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Old 06-04-2008, 07:18 AM #1
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Default question - basic circuit design

I have a question about current regulation on a single power-source board that runs either a laser, an led, or both. *See my (kinda ugly) drawing for reference. *



The rotary switch has 4 modes:
Off (not shown)
1 - laser only
2 - both laser and led
3 - led only

Diodes prevent current from Mode 1 or Mode 3 from backing onto the Mode 2 paths. *

Assume the target current levels are 70mA for the laser and 350 mA for the LED. *

For single mode (laser or led) the DDL circuits are easy to design: just choose the sense resistor and pot that provide the current you want for that side. *

My question: *in the drawing, in DUAL MODE, will the DDL's still cause the target current levels to be delivered to each side, or have I completely missed something? *

From a power-delivery-design standpoint, I don't know what to expect for dual mode. *

I guessing that either it will work fine because each DDL will only pull what it needs to deliver its constant current level...

OR

I'm clueless, and current delivery will be totally lopsided because of some "path of least resistance" issues, or something like that. *

Anybody care to shed some light on this for me. *THANKS!


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Old 06-04-2008, 06:09 PM #2
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Default Re: question - basic circuit design

It looks like modes 1 and 3 will work just fine, but mode 2 has a couple considerations.

1. The diodes you have in there a voltage drop, which will add between .5 to over 1V to your total voltage requirement, depending on the current. Also most standard diodes are rated for 1A, make sure you don't exceed that, but they'll still get hot running even at half that.

2. You'll have to make sure your battery source can supply the total amount of current between the two drivers.

That's all I can think of at the moment.
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