Error designing laser circuit - please advise

0 points · submitted Feb 2026 by u/thisusedyet

Tried making my own circuit board to replace part of a laser alignment jig that was sent out to another office.

I used a ROHM semiconductor diode (RLD65MZT7-00A), and had a 15V 1A power supply lying around, so got a voltage regulator (ZMR250FTA) to step down from 15V to the 2.5 V 50 mA the diode is looking for.

That being the case, why did the laser diode light brightly for about 10 seconds, turn off, and get VERY hot when I plugged all this in?

...and as a secondary question, how do I ensure a Thorlabs S7060R diode holder stays flush to the board? There's a bulge on all three legs right underneath the plastic, it wobble towards the nonleg side when you go to solder it.

VR values

laser datasheet relevant values

https://preview.redd.it/qojv9vxiuakg1.png?width=651&format=png&auto=webp&s=c71f28e621fdb706bfcff0141550e0ee4d50715b

5 comments · 50% upvoted · other discussions

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ZanyDroid 4 points · Feb 2026

My first guess is it was supposed to be constant current driven.

Going to check back to see if my guess is right

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70wdqo3 5 points · Feb 2026

There's no guarantee that the laser will draw 50mA at 2.5V and there's nothing to limit it. The regulator is specified for that current but won't limit itself if the load draws more. As the laser heats up it may draw more current at the same voltage, resulting in thermal runaway.

Also, the regulator's absolute maximum allowable power dissipation is 500mW. Assuming the laser does draw exactly 50mA at 2.5V, that's (15-12.5)/0.050 = 625mW being absorbed by the regulator which has no heatsinking so it may have been the first component to fail.

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thisusedyet [OP] 1 points · Feb 2026

ok, so I need to math out a resistor?

EDIT:
Most likely between the terminal and the Input of the voltage regulator?

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thisusedyet [OP] 1 points · Feb 2026

Just to make sure I understand this correctly (since I blew it the first time) - if I were to stick a less than or equal to 50 ohm resistor to soak up the excess 125 mW at 2.5V, that should keep things from burning out, correct?

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al2o3cr 2 points · Feb 2026

The forward voltage for that laser is specified as 2.3V "typical", and the I-V curve is exponential, so applying 2.5V without current limiting is likely causing it to try to draw 100+ mA

The simplest fix would be a series resistor between the regulator and the laser - just like driving an LED

A constant-current driver would be the next upgrade

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