issue with cascading voltage regulators

beatnik · Jan 18, 2016

i am trying to set up a simple power supply with two outputs +24 and +12 VDC

i am using a lm317 for the 24v output and a 7812 for the 12v

i have been trying to feed the 7812 input from the lm317 output, thus cascading the two regulators, but this way it doesn't work. the output of the 7812 stays about 1.5v and the lm317 regulator gets very hot

i also tried feeding the 7812 straight from the rectified dc supply but it's not working either. i guess it's because the 37v i'm getting after the diode bridge exceeds the maximum input voltage rating of the 7812

is there a way i can make these two working together ?

i got the idea from the gssl power supply which uses a similar approach but works on different voltages, 15 and 12

i hope i can make this work because i have the pcb already done ! i just assumed it would work but again my lack of theory has proved me wrong

Andy Peters · Jan 18, 2016

beatnik said:
i am trying to set up a simple power supply with two outputs +24 and +12 VDC

i am using a lm317 for the 24v output and a 7812 for the 12v

i have been trying to feed the input from the lm317 output, thus cascading the two regulators, but this way it doesn't work

Define "doesn't work." I assume you're missing parts of your sentence, which should read, "I have been trying to feed the input of the 7812 from the LM317 output ..."

-a

beatnik · Jan 18, 2016

yeah i wasn't very clear now i edited the first post

mjrippe · Jan 18, 2016

Double check the pinout of the two parts and your board layout. 7812 is In/Gnd/Out, 317 is Adj/Out/In

JohnRoberts · Jan 18, 2016

you should be able to cascade those parts...

how does the lm317 look with 7812 out of the circuit?

If 317 looks good try adding a resistor in series with the 7812 input (100 ohm?) to see what it is doing...
Maybe 7812 is faulty or connected wrong.

JR