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Old 10th November 2011, 07:50 PM   #1
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Join Date: Nov 2011
Default advice needed for changing PSU voltage

I am new to this forum, but already read so much useful advice on here.

I have bought a 30w linear power supply from Ebay, its marked as Ordo by Muse audio.

Only problem is it has a 12 volt output and I need a 5 volt supply for my Squeezebox.

My knowledge of electronics is limited, but I can identify components and solder so have no problem changing components, I just won't know why I am changing them :-)

It has an LM1084 IT-ADJ regulator and a variable resistor in front of it, marked as 922CD W103.

Is it as simple as adjusting the screw on the variable resistor until I get a 5 volt output? or do I need to change any other resistors?
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Old 10th November 2011, 10:50 PM   #2
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
Yes , that is probably all you need to do , just hook up your DMM on DC and slowly turn the trim pot till you get to 5V , You might have to connect a load to the output as most regulators need a certain amount of current draw to maintain regulation ....... and depending on the current draw of the curcuit you might need to put a (bigger) heatsink on the regulator as it will be dropping a lot more voltage at 5v than at 12v ......

Cheers
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Old 12th November 2011, 11:53 PM   #3
! is offline !  United States
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Agreed, it's very likely you will need a substantial heatsink increase dropping down to 5V output, probably won't even fit where the current 'sink is so you may have to improvise where to mount it and the regulator.

Sometimes a POT is used to make fine adjustments on output of a transformer rather than to provide full rail ( minus forward voltage) to ground range in output voltage. In other words even with the POT adjusted all the way down as low as it can go, it may not get down as low as 5V and if it doesn't then you may need to calculate new resistor values for the other resistors in this voltage divider portion of the circuit. The LM1084 datasheet typical use schematic will give an example of this voltage divider scheme and math to arrive at the output voltage required (note it does not have to be 5.0000V exact, having clean output that is off by a couple tenths of a volt shouldn't matter, if you can design away the potentiometer you have one less mechanical part which might have contact issues later).
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