dimming with potentiometer

i want to make a minim*g terralux ministar 2 dimmable with a potentiometer
i tested this (outside the m*g) with a 100ohm potentiometer (between batteries and tailcap) and it seems to work very well
but there are hardly any lights to speak of with such a feature, why not? what are the main problems?

I often wonder the same myself, my only guess is that they haven't caught on and haven't found a sturdy enough pot they thought would hold up as well as switches do. Also I found with LEDs I rarely ran a light that I was playing around with a pot on at more than 2 or 3 different light levels so having infinite levels was an extra feature I would rarely use.

ModaMag uses them on his MineTrios and Quads and FatMags. I have seen two. One sticks out a bit which makes for a nice anti rolling mini arm. The other is one that I may be doing on mine,it is filed down flush to the nut right above the mag button.

Not to act uppity or anything, but if you do a SEARCH you'll find a similar light was made about a year and a half ago... /ubbthreads/images/graemlins/grinser2.gif /ubbthreads/images/graemlins/grinser2.gif /ubbthreads/images/graemlins/grinser2.gif

Sold it right after.

Yup, that's pre modamag. Too bad you're so far away or I'd me over at your place drooling over all your lights /ubbthreads/images/graemlins/grin.gif

ahh yes, i remember seeing that one before
i think it's such a useful feature, that i wonder why not more designs use a pot dimmer

i want to fit mine into the space behind the contact spring in the minim*g tailcap, with the axis sticking out at the end
the best i found was this one which is 6mm diameter and can handle 1W (rated 0.5W but survived my test) but the problem is that it is quite fragile, has no axis and it's hard to attach one reliably /ubbthreads/images/graemlins/icon23.gif
guess i'll just have to search for a better one

Bad link Panzer.

ok, edited the link

but i can't use that one anyway, so it doesn't really matter /ubbthreads/images/graemlins/wink.gif

You don't see them with halogen lights because the bulb life drops if the voltage is too low. Has to do with the way the metal is redeposited on the filament.

You don't see pots on a lot of LED lights because the two stage tailcaps use a much smaller, singel value resistor. A wirewound resistor can be easily incorporated in a new design, but it's hard to work into a retrofit without being an obvious addon.

Many of us have done some fun things with pots, but unless you are building your own tailcap it's not easy to do.

Daniel

I don't use them because pots capable of handling 125W are really, really large. /ubbthreads/images/graemlins/grin.gif

Wilkey

I don't really see the point of using pots in flashlight. Yes, the pot would limit brightness, but would not necc save you battery power. From what I understand pots dissipate power by converting it to heat, not an extremely useful feature unless you need a hand warmer.

Plus most pots do not allow a 100% electron flow even when fully open, they average like 90%, so your top end would be limited too.

if you NEED dimmable, they might be useful, if you're trying to save battery power, you're better off looking into PWM circuits (which, when properly configured wouldn't use much power) or a simple 3 way switch and a resistor. (off, on, low)

my $0.02

I have used 'pot' controlled solutions for several drivers including the VIP driver and the FATMAN driver.. if you can find a voltage divider in the circuit for the teralux and incorporate a high voltage high resistance solution you'll have much better luck changing the output..

somebody used a high power magnet and saturated the inductor of a light to control output somebody else used a magnet and a hall effect... I have some very clever solutions to infinitely control the output of a light using a 'twist' (between body and bezel).. so that it's completely sealed by the o-ring still and there is no noticeable change in the appearance of the light.. and wastes no energy just heating a resistor like the tailcap resistor solutions.. which are 'ugly' in the nature of pulling the driver out of regulation 'on purpose' that just rubs me and several i know the wrong way even if it works /ubbthreads/images/graemlins/wink.gif

I would suspect efficiency is a big problem with using a 100ohm resistor inline with the battery... i would bet you can drive the efficiency below 50-60% with such a solution when if you can use a high-value resistor (like 10-100k).. inside the driver you can get 70-80%.. it can make quite a difference on the runtime.

I have come up with some very nice 2 and 3 way solutions as well that i'm hoping i can share with CPF soon.. they are designed to work directly with the likes of a bad-boy or wizard and in the likes of a CNC123.. 'twist' for 2 different levels... it should work on a minimag as well... i'm betting the terralux has a .1ohm Rsense that could be paralleled with a switched resistor to change the feedback voltage and hence the output... think about ways to put a 3-way swtich ala VIP into some space in the head behind the reflector.. and figure out 3 levels with 3 different Rsense resistors... else find the voltage divider that feeds back into the switch and you can use much higher level resistors w/o monkeying with low value resistors (much less reliable).

and.. keep up the creative thinking and create something none of us have thought of.

-awr

Hey Andrew. let me know if you find a way to dim a Wizard. It does not lend itself to dimming through input voltage changes (ala two stage tailcaps) becasue of the buck/boost design. The wizard just stops working if the volatge falls too low.

Daniel

So what's the difference between a Potentiometer (which is a volume knob on my guitars) a Rheostat and the nFlex? As far as theoretical differences.

I believe rheostats are a wirewound resistor with a wiper arm on it while non rheostat potentiometers are typically a flat surface resistor with a wiper on it. They usually use rheostats in heavy duty applications.

[ QUOTE ]
Ginseng said:
I don't use them because pots capable of handling 125W are really, really large. /ubbthreads/images/graemlins/grin.gif

Wilkey

[/ QUOTE ]

Does that mean the Polaris wont come with that feature? What if we WANT a monstrosity of a potentiometer jutting from the side of our flashlights? /ubbthreads/images/graemlins/wink.gif

Jeff

thanks for all the helpful comments guys!

andrewwynn:
i am aware of the circuit being the most effective component to include a dimming function, but my electronic knowledge is too limited for meddling with these
(well, i built a robot that drove towards a light source once, but that's almost 10 years ago, and was more a try and error solution using scrap parts, it worked, but only for about 20 seconds and then the transistors blew /ubbthreads/images/graemlins/frown.gif )

but i'm a fanatic do it yourself guy and always eager to learn and my recent interest in flashlights has also brought back my electronic curiosity /ubbthreads/images/graemlins/smile.gif

at least i can stop my hunt for smaller pods now, and rather use the time to catch up on electronics
any pointers for good online sources?

Most of the potentiometer (pot) adjustable converters you see or read about on this site use the pot in a control loop on the driver board and not in the main current path through the LED. The key difference is that the magnitude of the current in the control loop is in the microamps range. The magnitude of the current in the LED is in the 100s of milliamps range. That's about 4 to 5 orders of magnitude difference. Since the loss in the pot is expressed as I^2*R the losses are much greater when pot is in the high current path. This may not be an issue if you are running off the mains but if you are running on batteries it can be a big issue. Hope this helps.

Paul

technically the only difference between a rheostat and a potentiometer is there are only two taps on a rheostat vs 3 on a potentiometer..,. either can be made out of any material.. it is not likely to find potentiometers built as large as you will find a rheostat so there is a connotation of 'big' when you hear rheostat and likewise it's very unlikely to find smd rheostats since a potentiometer will work in their stead any time... and i've personally never seen a small rheostat.

SMT soldering would be involved for a more clever solution to dimming on such circuits, and a much easier solution for most users would be along the line of the 100ohm pot you describe... just make sure it's rated high enough current for your design.

gadget: i have a 3-level design for wizard.. some elements are still 'top secret' I haven't decided yet to make it public knowledge.. fire me an email maybe we could work out something.. [email protected]

teststrips... a larger percent of the power is lost to heat when dimming in the way described in this thread, however a much lower overall current is demanded from the battery so it does extend battery life as well.. eg.. at full power maybe you get 90% efficiency and 2 hrs.. and at low power you get 60% efficiency and 5 hrs.

panzer.. i will likely have a wizardII converter as well as a badboy nextgen in house very soon to do testing and will discover if a high-voltage low current solution is doable.. otherwise.. usually the only way to change the output power on those lights is with changing the Rsense resistor which is a very low resistance ( like 1/10th of an ohm).. you can have multiple resistors with switches, but a switch alone is often 1/10th of an ohm, so it's tricky... it's very likely possible to break the feedback loop from the Rsense to the circuit.. and put a potentiometer in place in parallel with Rsense.. and feed the wiper to the circuit.. changing that pot will vary the output level.. but it takes some tweaking of the limits to make it a 'smoother operator' (you don't want to be able to set it to zero).

-awr