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Redshift187 28th June 2012 09:59 PM

What's wrong with this circuit?
1 Attachment(s)
I tried to design a simple dual polarity shunt regulated power supply using TL431's, but I get way too much heat and even a little smoke. I'm sure I've got something wrong on the negative side, but the web is seriously lacking in example circuits for this design.

whizgeek 28th June 2012 10:28 PM

1st, put the divider resistors on the output (regulated)side of the cct.

TL431D = 725mW or 100mA maximum at 25 degC
If you are looking for 5v out, its the 100mA limiting factor.

with 75 ohm resistors, means you cant have more than 7.5v across the resistor, or max of 12.5v on the supply capacitors.

noSmoking 30th June 2012 01:52 AM

1 Attachment(s)
google tl431 calculator( if this doesn't load )

Redshift187 30th June 2012 01:59 AM

Whiz: Sorry, I'm not sure what you mean about the divider resistors. Also, how do you arrive at 7.5V and 12.5V?

Smoking: Thanks, but I'm unable to run that. I don't have MS Office, and it doesn't do anything in OpenOffice even with macros enabled.

noSmoking 30th June 2012 02:27 AM

hi Redshift187,
so how much power are you using going in?
and your wanting + and - out ,at what voltage ?
and how much amps does your circuit need?

Redshift187 30th June 2012 02:28 AM

I've got about + and - 17V going in, anywhere between +/- 5V to +/-10V out will be fine. Almost no current at all, it's an opamp driving a line level signal to an amp.

noSmoking 30th June 2012 02:49 AM

see if you can open this
StabDesign.exe or see if you can google it ,it is a cal that does them all.
[[IMG],GIF89a%12%00%12%00%B3%00%00%FF%FF%FF%F7%F7%EF% CC%CC%CC%BD%BE%BD%99%99%99ZYZRUR%00%00%00%FE%01%02 %00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%0 0%00%00%00%00%21%F9%04%04%14%00%FF%00%2C%00%00%00% 00%12%00%12%00%00%04X0%C8I%2B%1D8%EB%3D%E4%00%60%2 8%8A%85%17%0AG*%8C%40%19%7C%00J%08%C4%B1%92%26z%C7 6%FE%02%07%C2%89v%F0%7Dz%C3b%C8u%14%82V5%23o%A7%13 %19L%BCY-%25%7D%A6l%DF%D0%F5%C7%02%85%5B%D82%90%CBT%87%D8i7 %88Y%A8%DB%EFx%8B%DE%12%01%00%3B[/IMG]

noSmoking 30th June 2012 02:53 AM

don't know what that was but Google Stabdesign.exe

Redshift187 30th June 2012 03:10 AM

Thanks noSmoking. I got all the resistors right except the 75R should be 210R. But I don't think that would cause smoke. Also, it really doesn't matter if the resistor divider is on the left or the right of the TL431 in the schematic, it's connected to the same place.

didiet78 30th June 2012 03:39 AM

I think 75R could cause smoke, its limiting cuurent for TL431 (shunt reg). 226mA is too much for TL431. It safer using 210R.


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