andrewwynn
Flashlight Enthusiast
Update: found the chip to make this driver.. read on!
==
Hey folks.. working on turning the 'nano' driver into a voltage regulating FET controlling constant voltage driver for use in hotwire lights.
After Modamag did his testing on the Maglite switch and i did follow-up testing on a couple switches here.. and measured 0.15ohm to 0.16ohm...that equates to about a 1/2Volt loss on the switch in a Mag85.. and that equates to about a 200 lumen drop in output! craziness.
So... enough of that i'm goin' FET found a most excellent FET with RDSon of 0.006 ohm with Vgate=10 and 0.009 ohm with Vgate=4.5. (They are $7-8).. so i looked some more and found a much more common FET that is 0.026ohm at vGate of 5V... $2.30
So.. going to build an FET-based driver for my hotwires.. was going to just use it as a pure switch but with like $1 more in parts i can have it regulate the voltage at least just a little bit (i.e. hold the voltage to 11V or less with the mag85 or the 1331 bulb to keep it from popping when the Vbat is 12.7).
In any event, my idea was to take a 100kohm pot and trim the Vbulb and put the wiper into the input of the 35mV LDO driver MrAl designed that i use in the nano... using the 100k pot i use to trim the amplifier bias i should be able to find an input wiper voltage that is a little higher than the 4/100ths of volt i typically put into that circuit.. the amp bias can decrease the gain and i can put in some higher input voltage.. maybe 1/10th to 1/5th of volt.. that would be a 1.2:100 ratio on the trimpot.. maybe i'll use a 200k pot, or better use like a 2K resistor in series with 100k pot or something to set the feedback voltage.
In any event using this method i should be able to have a ludicrously low drop out voltage.. even at 3.3A of an 1185... 0.026ohm means 9/100ths of a volt drop... for the 1185 i'm considering using the $7 FET which would be a 2/100th of a volt drop on the FET...
I will be rewiring the switch of course and having some thick wire like 18 or even 14ga wire hooking things up.. and hope to wire it up such that the switch on the body actually just supplies the power to run the amp and w/o power applied the FET i'll put a resistor to pull down the gate and shut off the FET which will actually also be used as the power control.
The FET i found is N channel i think.. needs positive gate to turn on... goes on the 'neg' side of the bulb.. i don't have a problem for my designs with that.. i think i'd be interested in a P channel solution for 'the public' so they can't short out the battery if the switch is off and some how short the bulb pin that is always live.
I think i have a basic ckt design at home i'll try to post that to get thoughts.
Oh.. i have another design idea using the National voltage reg. part, but the dropout voltage is way to high for most designs, but ONE.. i want to drop 14.4V to 12.6 to run an 1166 bulb, and a 'stock' part will do wonders for that design...
I was trying to find a switcher or anything that will just take a feedback from a pair of resistors to set voltage but can run an EXTERNAL FET.. found a couple from National with 'low and high' FETs i didn't understand how those work..
I would love a single chip with 2 resistors to set voltage and external FET solution... or build one based on the nano ckt.. it's absolutely wonderful and should do the trick for what i want to do.
Thanks in advance.
-awr
==
Hey folks.. working on turning the 'nano' driver into a voltage regulating FET controlling constant voltage driver for use in hotwire lights.
After Modamag did his testing on the Maglite switch and i did follow-up testing on a couple switches here.. and measured 0.15ohm to 0.16ohm...that equates to about a 1/2Volt loss on the switch in a Mag85.. and that equates to about a 200 lumen drop in output! craziness.
So... enough of that i'm goin' FET found a most excellent FET with RDSon of 0.006 ohm with Vgate=10 and 0.009 ohm with Vgate=4.5. (They are $7-8).. so i looked some more and found a much more common FET that is 0.026ohm at vGate of 5V... $2.30
So.. going to build an FET-based driver for my hotwires.. was going to just use it as a pure switch but with like $1 more in parts i can have it regulate the voltage at least just a little bit (i.e. hold the voltage to 11V or less with the mag85 or the 1331 bulb to keep it from popping when the Vbat is 12.7).
In any event, my idea was to take a 100kohm pot and trim the Vbulb and put the wiper into the input of the 35mV LDO driver MrAl designed that i use in the nano... using the 100k pot i use to trim the amplifier bias i should be able to find an input wiper voltage that is a little higher than the 4/100ths of volt i typically put into that circuit.. the amp bias can decrease the gain and i can put in some higher input voltage.. maybe 1/10th to 1/5th of volt.. that would be a 1.2:100 ratio on the trimpot.. maybe i'll use a 200k pot, or better use like a 2K resistor in series with 100k pot or something to set the feedback voltage.
In any event using this method i should be able to have a ludicrously low drop out voltage.. even at 3.3A of an 1185... 0.026ohm means 9/100ths of a volt drop... for the 1185 i'm considering using the $7 FET which would be a 2/100th of a volt drop on the FET...
I will be rewiring the switch of course and having some thick wire like 18 or even 14ga wire hooking things up.. and hope to wire it up such that the switch on the body actually just supplies the power to run the amp and w/o power applied the FET i'll put a resistor to pull down the gate and shut off the FET which will actually also be used as the power control.
The FET i found is N channel i think.. needs positive gate to turn on... goes on the 'neg' side of the bulb.. i don't have a problem for my designs with that.. i think i'd be interested in a P channel solution for 'the public' so they can't short out the battery if the switch is off and some how short the bulb pin that is always live.
I think i have a basic ckt design at home i'll try to post that to get thoughts.
Oh.. i have another design idea using the National voltage reg. part, but the dropout voltage is way to high for most designs, but ONE.. i want to drop 14.4V to 12.6 to run an 1166 bulb, and a 'stock' part will do wonders for that design...
I was trying to find a switcher or anything that will just take a feedback from a pair of resistors to set voltage but can run an EXTERNAL FET.. found a couple from National with 'low and high' FETs i didn't understand how those work..
I would love a single chip with 2 resistors to set voltage and external FET solution... or build one based on the nano ckt.. it's absolutely wonderful and should do the trick for what i want to do.
Thanks in advance.
-awr
Last edited: