Viper22A modify 12V SMPS to 5V output

Thread Starter

eyesee

Joined Oct 19, 2013
77
Is it possible to modify a 12V SMPS adapter based around a Viper22A and TL431AA to output 5V instead?
 

Ian0

Joined Aug 7, 2020
6,677
Change the feedback resistors to give 2.5V at the TL431 input, and take half the turns off the secondary.
 

Dodgydave

Joined Jun 22, 2012
10,590
Yes it can be done, there are usually two resistors that set the voltage to trip the TL431, they go across the output of the supply and the centre goes to the reference pin on the TL431.

In the example below, lower the 18K resistor or increase the 4K7 resistor, so the zener trips early..1551030053_bandicam-2019-02-24-20-30-50-823.jpg
 

Thread Starter

eyesee

Joined Oct 19, 2013
77
Yes it can be done, there are usually two resistors that set the voltage to trip the TL431, they go across the output of the supply and the centre goes to the reference pin on the TL431.
So rewinding the transformer isn't necessary?
 

Dodgydave

Joined Jun 22, 2012
10,590
It will only be necessary if the viper uses the winding to self power itself when it's kicked up, like in the example if the output drops too low the viper won't work.

If it's easier to rewind it do it.
 

Ian0

Joined Aug 7, 2020
6,677
It will only be necessary if the viper uses the winding to self power itself when it's kicked up, like in the example if the output drops too low the viper won't work.

If it's easier to rewind it do it.
But it DOES need the auxiliary winding to supply it, because it only has a "startup" current source.
You might just be in luck if the auxiliary winding is quite a high voltage, because Vdd can be anywhere from 9V to 34V.
If the existing auxiliary supply is >21.6V, you don't need to rewind the transformer. If it's <21.6V you do.
 

ronsimpson

Joined Oct 7, 2019
2,520
Yes it can be done,
Moving the voltage from 12 to 5 is a long way. There is a 240 ohm resistor that limits the current into the LED opto isolator. At 5V there is likely not enough current to send feedback to the IC.
The top lead of the TL431 cannot drop below 2V. It really needs 1mA of current to work. The top of the TL431 if often designed to work at 1/2 supply or 6V. Now that the output is 5V we only have 2V to 5.5V to work in. (I forgot the LED needs 1.5V) Really we have a very small window of voltage to work in. The 240 will need to be adjusted to the LED current is the same as back at 12V.
1667067636125.png
and the AUX winding will be wrong. Thanks Lan0.
 

Ian0

Joined Aug 7, 2020
6,677
Also, it will lower the flyback voltage, and increase the time it takes the inductor current to reduce zero. Then it might end up in continuous-conduction mode and diode reverse recovery times suddenly become really important.
 

Dodgydave

Joined Jun 22, 2012
10,590
Moving the voltage from 12 to 5 is a long way. There is a 240 ohm resistor that limits the current into the LED opto isolator. At 5V there is likely not enough current to send feedback to the IC.
The top lead of the TL431 cannot drop below 2V. It really needs 1mA of current to work. The top of the TL431 if often designed to work at 1/2 supply or 6V. Now that the output is 5V we only have 2V to 5.5V to work in. (I forgot the LED needs 1.5V) Really we have a very small window of voltage to work in. The 240 will need to be adjusted to the LED current is the same as back at 12V.
View attachment 279539
and the AUX winding will be wrong. Thanks Lan0.
This drawing is an example i cut from the internet, we don't know his psu design!
 

ronsimpson

Joined Oct 7, 2019
2,520
If you want to move from 12V to 13 or 10 I would say go for it.
12V to 5V, no.
I go to the old electronics store and get old phone chargers for USB. 5V 1 to 3A very low cost.
 
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