Strange results from DDL driver

Ive built the DDL driver as specified at Laser driver - It can be done.

I will be using it to drive an 808nm 300mW LD. Ive determined according to consensus on this forum that about 400mA is optimal for this diode, and I built the driver with 450mA as a maximum. Everything is functional and seemed to be working perfectly with the test load until I noticed something I have not been able to understand.

My potentiometer is set at 2Ohms which is its minimum, I have two 1.5Ohm resistors in parallel adding 0.75 Ohms of resistance for a total of 2.75. Using the same calculation as on the above page:

Reference Voltage of LM317: 1.25
Ohms of resistance: 2.75
(1.25/2.75)*1000 = roughly 454mA as a maximum current.

When I measure the resistor on the test load, sure enough I find about 460mV, however, this value seems to fluctuate in a strange way. It starts out reading around 700mV, and it seems like it may be settling on 460 once the LM317 begins to heat up. It gets quite hot. Even then, though, it still reads anywhere between 700-780 at times. Im not sure what determines this.

I am even wondering if Ive damaged my multimeter, because earlier today (before this problem arose) I connected the multimeter to the test load while on the 200mV setting, which was one setting too low. It should have been on 2000mV to read the 450mA I was expecting. Could this have been the problem?

I forgot to mention, once the test load reads 460mV, it stays there and doesnt fluctuate at all. The value only changes after ive removed the multimeter leads and found a stable value again. The same goes for the other value, when it reads around 700mV it doesnt seem to change at all unless I remove the leads and try again.

Did you try to cool the Lm317 with a heatsink?

Yes, the LM317 has a large heatsink bolted to it with thermal conductive gel, and the heatsink gets quite hot during operation.

I also forgot to give you some information about the power source. It is a Ni-Cd rechargeable battery from an RC car, outputting about 7.5V. On the Laser drive site listed above it says that you could put 12V into the thing without worry, as the LM317 will dissipate whatever voltage it cannot use into heat. In my system there should only be about 2 extra volts here, which is why its strange that it heats up alot.

Do you probably have a picture of this setup when you are measuring the circuit, or just a picture to see what could have gone wrong?

BTW 2V*0.8A =1.6W still

Ive taken a photo of the complete circuit. Here it is:



My camera is not the best, so Ive also drawn up a simple circuit diagram. The circuit is soldered onto stripboard, according to the included indication of connectivity.

circuit diagram.jpg

(I couldnt upload it here because of the images dimensions.)

I am thinking the trim pot is not rated for the power your expecting it to pass...

I think you may be right, hmm...

This is an 89PR100
I found a data sheet here:
89PR100 - BI Technologies - datasheet

If this is the case, what is happening? The resistance of the pot is going down? It has a minimum of 2 Ohms and at this setting the current should be 400 but is often 720, this would mean that its only got 1 Ohm of resistance at that point. Does that happen when you overload something? If that were the case, would the resistance be normal when the circuit is off?

I still read the correct resistance from the potentiometer when the circuit is off. 2 Ohms at minimum setting.

The resistors are each 1 Watt, so they are fine I think.