# Newbie on a trimmer/trimpot watts and temperature for a laser driver

Discussion in 'LEDs and Optoelectronics' started by Hector Roldan, Feb 23, 2016.

1. ### Hector Roldan

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Feb 23, 2016
Hi there, I've been building stuff for years but as the usual DIY guy, I lack specific knowledge on certain stuff. I always research before asking a question (I'm not lazy and I do respect people time). But I'm having no luck on my search queries now. Sorry if this seems long, I'm just providing the details:

I built a CNC laser cutter/burner/engraver, it works quite well. Sure I also built the laser driver using the LM317. It got hot so I placed a heat sink and a fan, it does wonders, and I'm using 1 watt and 5 watts resistors to avoid heating (old problem with the usual resistors for this driver). The thing is, I originally built a laser driver for each laser diode I have, and now I'm building a variable resistor driver that I could use with all my laser diodes providing the current I need for each case.

So, the usual resistors got hot. The new ones, big, thick, don't get hot.
But the variable resistor/trimmer/trimpot is too small and couldn't get a "1W" version on any store. I suspect it will get hot regardless of the fan above, and I know heat reduces efficiency. The driver would be feeding constant current to the laser for extended periods, not just seconds, so that's my concern (had no problems with the big resistors).

My question, and I would like your help on this, is: What about using another resistor in parallel?
I read that if you only have regular resistors and they get hot, and you can't get 1W or 5W resistors, you can use two in parallel and do the math to get the exact resistance, having two in parallel reduces the pressure/load. I was using 5W version for an 2.6ohm resistor with no problems, so I want to combine a 100ohm variable resistor with a 10ohm 5w resistor in parallel to avoid so much load on the small variable resistor, does the explanation make sense? would you recommend this? I see no way to cool down the trimpot but using a fan.

2. ### Alec_t

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Jul 7, 2015
I wouldn't recommend it. If you were to adjust the variable resistor to a low resistance value you would divert a lot of current through it and cook it. It would be better to make a constant-current source from a power transistor and use the variable resistor to control the transistor current. That way, only a small current flows through the variable resistor.

Hector Roldan and davenn like this.
3. ### Hector Roldan

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Feb 23, 2016
Thanks that's really something, will try.

4. ### AnalogKid

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Jun 10, 2015
Using multiple resistors in parallel or series to distribute a thermal load works very well, but only if the resistors are closely matched in value. If they are 10:1 apart in value, they will be 10:1 apart in heat load.

ak