DIY Laser Power Meter

[IgorT]

Exactly.  You need the thermal mass to conduct the heat from the thermopile disc.  Reducing that mass will degrade the performance.

I see. Thats what i was afraid of. But it's ok. It will be slightly larger, but still not that bad.

Thanks!


Oh, i also noticed mine had 53.6° C written on it. Is that the temperature it was supposed to be warmed up to on the original boards, for maximum accuracy?

 

[knimrod]

[quote author=knimrod link=1200112201/360#364 date=1207319499]Exactly. You need the thermal mass to conduct the heat from the thermopile disc. Reducing that mass will degrade the performance.

I see. Thats what i was afraid of. But it's ok. It will be slightly larger, but still not that bad.

Thanks!


Oh, i also noticed mine had 53.6° C written on it. Is that the temperature it was supposed to be warmed up to on the original boards, for maximum accuracy?[/quote]

I think it must just be the temperature that it was regulated to. I don't believe the temperature regulation was there for accuracy but to prevent drift. The regulated offset from the heated mass was simply nulled out in the measurement.

 

[IgorT]

I think it must just be the temperature that it was regulated to.  I don't believe the temperature regulation was there for accuracy but to prevent drift.  The regulated offset from the heated mass was simply nulled out in the measurement.

Yeah, that's what i meant.

With more powerfull lasers, the heat would slowly start saturating the alluminum block, and the reading would slowly drop.
But if the temperature of the block is kept constant, the heatflow into the block will be constant as well.

With the low powers we are measuring this doesn't really matter so much.


Is that diode pressed into the back the heating element? And the big transistor must be a thermistor, to regulate the current into the heater, right?



The building is slowly progressing. I'm having trouble fitting the thermopile next to the DMM. The enclosure is 1mm too small. I calculated everything, so it would fit exactly, but the thermopile is 1mm larger than expected. I'm currently making the enclosure wider from the inside. It'll be VERY tight, but i'm gonna make it work.

 

[knimrod]

[quote author=knimrod link=1200112201/360#369 date=1207327250]I think it must just be the temperature that it was regulated to. I don't believe the temperature regulation was there for accuracy but to prevent drift. The regulated offset from the heated mass was simply nulled out in the measurement.

Yeah, that's what i meant.

With more powerfull lasers, the heat would slowly start saturating the alluminum block, and the reading would slowly drop.
But if the temperature of the block is kept constant, the heatflow into the block will be constant as well.

With the low powers we are measuring this doesn't really matter so much.


Is that diode pressed into the back the heating element? And the big transistor must be a thermistor, to regulate the current into the heater, right?



The building is slowly progressing. I'm having trouble fitting the thermopile next to the DMM. The enclosure is 1mm too small. I calculated everything, so it would fit exactly, but the thermopile is 1mm larger than expected. I'm currently making the enclosure wider from the inside. It'll be VERY tight, but i'm gonna make it work.[/quote]

There is a temperature sensor epoxied into the sensor module. There really is no heating element per se. I think the heat is generated with the TIP-120 transistor mounted to the module.

My apologies if my measurements were off a little.

 

[IgorT]

My apologies if my measurements were off a little.  

Don't worry Knimrod. I intentionally bought a small box. I wanted to make it as small as possible. The thermopile is 51mm, but my LCD is so small, that it will actually squeeze in there next to it.

I grinded the walls of the box away a little from the inside, and it fits now! I just have to be more exact than usual. If it should still turn out too small, i will put the thermopile into the back of the box, which will provide additional shielding from other heat sources.

I'm almost done. Pics soon!



EDIT: The meter works!!! Resulting in some good and some bad surprises of course... Need to sleep tho. Pics tomorrow.

 

[IgorT]

Knimrod.. I was wondering.

I would like to put an EL-foil behind my LCD. It has a driver circuit, which makes 190V peak to peak. The waveform is somewhat filtered, to prolong the EL foil's life and reduce the noise it makes.

I would have to make a 7805 to regulate the voltage for the driver, and i would run it off the same supply as the LCD. This is the datasheet of the driver: http://www.supertex.com/pdf/datasheets/HV857.pdf The switching frequency of the driver is 80kHz and the HV on the foil is ~240Hz.

Do you think, this high AC voltage and the noise could affect the the LTC, since it's so sensitive?



EDIT: Since many of you are using illuminated LCDs, do you perhaps know, if they're EL-Foil or LED illuminated? EL foil illumination is more even, and looks slightly grainy from up close.

 

[IgorT]

My meter just died. It was working fine, then it just stopped. I measured the output, and it shows 8.4V or so.

Looks like my LTC is dead. Should have bought a spare. I probably overheated it, while pulling it out of the board.

 

[phenol]

short its input to gnd and test the output. it should be ~0V

 

[IgorT]

Did that, doesn't work. I get 8.14V in both cases.

I'm pretty sure it's dead.

 

[danq]

My meter just died. It was working fine, then it just stopped. I measured the output, and it shows 8.4V or so.

Looks like my LTC is dead. Should have bought a spare. I probably overheated it, while pulling it out of the board.

Same thing happened with mine - in my case I overpowered it. What are you using for power source?

DanQ

 

[IgorT]

Two 9V Ni-MH rechargables. They contain 7 Ni-MH cells, so the voltage is around 8.4V, but can get to 9.8 when full.

But i measured them, and they were at 9.2V. Regular 9V batteries can also have up to 10V when new.

Do you think i should put a couple more diodes into the circuit, just in case? After the two diodes already in the circuit, the total voltage is 17.6V, but the batteries are not completelly full at the moment.

 

[knimrod]

I've run mine at +/-12V for a short time with no ill effects.. I realize the AMR is 18V but there is still usually some margin beyond that. I've abused the heck of these op-amps and have yet to experience a failure. You may want to re-verify power at the pins 4 and 7. Also ground both input pins (3 & 2) and re-check the output pin (6). Does the op-amp get hot?

 

[IgorT]

It's dead. I'm just gonna have to buy a new one.

I think it might have gotten shorted, while i was testing it, and the circuit was out of the box, on the table. There must have been a short piece of a resistor leg or even a tiny drop of solder, that i overlooked, and it burned out, because i think it did get hot at that point.

I immediatelly disconnected it, checked everything and powered it back on, and it continued working for an hour in the enclosure, so i thought it was fine, but then i turned it off and later back on again, and it was dead. I would have expected it to die immediatelly tho...



I have an unrelated question about it's operation..

Since one of the batteries also powers the LCD (and the batteries themselves don't behave completely the same), the voltages slowly drift appart during use. Does this affect the output in any way? Would it be more consistent if i used a dual LM78L05 power supply? Or is this what the offset circuit corrects?

 

[knimrod]

It's dead. I'm just gonna have to buy a new one.

I think it might have gotten shorted, while i was testing it, and the circuit was out of the box, on the table. There must have been a short piece of a resistor leg or even a tiny drop of solder, that i overlooked, and it burned out, because it did get hot at that point.

I immediatelly disconnected it, checked everything and powered it back on, and it continued working for an hour in the enclosure, so i thought it was fine, but then i turned it off and later back on again, and it was dead. I would have expected it to die immediatelly tho...



I have an unrelated question about it's operation..

Since one of the batteries also powers the LCD (and the batteries themselves don't behave completely the same), the voltages slowly drift appart during use. Does this affect the output in any way? Would it be more consistent if i used a dual LM78L05 power supply?

Can you get a replacement LTC1050 locally? I'll send you a couple if necessary.

The power supply voltages do not need to be regulated or even symmetrical for that matter. Since everything is "ground" referenced, as long as you stay within the common mode range, you're good.

 

[IgorT]

Can you get a replacement LTC1050 locally?  I'll send you a couple if necessary.

That's very kind of you. I hope i can get them locally. I looked online, and found it at one store in another city, but it was the 14 pin version, and they have LTC1051 8 pin dual version. It's not pin compatible, but i could rewire it.

The store in my city doesn't have a website, but if this is an older and fairly common IC, they should have it. If not, there is another store, and if that doesn't help, they can order anything from Farnell.

Thank you for the offer tho. These are quite expensive here from what i've seen. Twice the price you listed at the beginning of this thread. If i can't find it, i would be very gratefull, if you could sell me some. They would probably get here sooner, than if the store orders them from Farnell.


But i really hope i can get it up and running tomorrow.

 

[IgorT]

I'm having trouble finding the LTC1050.

I read the LTC is a direct replacement for a 7650 or 7652, but i can't find the datasheets.

But our stores do have something called ICL7650. Is this it? Would it work in place of the LTC, if i use those two external capacitors?