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Old 12-15-2012, 04:33 PM #1
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Default Voltage discrepancy between energy sources

Firstly, this is roughly what I'm intending to build.

Ugly resistor "farms" to make up for my lack of components.

My main concern is the discrepancy in voltage between the 2X18650cell battery (7.4V) and the WallWart (9V). Should this be a cause for concern?
Besides the extra power lost at the LM317; heatsinking is no problem. I am worried because I am not sure what will happen if I connect/disconnect the input to DC jack while the laser is in operation.

Tempted to add 2 1N4001s into the wallwart to drop it's voltage to 7.6V...

Also, how much capacitance should I use for the input capacitor?

in advance!

Cite:
"Safety" Pin idea
"Ground" Screw idea -> Ground Screw:The Sequel

Edit: I realise the picture is too small, sorry. Scanner DPI limitations.


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Last edited by ilovegm; 12-15-2012 at 04:35 PM.
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Old 12-15-2012, 07:34 PM #2
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Default Re: Voltage discrepancy between energy sources

It shouldn't cause any problems, as far as I'm concerned. The extra voltage will just be dissipated as extra heat, though just to be 100% sure I'd add some more capacitance before the regulator, and maybe 0.1 uF of capacitance after the regulator.
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Old 12-15-2012, 08:11 PM #3
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Default Re: Voltage discrepancy between energy sources

Ideally, you'd have an input voltage that is the output voltage + dropout-voltage of the voltage regulator. If you have less than that voltage, the voltage regulator doesn't work. If you have more than that, and the voltage regulator has to drop the extra voltage and will heat up more. It won't affect the operation of the driver with respect to your laser diode, but you may need to heatsink the LM317 a bit more.

Adding those diodes in series with the power supply will work too, so you can do that rather than increase the heatsinking.
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Old 12-16-2012, 12:52 AM #4
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Default Re: Voltage discrepancy between energy sources

Also, you wired it incorrectly: When using a lm317 or something similar as a current source, the laser diode conntects to the ADJ pin, not the OUT pin. The latter is the output in the intended purpose of the regulator, i.e. supplying a constant voltage, not a constant current.
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Old 12-16-2012, 02:13 AM #5
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Default Re: Voltage discrepancy between energy sources

Quote:
Originally Posted by Benm View Post
Also, you wired it incorrectly: When using a lm317 or something similar as a current source, the laser diode conntects to the ADJ pin, not the OUT pin. The latter is the output in the intended purpose of the regulator, i.e. supplying a constant voltage, not a constant current.
AH! Thanks for spotting it, I labelled it wrongly because I misread the datasheet and took the adj as middle pin.

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Old 12-24-2012, 04:47 AM #6
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Default Re: Voltage discrepancy between energy sources

Your SP0T should be SPST if you intend on adding the necessary ground screw
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Old 12-26-2012, 02:36 PM #7
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Default Re: Voltage discrepancy between energy sources

Cy: Good call! I've seen several people here use Earth screws, but fail to realize that Earth screws are only good for switch-mode regulators at frequencies lower than 1kHz, and that they should actually be using Ground screws for the low frequency signal grounding.

Here's a good tutorial that will help you in your endeavor for a stable laser driver circuit: Ground Screw
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