changing SMPS voltage from 24 to 12

Thread Starter

medman

Joined Sep 27, 2017
2
Hi AAC
I am new to electronics, newly retired physician, studying electronics and repair, have a 24v, 15a SMPS, trying to lower the voltage to 12v, it is based on push pull have-bridge topography and TL494, what I did was de-solder the transformer (inner layer the primary, then 2 layers of 4-copper wire secondary forming center tapped 13 turns, then thin 13 turn double direction wire going to the thermal protection circuit, then outermost 21 turn auxiliary winding).
I shortened the secondary wires to 6 turns, and reassembled.
I am now getting 22 volts at the secondary and rectifier diode, and hence in the output, voltage at pin 1 of the TTL464(+error amplifier) and pin 3(FB) are both 2.5, tried changing the voltage divider network going to pin 1 but same voltage I get, some help will be much appreciated
 

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Dodgydave

Joined Jun 22, 2012
11,307
Temporarily remove R25 in series with the adjuster preset, this will put pin 1 at positive supply , and should lower the output to the minimum, this should prove the circuit is working.
Then you can alter the resistor R25 to give you the desired output, and tweak it with the preset SVR1.
 

Thread Starter

medman

Joined Sep 27, 2017
2
Temporarily remove R25 in series with the adjuster preset, this will put pin 1 at positive supply , and should lower the output to the minimum, this should prove the circuit is working.
Then you can alter the resistor R25 to give you the desired output, and tweak it with the preset SVR1.
Hello DodgyDave
Yes it worked, I can't thank you enough, I put a 10K pot and found out it needs 8.7K so I made it out of 2 resistors and it is working, thanks again, but I need to know why when no resistor was there, there was a squeaking sound coming from the transformer in powering on/off, and it gave 4volts, but when powering off it overshooted to 5v then zeroed
 
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