are getting as hot as I would expect with 5 amps on it. I’m thinking a resistor in-line with the output collectors now or other methods you guys can suggest. With no load everything is cool.
 
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ScottWang

Joined Aug 23, 2012
7,063
You need to in parallel one pair or two in pairs for Q12 and R26, the schematic already noted that in your first post, maybe you can add the fan to cool down the Q12.

The better thing is that the each power transistor like Q12 only flow through less 2A.
 
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