Power supply help

-KoMoD0- · 23rd November 2006, 5:53 PM

Basically i made a variable DC power supply using a bridge rectifier and a LM317t IC, plugged it in and it was working fine, powered up a few things and noticed the heat sink was so hot it was melting the plastic above it and when i touched it it burnt me. It was a pos clip on type so i pulled a massive heat sink out a blown computer PSU and used it. Screwed it to the chip and hot glued it to the board (on the ends) - now it doesn't work. Multimeter was showing weird things (showing 5v and it slowly dropping after it was switched off, then doing nothing and giving me 0.03v).

Im off to buy another IC, any idea how this happened and could i have killed something else.

btw, dse gave me the wrong caps, i asked for 16v and they gave me 10v = loud bang and bad smells. idiots

Cheers

stewpot · 23rd November 2006, 6:37 PM

Are you shorting the mounting tab onto anything?

-KoMoD0- · 23rd November 2006, 6:39 PM

no unless hotglue is conductive

nux · 23rd November 2006, 6:41 PM

The heatsink attachment is Vout (not ground like other Vregs) so if you shorted it to ground or something you might have killed it.

The heatsink shouldn't be getting really hot unless you are pulling a lot of current and/or dropping a large voltage across it. What input voltage are you using?

-KoMoD0- · 23rd November 2006, 6:49 PM

Input is 16v AC 1.25A

I was pulling 9v 1A

Goth · 23rd November 2006, 7:29 PM

Quote:

Originally Posted by -KoMoD0-

Input is 16v AC 1.25A

I was pulling 9v 1A

Thus the input DC voltage is about 23V, and the voltage drop across the regulator is about 14V.

Assuming a temperature rise of 40 degrees is acceptable, you need a heatsink thermal resistance of about 2.9 degrees per watt or better, which is rather chunky.

Something like Jaycar's HH-8570 would do it.

-KoMoD0- · 23rd November 2006, 11:25 PM

Changed it and its still dead

Im getting 25v across the in and out legs (1st and 3rd pin's), could this mean ive killed the pot somehow as i did use some hot glue on it?

dohzer · 23rd November 2006, 11:33 PM

Quote:

Originally Posted by -KoMoD0-

Changed it and its still dead

New sample order time?

-KoMoD0- · 23rd November 2006, 11:35 PM

Its DIY, dont want to do it again as i had to use of of those shitty dalo pens

nux · 23rd November 2006, 11:37 PM

Check the pots resistance with a multimeter, show us how you've connected it all. Are you sure you haven't shorted any of the pins together (especially the heatsink)?

-KoMoD0- · 23rd November 2006, 11:43 PM

Ill do it tomorrow, heat sink is just connected to ic and is touching nothing else

It only gives me 1.25v on the output now

dakiller · 24th November 2006, 12:06 AM

Sounds like the adjust pin is shorting to ground (dead pot, bad wiring somewhere?), it certainly isn't dead

Goth · 24th November 2006, 1:10 AM

Quote:

Originally Posted by dohzer

New sample order time?

It's a LM317.

Even if you could get it as a sample from any of the manufacturers that do samples, you'd never be justified in sampling it.

-KoMoD0- · 24th November 2006, 5:05 PM

Its classed as junk atm, changed pot, double checked all wiring. Only think i haven't touched is the transistor, caps and diodes

wabbit · 24th November 2006, 5:37 PM

Output of 1.25 volts suggests that your "Adjustment" terminal is grounded (connected to the negative supply) somehow. Can we see a circuit diagram please?
Cheers
Rob

23rd November 2006, 5:53 PM	#1
-KoMoD0- Member Join Date: Dec 2005 Location: Western Sydney Posts: 7,735	Power supply help Basically i made a variable DC power supply using a bridge rectifier and a LM317t IC, plugged it in and it was working fine, powered up a few things and noticed the heat sink was so hot it was melting the plastic above it and when i touched it it burnt me. It was a pos clip on type so i pulled a massive heat sink out a blown computer PSU and used it. Screwed it to the chip and hot glued it to the board (on the ends) - now it doesn't work. Multimeter was showing weird things (showing 5v and it slowly dropping after it was switched off, then doing nothing and giving me 0.03v). Im off to buy another IC, any idea how this happened and could i have killed something else. btw, dse gave me the wrong caps, i asked for 16v and they gave me 10v = loud bang and bad smells. idiots Cheers

23rd November 2006, 6:37 PM	#2
stewpot Member Join Date: Sep 2001 Location: NW Tas Posts: 1,165	Are you shorting the mounting tab onto anything? __________________ FOLDING LIKE A MOFO ============ TRADING HISTORY ============ COMIC SANS GETS ME HOT

23rd November 2006, 11:25 PM	#7
-KoMoD0- Member Join Date: Dec 2005 Location: Western Sydney Posts: 7,735	Changed it and its still dead Im getting 25v across the in and out legs (1st and 3rd pin's), could this mean ive killed the pot somehow as i did use some hot glue on it? Last edited by -KoMoD0-; 23rd November 2006 at 11:35 PM.

24th November 2006, 12:06 AM	#12
dakiller (Oscillating & Impeding) Join Date: Jun 2001 Location: SE Melb Posts: 6,151	Sounds like the adjust pin is shorting to ground (dead pot, bad wiring somewhere?), it certainly isn't dead __________________ In memory of Cheers Z

23rd November 2006, 6:39 PM	#3
-KoMoD0- Member Join Date: Dec 2005 Location: Western Sydney Posts: 7,735	no unless hotglue is conductive

23rd November 2006, 6:41 PM	#4
nux Member Join Date: Jan 2002 Location: Melbourne Posts: 16,790	The heatsink attachment is Vout (not ground like other Vregs) so if you shorted it to ground or something you might have killed it. The heatsink shouldn't be getting really hot unless you are pulling a lot of current and/or dropping a large voltage across it. What input voltage are you using?

23rd November 2006, 6:49 PM	#5
-KoMoD0- Member Join Date: Dec 2005 Location: Western Sydney Posts: 7,735	Input is 16v AC 1.25A I was pulling 9v 1A

23rd November 2006, 11:35 PM	#9
-KoMoD0- Member Join Date: Dec 2005 Location: Western Sydney Posts: 7,735	Its DIY, dont want to do it again as i had to use of of those shitty dalo pens

23rd November 2006, 11:37 PM	#10
nux Member Join Date: Jan 2002 Location: Melbourne Posts: 16,790	Check the pots resistance with a multimeter, show us how you've connected it all. Are you sure you haven't shorted any of the pins together (especially the heatsink)?

23rd November 2006, 11:43 PM	#11
-KoMoD0- Member Join Date: Dec 2005 Location: Western Sydney Posts: 7,735	Ill do it tomorrow, heat sink is just connected to ic and is touching nothing else It only gives me 1.25v on the output now

24th November 2006, 5:05 PM	#14
-KoMoD0- Member Join Date: Dec 2005 Location: Western Sydney Posts: 7,735	Its classed as junk atm, changed pot, double checked all wiring. Only think i haven't touched is the transistor, caps and diodes

24th November 2006, 5:37 PM	#15
wabbit Member Join Date: Sep 2003 Posts: 408	Output of 1.25 volts suggests that your "Adjustment" terminal is grounded (connected to the negative supply) somehow. Can we see a circuit diagram please? Cheers Rob