Simple Single Battery LPM

[ped]

Thats a superb LPM , I like the neat , no fuss approach.

Very tidy well done.

 

[brianpe]

Wow... that's a really compact and sleek setup!

I'm working on creating one myself, and I have a couple of questions.
- How much current does your voltmeter consume by itself?
- How much current does the OPHIR consume? (I heard it's 5mA, but want to just verify)
- How much current does the total setup consume (at your supply of 9V)?

These are excellent questions and if I was a good engineer I'd have answers for them, but I never measured the draw. My LPM is in my garage in my house, which is currently being remodeled, so getting to it is a challenge. I will try to get to it and measure currents on it this weekend and reply back to the forum. Until then, here are some somewhat made up answers based on data sheets:

The voltmeter current draw is listed at 200ma.

I couldn't find current draw for the sensor anywhere (the board will provide up to 100ma for both + and - supplies). Let's assume you're right at 5ma for the sensor (I'm assuming 5ma for + and 5ma for - supplies). That's 10ma total + a couple of resistors on the board to ensure the minimum stable load of 4ma for each side of the converter. The converter is also running a 7805 and 7905 regulator. I don't have efficiency ratings for those but their quiescent current is 5ma and the draw is so low that's probably close enough. That gives a total of 28ma out of the converter. The converter is rated to be 79% efficient, so that ends up being a total draw into the converter of 35ma.

Finally, there's a larger 7805 supplying the voltage to the converter and the voltmeter. Assuming the 5ma quiescent current again (which is wrong, but I don't know the efficiency) you have a total of 240ma of draw.

Any bets as to how far off the actual current draw is when I measure it this weekend?

 

[Tw15t3r]

These are excellent questions and if I was a good engineer I'd have answers for them, but I never measured the draw. My LPM is in my garage in my house, which is currently being remodeled, so getting to it is a challenge. I will try to get to it and measure currents on it this weekend and reply back to the forum. Until then, here are some somewhat made up answers based on data sheets:

The voltmeter current draw is listed at 200ma.

I couldn't find current draw for the sensor anywhere (the board will provide up to 100ma for both + and - supplies). Let's assume you're right at 5ma for the sensor (I'm assuming 5ma for + and 5ma for - supplies). That's 10ma total + a couple of resistors on the board to ensure the minimum stable load of 4ma for each side of the converter. The converter is also running a 7805 and 7905 regulator. I don't have efficiency ratings for those but their quiescent current is 5ma and the draw is so low that's probably close enough. That gives a total of 28ma out of the converter. The converter is rated to be 79% efficient, so that ends up being a total draw into the converter of 35ma.

Finally, there's a larger 7805 supplying the voltage to the converter and the voltmeter. Assuming the 5ma quiescent current again (which is wrong, but I don't know the efficiency) you have a total of 240ma of draw.

Any bets as to how far off the actual current draw is when I measure it this weekend?

I'd say your calculations shouldn't be far off! I have a circuit in the design stage as well, and I did the calculations the same way you did, except that I added in extra leakage current where ever there was a potential divider. Trouble is, the estimated current I have as a final figure is rather close to my design limit, causing me concern.

Apparently, according to a certain A/D conversion chip I looked up that's meant to power 7 segment displays, each segment uses 8mA of current when turned on. So you should see higher power consumption when the reading is of certain figures (mostly involving 8s). For yours, since no one goes to 8W currently, the max consumption would occur at 2.88 or 3.88 (same number of segments), and so should consume a maxmimum of 20 x 8 = 160mA from just the LEDs alone. I guess why it's rated 200mA is probably because of 18.88 when displayed, is 24 segments, so 192mA (assuming your meter only reads till 19.99). Add on a negative sign, and that's exactly 200mA!

So... Indeed, your estimates should really not be far off. I'd say give or take 10mA?

Thanks for going through the calculations for me! +1

 

[lasersbee]

Nice clean design..
One question.
Why does your Circuit/DPM only read in 10mW increments ??


Jerry

 

[brianpe]

Nice clean design..
One question.
Why does your Circuit/DPM only read in 10mW increments ??


Jerry

Good question, Jerry. The meter is only 3 1/2 digits. The range is selectable but I fixed at this range because it seemed more generally useful. A 4 1/2 digit meter would have been better.

 

[cheech226]

nice work! i could handle 10mW resolution above a watt easy.

 

[brianpe]

Ok. I just measured the current draw. With no input, the Ophir sensor draws 3.1ma on each rail, or 6ma total. The claim of 5ma sounds right on. I imagine it will go up slightly with a significant laser input and probably maxes at 5ma for each rail. The total draw for the circuit was only 150ma with it was reading all zeros. I can see that going up or down depending on the reading but it was a little lower than I expected.

On the subject of accuracy it would have been much better to have it auto-select the decimal placement. Below 1w, 3 digits of precision. Above, two digits. Homework for whoever makes the next board.

 

[Tw15t3r]

Thanks for the info! (would +rep but can't yet)
I haven't seen any auto deimal selection circuits online, but I bought a voltmeter off ebay that does 0.000V to 3.300V so that's no prob for me! There are also ones which do 0.000 to 30.000 which is enough for even a CO2 laser.
Why deal with less precision the higher you go when you can have more regardless of how high you go?

 

[LaserCo]

You did a very nice job on this project brianpe, 7 out of 11 posts were here so I will say right here:
Welcome to Laserpointerforums.

 

[brianpe]

You did a very nice job on this project brianpe, 7 out of 11 posts were here so I will say right here:
Welcome to Laserpointerforums.

Thanks, LaserCo. I certainly spent enough time lurking on here before building or posting anything. This place is a wealth of information. It feels good to finally contribute.