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# DIY Thermal LPM for under \$50

#### gregma

##### 0
Not the case here, if you have a laser beam with a power of 1 watt, if you pick off half of the beam you only have 500mW. Were that not the case you would break the laws of thermodynamics =P

So for anyone skimming, the ENTIRE beam must be on the TEC for an accurate reading.

I stand corrected! Thank you for the clarification. I'll learn these things eventually.

#### Hibro

##### 0
Can some one please tell me if what is the preferred voltage and current of the TEC?

#### ARG

##### 0
Can some one please tell me if what is the preferred voltage and current of the TEC?

It's a balance. If the voltage is too high there will be some non-linearity problems to deal with because there are more junctions. If the voltage is too low you'll have to set the gain on your amplifier really high which will amplify unwanted noise.

Generally the lowest voltage you would want is about 1V.

#### gregma

##### 0
Question about this new circuit. It's been quite a while since I've read them and there is one point that is confusing me trying to breadboard the circuit waiting for the pcb. I see several +9v and several -9v. I also see 9v1+, 9v2+, 9v1- and 9v2-. I'm thinking these last are the two separate 9v batteries But it looks like the + on 9v2 is connected to the - of 9v1?

#### MarioMaster

##### 0
Question about this new circuit. It's been quite a while since I've read them and there is one point that is confusing me trying to breadboard the circuit waiting for the pcb. I see several +9v and several -9v. I also see 9v1+, 9v2+, 9v1- and 9v2-. I'm thinking these last are the two separate 9v batteries But it looks like the + on 9v2 is connected to the - of 9v1?

Yup that's correct. The new circuit requires two 9v batteries. They are used to provide a larger voltage span for the LM358. The old design had the output of the amp running right at the negative rail which although usually works can cause some voltage offset or an inability to fully zero the meter.

The two 9v wires are connected together so the second battery provides -9v to the LM358's negative rail input.

#### gregma

##### 0
makes sense. Next question. I finally have all the components, and am ready to breadboard while I wait for the pcb. Any way to use say a wall wart rather than two batteries?

#### MarioMaster

##### 0
makes sense. Next question. I finally have all the components, and am ready to breadboard while I wait for the pcb. Any way to use say a wall wart rather than two batteries?

There are a few options, you could use two wall warts (or a PSU with +/- voltage outputs)

The other option is to use an isolated switching converter, but these tend to be noisy and require extra filtering components. There are also charge pump type chips that will also do this.

You can use the isolated output and ground the positive output (9v2+), making the negative output negative to ground. That will be your 9v2-

#### gregma

##### 0
There are a few options, you could use two wall warts (or a PSU with +/- voltage outputs)

The other option is to use an isolated switching converter, but these tend to be noisy and require extra filtering components. There are also charge pump type chips that will also do this.

You can use the isolated output and ground the positive output (9v2+), making the negative output negative to ground. That will be your 9v2-

I'll look into that. I know that the DMM will require a separate vcc in, so I'm trying to find circuits that will take a wall wart of some voltage/amperage output and putting out +9v, -9v, +9v. Proving to be difficult, but will really make for a neat package that doesn't depend on batteries if I can figure it out.

#### DutchLaser

##### 0
Huge bump.

I am back from gone and want to make this LPM.
I do have al the components.
But now the V2.0 schematic (post#541) from Mariomaster is gone!:banghead:

Who has it? Mariomaster hasnt been online for a long time.

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#### MarioMaster

##### 0
Hi guys, I have re-hosted the v2 schematic image on the original post.

#### paul1598419

##### 0
Cool, MarioMaster. The last thing I expected to see was this thread revived. It might come in handy for some new members that don't already know how to do this kind of work. + Rep.

#### lasersbee

##### 0
Hi guys, I have re-hosted the v2 schematic image on the original post.
I see you are using a split supply now...
BTW... there seems to be a short on the
posted schematic...

Jerry

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#### hoo7h

##### 0
Mario you are a legend!
I am looking forward to try and build one of these and learn some more about circuits along the way.

#### MarioMaster

##### 0
I see you are using a split supply now...
BTW... there seems to be a short on the
posted schematic...

Jerry

Well yes if you mean by now you mean 2015 when I originally posted the v2 schematic (post #541)

I do not see any shorts on the board, maybe you are seeing the 9V1- and 9V2+ being connected? This is intentional.

Anyway I think after taking a look at this project with fresh eyes it could really do with a revamp. I would also like to put together a complete BOM with part numbers so you can find the parts more easily. 15mm TECs are also much easier to find these days.

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#### paul1598419

##### 0
Just had a look and I also see the optional low battery indicator. Might as well since you have an unused OP amp just sitting there. I have all those parts already in my stock. What I don't have is a need to build it.