DIY inexpensive thermopile sensor mod from coherent board

[drlava]

The coherent power board discussed previously could be a good candidate for a laser power sensor, with only the need for a resistor and a multimeter with a mV range.

This board has popped up as surplus recently and I was sent one to see if it could be useful as a laser power meter. I looked up datasheets, drew circuits, and in general tried to figure out the purpose of each part. The board functionality can be divided into what appear to be 4 general areas:

- Laser diode driver
- TEC controller/power supply
- Thermopile amplifier for power readings
- Thermopile preheater/temperature regulator

The sections are very roughly outlined here:


High-resolution
The left board edge connector appears to be power supply/control and the right edge connector appears to be intended to connect to the head umbilical.


In order to use it as a power meter, with a 1mV/mW response, you need:

- a 1.5k resistor (or a 5k variable resistor if you're going to calibrate it yourself)
- a 12V power supply capable of sourcing 12V 100mA and -12V 50mA (or two regulated 12V wall-warts with isolated secondaries)

Instructions:
Connect a 1.5k resistor (5k variable resistor for calibrating if you want) across R15 in the Pile amp section, solder a lead to the square pad of CR6 in the Pile amp section to connect to your multimeter.
Disconnect the multicolored heater cable from the back of the sensor. Put your multimeter in 200mV mode for measurements up to 200mW. Fortunately for us, the unit has easy-to access test point wire loops coming up off the board. Power the unit, connecting your supply ground to TP19, +12V to TP21, and -12V to TP20. If the output to your meter starts off slightly negative, measure the difference with and without the laser.

For added stability, attach a tube to the front of the sensor to limit air currents interfering with the measurement.

[The_Doctor]

Or you could just use a 5VDC to 24VDC isolated power converter module and a dual op-amp and a couple of pots and a few passive parts to get a single range to over 20 watts, with a resolution capable of fractions of a milliwatt, and a cheap sensor off eBay or made as I described in a thread here somewhere. Whole board runs off a simple 5V supply, gives accurate zero setting, is built cheaply from very standard parts, is tiny enough to fit in a matchbox, and will soon be public domain because I'm going to publish the circuit here complete with PCB artwork and parts list and schematic soon. Might even have a few pre-drilled PCB's and assembled units for sale.

[drlava]

Or you could just use a 5VDC to 24VDC isolated power converter module and a dual op-amp and a couple of pots and a few passive parts to get a single range to over 20 watts, with a resolution capable of fractions of a milliwatt, and a cheap sensor off eBay or made as I described in a thread here somewhere. Whole board runs off a simple 5V supply, gives accurate zero setting, is built cheaply from very standard parts, is tiny enough to fit in a matchbox, and will soon be public domain because I'm going to publish the circuit here complete with PCB artwork and parts list and schematic soon. Might even have a few pre-drilled PCB's and assembled units for sale.

Yes, one could build yours, or buy yours, or even buy someone elses. This was only presented as an option that requires minimal effort and expenditure, and uses a professional meter head.

And.. my sig will be gone soon, so don't destroy me.

[The_Doctor]

And.. my sig will be gone soon, so don't destroy me.

Ha! That had me going for a moment, I'd completely forgotten my sig on this board.

Re meter heads, that's the ideal, those often turn up without meter circuits. That's why I made my circuit. Is especially useful given that making the entire sensor head is trivial for anyone with a lathe and a block of aluminum and some activated charcoal powder. The only awkward thing is the calibration.

My ideal is to make the circuit so small it can be tucked into a tiny cavity in the back of the sensor block. Then there can be four sockets (many already have two for calibration, two for signal) wired as ground, cal input, 5V input, signal out for 1mV/mW at up to 20 or 30 watts. It should be easy enough to punt those out at a SMALL fraction of what Scientech and the likes are used to charging for them.

[mixedgas]

I've got a spare 210 board setting here for the first person who agrees to take 30 minutes to RE it and post the schematic. Just add 100 uA meter movement and Sensor and two 9V batteries. I'll pay low and slow shipping to anywhere in the western world. I'm too damn busy at the university at the moment to do anything with it.

Steve Roberts

[marconi]

Send it here Steve, I'm always a gluten for punishment.

[mixedgas]

Send it here Steve, I'm always a gluten for punishment.

Done! On its way

[Laser Ben]

I just bought one this morning so hopefully it'll be here this weekend and I can RE it.

[dar303]

How is the preheater used in the original application?
Is it used to heat the thermopile above ambient temperature like a crystal oven or used to calibrate in some way?

[drlava]

How is the preheater used in the original application?
Is it used to heat the thermopile above ambient temperature like a crystal oven or used to calibrate in some way?

In this case, the heater merely keeps the head at a constant temperature whether it's reading 50mW or 3W of input power. This serves to increase the measurement accuracy somewhat.