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Varying voltage of Computer power supply

Hello,

Can the output voltage of computer power supplies be varied. I would
like to get about 7 volts from the 5v output. Would putting a
potentiometer on the brown sense wire fool the supply into putting out
more voltage.
There are two pot's on the board of a number of supplies that I have.
What are they for. They vary the voltage a small amount when twisted.

Andy
 
N

Nico Coesel

Jan 1, 1970
0
Hello,

Can the output voltage of computer power supplies be varied. I would
like to get about 7 volts from the 5v output. Would putting a
potentiometer on the brown sense wire fool the supply into putting out
more voltage.

That is not going to work. The potmeters are probably for fine
adjustment. You'll need to modify the feedback loop. This means that
all output voltages will go up (or down)!
 
W

w_tom

Jan 1, 1970
0
Can the output voltage of computer power supplies be varied. I would
like to get about 7 volts from the 5v output. Would putting a
potentiometer on the brown sense wire fool the supply into putting out
more voltage.

Usually, a sense wire internally connects to the power wire via a
diode. If a sense wire is broken, then it will never see more than
0.7 volts difference from the power wire; power supply will not
increase voltage into runaway.

Better is to learn the supply's controller circuit, find resistors
that regulate voltage, and replace / shunt those resistors. But
another problem exists. Properly constructed power supplies must have
other circuits so that voltage cannot exceed maximums. Better is to
lower 12 volts to 7. Increasing 5 volts to 7 will trigger overvoltage
protection circuits that all minimally acceptable supplies must
contain.
 
D

D from BC

Jan 1, 1970
0
Usually, a sense wire internally connects to the power wire via a
diode. If a sense wire is broken, then it will never see more than
0.7 volts difference from the power wire; power supply will not
increase voltage into runaway.

Better is to learn the supply's controller circuit, find resistors
that regulate voltage, and replace / shunt those resistors. But
another problem exists. Properly constructed power supplies must have
other circuits so that voltage cannot exceed maximums. Better is to
lower 12 volts to 7. Increasing 5 volts to 7 will trigger overvoltage
protection circuits that all minimally acceptable supplies must
contain.

Reminds me of the time I contacted Phihong and asked how I can get 3
more voltages out of one of their switching supplies.
I actually got practical info and got the supply working with 3 more
volts.


D from BC
 
C

Chris Jones

Jan 1, 1970
0
Hello,

Can the output voltage of computer power supplies be varied. I would
like to get about 7 volts from the 5v output. Would putting a
potentiometer on the brown sense wire fool the supply into putting out
more voltage.
There are two pot's on the board of a number of supplies that I have.
What are they for. They vary the voltage a small amount when twisted.

Andy


You will be better off tracing the schematic first.

As another poster explained, you will have to change the feedback circuit
AND also modify the over-voltage cut-off circuit that would normally
trigger before 7V to shut down the power supply to protect the motherboard
etc.

The circuit may not be very happy running at 7V - that's 40% above normal.
Check the voltage rating of the electrolytic capacitors on that rail,
amongst other things. If you won't be loading it heavily and you don't
require very high reliability then you might get away with it. Otherwise
you could possibly reduce all of the supply voltages until the "12V" rail
measures 7V instead. Make sure the fan goes fast enough with the voltage
reduced - you might have to make some changes to fix that, e.g. put the fan
between the +7V rail and one of the -ve rails.

If you only require a small current then you could use the power supply
un-modified, and add a linear regulator to reduce the 12V rail down to the
voltage that you require.

Good luck, and be careful of the high voltage and things that might go bang
- safety glasses and an isolation transformer would be a good idea, and
don't put your hands near it when the power is on or the capacitors are
still charged up.

Chris
 
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