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Old 06-20-2010, 02:47 PM #1
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Default Configuring flexdrive for multiple output currents?

I've been trying to work out the resistor layout on a flexdrive V4, because with the new 445nm diodes I want to wire in a switch for high/low power.

I've determined that the flexdrive sets the current based on an input resistance, which is adjusted by soldering bridges across resistors to put them in parallel. From this I can gather that a lower input resistance tells the flexdrive to output a higher current. All good so far.

However, I've just spent half an hour trying to figure out how the potentiometer fits into the equation and all I got for my efforts was a headache from looking through the loupe.

Has anyone tried this mod before, or can somebody give some advice as to the best place to wire in a switch and a resistor if necessary? I'm looking to output about 750-1000mA at high power, and anything below 400mA would be nice for the low power option.


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Old 06-21-2010, 01:52 AM #2
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Default Re: Configuring flexdrive for multiple output currents?

Another issue was brought to my attention - adding wires to this part of the circuit may make it unstable. Is anyone able to comment on that?
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Old 06-21-2010, 04:49 AM #3
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Default Re: Configuring flexdrive for multiple output currents?

I would ask Dr. Lava. He knows these drivers better than anyone!!! He's a really nice guy and I'm sure he would answer your questions but sometimes he doesn't reply to PMs but once a week.

http://laserpointerforums.com/members/drlava/
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Old 06-21-2010, 06:12 AM #4
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Default Re: Configuring flexdrive for multiple output currents?

You'll probably want to use some other driver than that one. Dr. Lava's driver uses chips that use an oscillating voltage to be able to create the current source at the needed voltage. This can make them sensitive to the layout on the PCB, because at higher oscillating frequencies, traces (or wires) can act like capacitors or inductors, affecting the circuit. It may also not be good to be able to instantly switch the resistances at those points on the board as it could cause a rapid change that may affect how current is regulated, damaging the circuit. Another thing to worry about is that switches can have momentary disconnects when switched, which can affect the circuit as it requires a load at all times.

Ask Dr. Lava though. The circuit may still be robust enough to allow those modifications. It may just be cheaper and more effective to use a different current regulation system (like with transistors or something).
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Old 06-21-2010, 09:21 AM #5
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Default Re: Configuring flexdrive for multiple output currents?

Thanks Bionic-Badger. I've sent a PM to Drlava, I'll let you know his response if I get one.

The reason I ask is that I already have a flexdrive spare from another build and the flexdrives have proven to be very reliable in other projects.
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Old 06-21-2010, 10:23 PM #6
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Default Re: Configuring flexdrive for multiple output currents?

Reply from DrLava:

"Hi, the pot is in series with those 3 resistors, and the signal is DC. However, at startup there is a transient so you should use a twisted pair to minimize inductance. You can set up a switch between to power outputs, I would suggest using a second potentiometer that is switched in parallel with the existing one."

I'll give it a try and post my results back here in a week or so when the materials have arrived.
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Old 06-21-2010, 10:33 PM #7
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Default Re: Configuring flexdrive for multiple output currents?

Sounds good - it can be difficult to see the traces on his boards. As he said, twisted pairs and short, thin wires are vital, combined with strong solder joints. Also be careful to not switch modes whilst the driver is running, that could create a whole manner of bad consequences...
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