[SOLVED] problem with lm317 current regulator; current too high

[wr000m]

Hi,

I've been dabbling in electronics for a year or so, and am trying to get my first larger circuit to work properly.. The digital stuff seems simple enough, but analog stuff confuses me The project is a control circuit for two panels of UV leds for exposing circuit boards. Otherwise everything seems to work OK, but my LM317 regulators don't work as I want them to. I'm trying to achieve a 0.625A current through the led panels, but the LM317's pass through approximately 0.8A (much more with a higher voltage power supply). I've placed 2 1ohm resistors in series between the output and adjustment pins, per the example in the datasheet, but the voltage between the output pin and the adjustment pin doesn't stay at 1.25V.

I've tried adding filter capacitors to fix the problem -- I've tried placing a 0.1uF, a 100uF and a 1000uF filter capacitor before the regulator, and a 1uF capacitor after the regulator; these didn't have any effect. I've also tried replacing the regulators, and verified that they work in a voltage regulating configuration.

As a power source I'm using a cheap 16V laptop power supply I got off ebay -- could this be a problem?

Also -- the regulators are screwed to a piece of aluminum to act as a heatsink, and they don't get very warm.

Attached are pictures of the board etc.. The relevant part is a bit messy, since I've been soldering and resoldering it.

Feel free to criticize my board layout etc; I'm not very experienced at this yet

(EDIT: I just noticed it says join date 2006 -- apparently I've been lurking around longer than I thought. Still, I hadn't really made an actual working circuit until last year )

 

[KJ6EAD]

How many volts would your regulator have to produce at the output to deliver 625mA?

 

[wr000m]

Somewhere around 12v I guess, now it's at slightly above 14v.

 

[KJ6EAD]

How are your LEDs wired? At a glance, your emitter board looks like it wants almost 40V to drive 160mA.

 

[wr000m]

The leds have a voltage drop of around 3 volts; they're wired in 12 parallel strips of 4 leds in series, which should require about 12 volts.

 

[alexan_e]

I think I see a short circuit in your pcb



Alex

 

[KJ6EAD]

You could test the performance of the LM317 current regulation without the thermal effect of the mosfet by supplying a hard wired ground to the emitter board in place of the mosfet.

 

[wr000m]

alexan_e: Sure looks like it in the picture. I checked it with a magnifying glass, however, and it seems to be fine. Also, that part of the circuit works as planned

KJ6EAD: Thanks, I'll try that this evening and see what happens!

 

[ZASto]

For reliable LM317 operation, input voltage should be at least 3V higher than output voltage, sooooo, take your meter and check your voltage at the LEDs (output voltage). Your LEDs seems to have more than 3V drop in forward direction.

 

[wr000m]

The voltage at the leds is about 14.2 volts at the moment.

I was originally using a 20V power source, which led to the regulators running much hotter (the heatsink was about 80 degrees C). In this configuration the regulators let through about 1.3A, and the output voltage was only slightly higher than now. So the input voltage being too low wouldn't explain this I think? I switched to a lower voltage power source to avoid burning the leds while I tried to figure this out.

 

[alexan_e]

In your first post you have a picture of an oscilloscope showing some pulses (65KHz),
how are there pulses related with the leds,
do you use a constant drive for the leds of are they switching?

Alex

 

[wr000m]

Hi,

The oscilloscope picture show the output of the regulator; it's at 14.2V with those ~400mV spikes.. The LM317 is a linear regulator, but my power supply is a laptop power supply which I guess is switching. I really don't know what to make of it -- is it possible that the power supply is so noisy that it causes the LM317 to work incorrectly? That's why I tried adding the different capacitors. I guess I need to start reading up on the physics of it all if I want to figure this out..

 

[alexan_e]

You can try with a LM317 regulation (connected as voltage reg) between the psu and the current regulators on the board to reduce the noise/ripple
and feed a more constant voltage to the board and check if that fixes the problem.
About the other 20v source that you have mentioned, was it pulse too?

Alex

 

[wr000m]

Thanks, I'll try that. Yes, the 20V source was a similar laptop power supply I got for cheap on ebay.

 

[FvM]

According to the photo, you mounted both LM317 on a common heat sink without individual isolation. That can't work, because you are shorting the outputs.

 

[wr000m]

According to the photo, you mounted both LM317 on a common heat sink without individual isolation. That can't work, because you are shorting the outputs.

THANK YOU, that turned out to be the problem! This is just the sort of stupid mistake I assumed I'd made but couldn't figure out
I started with separate heatsinks, but with a higher voltage power supply, so the smaller heatsinks got quite warm -- I thought a big piece of aluminum would do a better job.. I only measured the current after this.