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Old 12-27-2010, 03:10 AM #1
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Default Dumbest question in this forum!

For the record, i feel very embarrassed asking this question considering that i'm majoring in EE.

So the question is why do we need a complex driver to regulate current?

I mean can't we just use a resistor to get the current we want?

For example, if i want to power a 1W 445nm laser diode. i can build a simple voltage divider and hook it up to 2x 18650 batteries in series setup to output 5 Volts. And then hook up a 5 Ohm resistor to the 5V output to get 1 Amp of current? And there i have it, 1Amp @ 5V. Shouldn't that power the 445nm diodo just fine?


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Old 12-27-2010, 03:32 AM #2
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Default Re: Dumbest question in this forum!

I'm not in electronics at all, let alone an electrical engineer, however I can say that batteries will lose current as they depreciate their source of power which I expect is a variable... Just tossing a resistor in front means variable in, variable out which kills our ability to condition the line for a consistent output to a diode.

Again, I could be waaaaaaay off base here, but just looking at the drivers as I have, it seems it's more about providing a consistent output than a reduced output. When the power drops on the battery in any way, so does the current on our diode.

Again, could be off and I expect someone who knows way more about this stuff than I to give you an educated answer... So I'm curious to see what they say. Lol
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Old 12-27-2010, 03:43 AM #3
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Default Re: Dumbest question in this forum!

The forward voltage of the 445nm diodes is very close to the voltage of a fully charged 18650 (4.2V). So, yes, you can power the diode directly off the battery with no regulation if you'd like.
2 18650s will output 8.4V, so that is probably not the best option.

The reasons drivers are used is:
  • They smooth out spikes in current/voltage from the power source
  • They can be easily set to output a specific current value and maintain it - regulation from a driver like the Micro Flex gets the most out of the battery without dropping in power as the voltage sags (like linear drivers or direct driven lasers)
  • Buck/boost drivers can be used with a wide variety of battery types unlike linear drivers or direct driven lasers
So, basically, drivers are used for protection, output stability, and for power source compatibility.

Last edited by RA_pierce; 12-27-2010 at 03:47 AM.
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Old 12-27-2010, 03:45 AM #4
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Default Re: Dumbest question in this forum!

Thats correct enigma. You want a constant current and with only a resistor as the current control device the diode would see spikes that can kill it.

RA beat me to it.
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Old 12-27-2010, 04:00 AM #5
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Default Re: Dumbest question in this forum!

Quote:
Originally Posted by RA_pierce View Post
The forward voltage of the 445nm diodes is very close to the voltage of a fully charged 18650 (4.2V). So, yes, you can power the diode directly off the battery with no regulation if you'd like.
2 18650s will output 8.4V, so that is probably not the best option.

The reasons drivers are used is:
  • They smooth out spikes in current/voltage from the power source
  • They can be easily set to output a specific current value and maintain it - regulation from a driver like the Micro Flex gets the most out of the battery without dropping in power as the voltage sags (like linear drivers or direct driven lasers)
  • Buck/boost drivers can be used with a wide variety of battery types unlike linear drivers or direct driven lasers
So, basically, drivers are used for protection, output stability, and for power source compatibility.
Wait i thought 18650 are 3.7V?
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So initially i thought 2x will give me 7.4V. and i can use voltage divider to get like 5V.

Anyways,
We built a AC to DC power supply last semester with variable DC voltage output. This voltage should be constant. So if consistency is all i'm looking for, then to replace the function of a driver, all i have to do is set the power supply to 5V and hook up a 4 Ohm resistor to get a constant current of 1.25A to power the 445nm laser diode? If so, that definitely make my life much easier.
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Old 12-27-2010, 04:09 AM #6
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Default Re: Dumbest question in this forum!

LEDs, like LDs, and peltiers are current devices, so they ought to be "driven" for best performance.

Heat has some to do with it.

Constant voltage source may put a little too much voltage across the depletion layer in the diode, creating heat.

Also, its easier to simply let the diode naturally drop whatever voltage it wants to, by controlling the current through the device.

You CAN simply use a battery and resistor on a laser for instance, but such a setup does not protect against capacitive spikes from battery terminals, dirty switches, and does not regulate current as the device heats up.

For LDs for instance, we measure output by power vs. current in, since the current through the diode is much more directly related to the output of the device.

Voltage is direct too, but not so proportionally like current is.

If you're going with the resistor route, remember that the resistor will be dropping voltage in series with the diode.
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Last edited by Meatball; 12-27-2010 at 04:11 AM.
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