Triple Switchable Output, Lab Style PL520 Build w/Custom Linear Power Supply

[aaronnoraa]

Just thought I'd share this PL520 build I did a few months back to add something to the sound of tumble weeds rolling through the lands of LPF builds lately

I wanted switchable output with the lowest output having some degree of eye safety (keeping in mind something should never actually be considered "eye safe" from any laser) so I gave it outputs of <5mw, ~40mw and ~110mw in a switching network I designed using a double pole switch that controls the output by changing between program resistors with the off position being the lowest setting, the other two positions adding parallel resistance (so, technically less resistance...higher current) on top of the base resistor set. Linear regulator is the LM317 with ripple smoothing, spike filtering and full bypass caps Eight Ways to Sunday. I found the G7 had a nice clean output and stuck with it, also tried the acrylic which looked good as well but the output fell noticeably and the 3 element just looked dirty (bad dirty).

Speaking of dirty, I couldn't stop laughing after looking at the picture of the LM317 mounted to the case before I soldered it. Seriously didn't end up like that on purpose.

Here's the pics...

 

[aaronnoraa]

That sunbathing LM317...

 

[aaronnoraa]

...and the rest of the pics of it finished in my eh, shop. I'm happy with the way the micro led power indicator turned out, simply drilled a hole and slapped on the backside.

 

[BobMc]

Very nice build! Love all the pics. :gj:

 

[badboybilly]

Nice job, very nice,
Yeah not many builds being posted lately, hopefully I'll have something to show-contribute tomorrow!

 

[BobMc]

Badboy with a new build thread. My hearts starting to beat faster.

 

[paul1598419]

Interesting. I haven't seen this kind of build using an LM317 like this before now. I see you prefer using proto-boards over printing circuit boards. That is fine for one of a kind builds like this. +rep.

 

[Cyparagon]

Physically switching the sense resistors is a ballsy move, man. You have no feedback for several milliseconds; caps or not, this is gonna cause some spikes.

 

[aaronnoraa]

Physically switching the sense resistors is a ballsy move, man. You have no feedback for several milliseconds; caps or not, this is gonna cause some spikes.

Somehow missed your post until now, sorry for the late reply. I turn the unit off before switching the output level..

 

[Benm]

Switching is not going to be that much of a problem really, the switches go open circuit between positions so there should only be power drops, not spikes.

The thing i dislike about it is mounting the lm317 electrically connected to the enclosure: the middle pin and tab of an lm317 are connected to it's output. The unit may still work, but i really prefer to have the case either isolated completly or connected to ground. If you put this on top of somthing that has it's case connected to ground it would short out the output. In this case it'd probably just turn the laser off and make the lm317 go into self-preservation shutdown, but it's not a good idea to begin with.

 

[aaronnoraa]

The thing i dislike about it is mounting the lm317 electrically connected to the enclosure: the middle pin and tab of an lm317 are connected to it's output. The unit may still work, but i really prefer to have the case either isolated completly or connected to ground.

The LM317 is insulated from the case, you can see the mica wafer in the picture of the regulator... the screw is also going through a teflon shield and washer isolating the securing side of the metal tab.

 

[Benm]

Ah, in that case no problem at all. I figured it was just some heatsinking pad you used there as the bolt is metal.

I know of the insulating washer being available, but i prefer to use nylon bolts and nuts when mouting things electrically insulated. It's not required by any means but more easily to identify as electrically insulated of anyone attemps to do further work on the circuit. I figure it cannot get hot enough for the nylon to be a problem given the internal thermal limiting in the lm317.