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Old 05-18-2009, 09:31 PM #1
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Default Math on building custom drivers? Help please.

Ok, my ultimate goal is to build a nice IR spotting laser for use with NVGs. Low power, like maybe 5mw.

What I'd really like to do is learn all the math involved in building the appropriate driver, so I can know exactly what rating of components to use.

The other thing to consider is that I'd like to make everything fit inside something no larger than a pocket flashlight like a pentagon.

Any help ya'll could throw at me would be most appreciated. Or if this has been answered in detail elsewhere, a quick point in the right direction would be just as good.

Thanks
-Andy


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Old 05-18-2009, 11:10 PM #2
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Though it might not really teach you anything, you might want to consider a flexdrive, since it's a 9X12 mm package, that will provide the proper current to almost anything and takes 2.75 - 5.5V.

You should check that out, see if it helps

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Old 05-19-2009, 02:00 AM #3
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Thanks, that drlava thing looks pretty good for what I need.

Now to learn enough not to break it trying to put it all together!
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Old 05-19-2009, 06:16 AM #4
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One word of advice: NEVER use batteries fresh from the charger. I managed to fry mine that way yesterday
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Old 05-19-2009, 05:33 PM #5
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One word of advice: NEVER use batteries fresh from the charger. I managed to fry mine that way yesterday
The laser or the batts?

Is it because the batteries are putting out a higher than listed voltage with a fresh charge?
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Old 05-19-2009, 05:47 PM #6
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The driver. And yes, as Bob_Boyce explained it to me, it's because fresh from the charger, the voltage is abnormally high for a while, so you should let it settle before connecting them to the driver, lest you fry it...
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Old 05-20-2009, 01:25 AM #7
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Hey ThunderBird89, you think we can still see some math? I'm also trying to figure all this out. I ordered a PHR sled yesterday and I want to learn how to build a driver for it instead of just buying one lol



Thanks
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Old 05-20-2009, 01:56 AM #8
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Well, here is your starting point for building a simple current regulated driver... It uses the LM317T linear regulator to regulate voltage or current. The LM317T uses an internal reference voltage of 1.25V, so using Ohm's law we can deduce a 10ohm resistor will give an output current of 125mA. (1.25volts/10ohms=0.125amps).
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Old 05-20-2009, 03:44 AM #9
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Quote:
Originally Posted by ThunderBird89 View Post
The driver. And yes, as Bob_Boyce explained it to me, it's because fresh from the charger, the voltage is abnormally high for a while, so you should let it settle before connecting them to the driver, lest you fry it...
Good deal. I'll be sure to test my batteries before using them.

Thanks for the link, lobster!
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Old 05-20-2009, 04:24 AM #10
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awesome! That does help pseudolobster. How would you incorporate a potentiometer in there? And how much resistance should it be?
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Old 05-20-2009, 04:57 AM #11
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hey bhank. go to rog8811's profile and check out his link in his signature. he has all the steps there. and how to make a dummy test load for setting the driver.
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Old 05-20-2009, 03:12 PM #12
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Well if it wasn't for the ICs in these drivers I could borrow my old circuits book from a friend and do the math if I knew what was needed...
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Old 05-20-2009, 03:28 PM #13
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Easy as they are to build, I wouldn't exactly recommend the 317 drivers, since they need more power to operate (~2V above the diode voltage), they are buck only, while the FlexDrive are buck-boost drivers that can take a much wider range of voltages for input.
Also, if it gets overvolted, the 317 driver shorts out, passing on the full spike, and killing the diode, while the Flex absorbs the overvolt, at the cost of it's life. For a cheaper diode this might not be much of a problem, but for a more expensive one, it might be a better idea to use the admittedly more complicated and more expensive Flex...

Just my two cents, though

Unfortunately I can't give you any math, since I suck at that. I can see why something is the way it is, but can't really explain it, let alone calculate it. I prefer to use online or offline programs to do that for me, the arithmetic coprocessor never makes mistakes

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Old 05-20-2009, 04:21 PM #14
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Quote:
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The driver. And yes, as Bob_Boyce explained it to me, it's because fresh from the charger, the voltage is abnormally high for a while, so you should let it settle before connecting them to the driver, lest you fry it...
I always check my freshly charged batteries with a multimeter before I stick them in my lasers and I have never once seen any such issue. That includes both protected and unprotected cells.
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Old 05-20-2009, 04:26 PM #15
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Mine were protected cells, and he said that because of 'Surface Charge', the actual voltage of a freshly charged NiMH cell, nominally a 1.5V, may reach up to 2V. Now, three of these (intended to power the laser with 4.5V), were apparently enough to fry it with 6V
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Old 05-20-2009, 10:03 PM #16
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Quote:
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...
Also, if it gets overvolted, the 317 driver shorts out, passing on the full spike, and killing the diode, while the Flex absorbs the overvolt, at the cost of it's life.
The LM317T has a max voltage of something like 35V... I don't even have an adapter capable of supplying that... Unless someone happens to accidentally wire together 20 batteries instead of 2, I highly doubt this will be a problem. Compared to a flexdrive, with a max voltage input of 6v, it's incredibly easy to overvolt.

For what it's worth, the 317 is very hardy and resilient compared to the flexdrive, which is really very finicky... Hook it up in reverse, hook the diode in reverse, power it open circuit, or overvolt it it will instantly kill the driver... Considering flexdrives cost more than double what we pay for diodes, it seems silly to want to try and protect your diode by killing an expensive driver.
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