Old 07-02-2008, 04:14 PM #1
Cyparagon's Avatar
Class 4 Laser
 
Join Date: Sep 2007
Location: Wisconsin
Posts: 9,293
Rep Power: 208829
Cyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond repute
Cyparagon Cyparagon is online now
Class 4 Laser
Cyparagon's Avatar
 
Join Date: Sep 2007
Location: Wisconsin
Posts: 9,293
Rep Power: 208829
Cyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond repute
Default Is there any reason this won't work?

Jfet based. is the capacitance sufficient? I'd rather not attempt stuffing a capacitor the size of my thumb into the thing if at all possible.
Attached Thumbnails
Is there any reason this won't work?-driver_001.png  


__________________
A problem well stated is a problem half solved.
Cyparagon is online now   Reply With Quote


















Old 07-02-2008, 11:10 PM #2
Junior Member
 
Join Date: Aug 2007
Posts: 29
Rep Power: 0
cdanjo is on a distinguished road
cdanjo cdanjo is offline
Junior Member
 
Join Date: Aug 2007
Posts: 29
Rep Power: 0
cdanjo is on a distinguished road
Default Re: Is there any reason this won't work?

What current are you trying to drive the LD with? The problem I see is that the variable resistor has to take the full current of the LD (since the current into the JFET gate is zero), so it's going to have to be a low value, like 10's of ohms. At that point you may as well not even use the JFET, becuse the voltage and current won't swing that much when you have over 2v of headroom anyway.

I don't see what the capacitance is for. My guess is 10uF should be okay -- at 50mA, it will charge and drain in about 1 ms, which is plenty short not to be annoying. I'd guess you could go as high as 100uF without really affecting turn-on/off delay, but I'm not sure why you would want or need to.

-Colin
cdanjo is offline   Reply With Quote
Old 07-03-2008, 02:49 AM #3
Cyparagon's Avatar
Class 4 Laser
 
Join Date: Sep 2007
Location: Wisconsin
Posts: 9,293
Rep Power: 208829
Cyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond repute
Cyparagon Cyparagon is online now
Class 4 Laser
Cyparagon's Avatar
 
Join Date: Sep 2007
Location: Wisconsin
Posts: 9,293
Rep Power: 208829
Cyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond reputeCyparagon has a reputation beyond repute
Default Re: Is there any reason this won't work?

I've seen all designs incorporate capacitors due to battery voltage spikes.

The problem with lithiums (as well as all batteries to some degree) is the voltage drops with usage, and I'd rather the output of the laser didn't drop with usage (as would be the case with a single resistor design)

I built it today and the JFET is dropping .8V and the resistor dropped a little less than 2V with the resistance value at about 15 ohms to achieve 120mA
__________________
A problem well stated is a problem half solved.
Cyparagon is online now   Reply With Quote
Old 07-03-2008, 03:11 AM #4
Class 3R Laser
 
Join Date: Oct 2006
Posts: 2,032
Rep Power: 32
pseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud of
pseudonomen137 pseudonomen137 is offline
Class 3R Laser
 
Join Date: Oct 2006
Posts: 2,032
Rep Power: 32
pseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud ofpseudonomen137 has much to be proud of
Default Re: Is there any reason this won't work?

Is there any chance of using some sort of regulator? Driving a diode directly like that is never the best idea - either you'll be overdriving it on the high end of battery charge, or underdriving it on the low end (or both!). With a regulator you could stop worry about the cap, and your output power would remain stable independent of your input (within reason and the specs of the regulator of course)
pseudonomen137 is offline   Reply With Quote
Reply





Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

























Privacy Policy | Advertising Disclaimer | Terms of Use


 


All times are GMT. The time now is 09:43 PM.


Powered by: vBulletin
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO
Privacy Policy | Advertising Disclaimer | Terms of Use
Copyright (C) 2017 Laser Pointer Forums, LLC