Desperate help for a 0-25v power supply project

[kentrol]

I'm short on time so any hel is greatly appreciated.
I'm using a lm317to-220 to regulate the output. The problem im having is as soon as i apply voltage to a load, my output will drop to 1.1 or 2.0v (when it should be 25V). Can anyone help me before i shove my soldering gun in my eye?

 

[BrownOut]

Before you poke our eye out, post your schematic.

 

[kentrol]

i'm trying but i cant really figure out how.

 

[BrownOut]

Click "Go Advanced" at the bottom of the text window. They use the paper clip at the top of the resulting window. The rest is easy. I doubt that anyone will be able to help with the information you've provided so far, but I could be wrong.

 

[kentrol]

i made this fast with diptrace, and i didnt have time for a full tutorial so ill explain what each things are incase you dont fallow:


U3 = 4700uF
U4 = .22uf ceramic
U1 = lm317
VR1 = 10K
R2 = 330Ω
Diodes = 1n4001

oh and u5 is a 10uf cap
I'm using a 10A 24Vac transformer

( the top part is the lm317, the bottom are 2 7805 To-220)

 

[kentrol]

bump
anyhelp out there??

 

[kentrol]

seriously, could use some help

 

[BrownOut]

OK this is a wild shot in the dark. Could it be that your're setting the output without the load, and then connecting the load and getting the low output because you should have set the output with the load connected in the first place?

 

[kentrol]

OK this is a wild shot in the dark. Could it be that your're setting the output without the load, and then connecting the load and getting the low output because you should have set the output with the load connected in the first place?

...that's possible.
maybe this might help:
i put a blue led across my output (forward voltage 3V) and the output maxed out at 3v. Then i put 2 in series and it maxed out at 6V. is that normal for the lm317...?O-o

 

[BrownOut]

If you're putting LED's across the output leads, then the output voltage will be determined by the LED's and not the regulator. A well working regulator should have blown the LED's up.

 

[kentrol]

i know i was flipping **** sideways when that happened. so i need to change my regulator?

 

[BrownOut]

Well, when I say regulator, I mean the whole circuit. You might have a wiring mistake for all I know. You're schematic looks OK, from what I know about the 317, but I'm not an expert and have never tried to make one work. I'm just looking at the datasheet to see if I can see anything.

 

[kentrol]

well im gonna swap out my 317 when im done fixing my blow leds
if you come up with any thing i might have missed, let me know.

thanks a lot!

 

[audioguru]

LEDs blow up when they don't have a current-limiting resistor. Your LEDs didn't so they blew up. LEDs are driven by current, not voltage.

R2 in your circuit is supposed to be 120 ohms or less for an LM317 or 240 ohms or less for a more expensive LM117. Yours is much too high. The datasheet tells you what happens when your R2 resistance is too high.
Since your R2 has a resistance too high then your pot also has a resistance too high.

 

[BrownOut]

I guess it's worth a try,but I didn't see anything in the National datasheet that set a requireent for R1. But there is a requrement for a minumum load, which i guess the lower value for R1 would satisfy. That's why I asked if the OP had the load connected while we was setting the output voltage.

 

[kentrol]

if i changed VR1 to 5K would that work?

 

[BrownOut]

It's a variable resistor. It won't make any real differnece.

 

[audioguru]

When R1 is 120 ohms then the requirement of a minimum 10mA load is satified, even without another external load. The minimum load for the more expensive LM117 is 5mA.

What good is a power supply that is built with the value of R1 too high so that its output voltage rises without an external load??

 

[Mr RB]

I think you might have a wiring problem. Connect your load so the output voltage drops right down, then check the voltages on Vin pin of the LM317 and Vout pin (both with respect to ground).

Make sure you wired LM317 correctly, it is NOT the same pinout as all the other common regulators like 7805 etc!!

 

[audioguru]

I think you might have a wiring problem.

no.
Didn't you notice that R2 is 330 ohms instead of the required 120 ohms or less for an LM317??