Single FET stepless dimming magic witchcraft voodoo

Flashlight Modding and DIY Parts
1

Ok guys. I'm no electronics wizard when it comes to solid state stuff so this has me very very confused. Here, look at this picture, it is exactly as simple as it looks:

That's it. That is an old junk LED, two cr123s, battery box, some wire, and some thing apparently called an "AOD436 N-Channel Enhancement Mode Field Effect Transistor." LED+ to BAT+, LED- to AO436 drain, AO436 source to BAT-. Can it get any simpler?

When the switch on the side of the box is turned on, nothing happens (duh). If I jumper from BAT+ to AO436 gate, LED turns on full brightness. And stays on after the jumper is removed (what?). Shorting the gate pin to the source pin turns the LED off, stays off until I... lightly brush the gate pin with my bare finger, making no other connections to anything else anywhere, and the LED smoothly ramps up to a low level and stays there. Touch the gate again and it ramps up some more. Hold the gate pin firmly and it ramps to full brightness and stays there after I let go. If I touch gate and source with my finger, it turns off.

So anyway, maybe this is completely normal expected behavior from this kind of circuit and if I knew half as much about silicon as I do about wires and relays this would all be completely ho-hum and mundane and no big deal, "that's just how they work, what do you mean everybody doesn't know that already?" kind of thing.

I think I'll go lie down for a little nap, I'm feeling kinda lightheaded. (current state of mind: :Sp + :party: + :love: )

Oh, and while I'm gone, somebody tell me how to get this kind of ramping/on/off control in a frickin' flashlight.

2

That’s pretty cool. I know gets are used for touch sensor switching with exits. Was interested in it but never got around to playing with them.
I would like to see the electronic gurus here have to say… I have literally oodles of fets to play with

3

My next question is: what made you think of trying this? Lol

4

Citing yourself, that’s just how they work :slight_smile: That’s actually a classical demonstration experiment for FETs.

The On/Off behavior is controlled by very little electrical charges that you carry around. Sometimes it may even switch on or off randomly, because the human body is an antenna for electrosmog.

5

Dude, I can not answer any questions about this, I got nothing. Somebody else build the same thing and then tell me what's going on!

I considered shooting video of it in action, but really, that would just make it look even more unbelievable. It's so stupid simple, anybody wants to see it in action go build one.

There has got to be some kind of... SOMETHING that could be put in a flashlight that would mimic whatever voltages are being applied by the finger touching the gate pin. GOT to be. If not somebody invent one.

Another trick: if I touch the BAT+ with one finger, and short across the gate and source pins with another finger, by varying the amount of pressure between gate & source I can make it go bright and dim at will. More pressure on the gate and less on the source it goes up. More on the source and less on the gate, it goes down and then turns completely absolutely off. It's nuts.

6

I could kind of start to wrap my head around it if it turned off when I let go. But it stays at whatever brightness it was when I let go and just sits there doing, say, super-micro firefly mode, until I touch something again.

Now I know what folks back in the '50s felt the first time they saw a jet airplane, and thought 'but that's impossible, planes can't fly without propellers!'

7

The gate is insulated from the other pins, so if nothing touches it, the gate charge (which controls the FET) just stays where it is, thus the drain-source-current (controlled by it) doesn’t change either.

8

The gate pin of the FET looks like a small value capacitor (connected across the gate and source pins). Depending upon the voltage on the gate pin, the FET acts like a variable resistance between the drain and source pins.

Touch between the gate and source pins and the capacitor and the gate discharges to 0 volts and the drain-source resistances goes very high. Touch between the gate a drain (Vbatt) and the cap charges up and the gate-source resistance goes very low. Normally FETs are operated either full on or full off. There is a small range of gate voltages that cause the drain-source resistance to be in some middle ground. That is where you see the LED dimming. That region is not very stable and changes with things like temperature and the phase of the moon. With nothing connected to the gate (just left floating) the gate can pick up stray voltages from the air and give weird results.

9

Yeah, I have noticed that the lower the light level, the more it's prone to flicker/do random things.

So... once it's actually connected to a proper circuit to make it practical, the magic goes away? But it's so pretty when it ramps up and down, noooooo :(

10

Connect a pot (like 100K or so) across the batteries. Connect the wiper of the pot to the gate. Voila, variable dimmer. But FETs don’t like to operate in that region between on and off. Depending upon the current, they can get toasty.

11

The FET didn't make much heat at all except when it was left at 100% for a while, at lower levels it did fine. I ran it on a single 18650 too and it acted the same.

The reason I was messing with this is for a direct drive SkyRay King with 5x XMLs, would these, or similar FETs, be OK for that setup with one per LED, switched on/off with the SRK's stock pushbutton (press and hold for ON, release for OFF)? I realize it'll lose the pseudo-latching behavior it has now once the gate pin is no longer floating like it is here. I was originally going to do it with a mechanical relay... lol.

12

OK, this is just a random thought, but why not wire the gate pin to an insulated touch button and see if it works as you want? If it does, then the button is your flashlight ramping control and the traditional on/off switch is your override, no?

13

I think the catch would be is that the more of anything you have hung off that gate pin, even if there's no electrical connection at the other end, the more it's susceptible to picking up stray voltage or bleeding it off when it's not meant to. Would be neat to have three separate touch pads though, each going to the gate, source, and BAT+. Touch the right combination of pads to ramp up or down or off. Don't think it would work that way in practice, though.

14

A modern power FET can take much abuse. You can pulse high current trough it or operate it at 120°C for a few days, it might not care. But you can instant-kill it with static electricity on the gate. I would rather not touch that to often without proper protection.
Btw. these FETs are mainly made for fast switching, so their characteristic curve is not optimized for linear applications.
But with some better suited ones and an operational amplifier, one could build a really nice adjustable linear current regulator - with proper heatsinking.

15

Got a link for a tutorial on something that would work? I know so little about this stuff I don't even know what to search for. Controlling it with a momentary on-off-on rocker switch, or a spring loaded momentary rotary switch... :love:

Just for fun:

16

You have to be very careful selecting FETs for operation in the linear region. Check the “Safe Operating Area” curves for DC operation characteristics. Most FETs will burn out due to pin-point hot spots if operated at even moderate power levels in the linear region. This is a main concern for selecting FETs for use in things such as variable DC loads, battery dischargers, and battery analyzers.

17

Very true!
These switching fets usually don’t even have a DC area in the SOA graph.
But there are some easy available good old friends like BUZ11 which do nicely in linear operaten. I just can’t think of any suitable for very low gate voltages right now. (As one would want for single li-ion cell application. BUZ11 is fine with ~5V)