N-Channel with PTN0405 booster touch works

[jrm850]

more testing. Yes the mosfet gets much hotter faster when used as touch switch compared to a regular switch. It must be hitting that region that other were talking about, the not fully on state.

I cooled it down with a wet sponge and did the test both ways and it gets hotter in touch mode.

? for mosfet gurus. Do N channels only use the gate to source res for pull down or drop out as opposed to pull up for P channels? And can a pull up resistor be used with an N channel?

OK TEST CONCLUDED: Just checked and with my 'genius setup' lol, with an atty attached to the circuit I have a continuous voltage draw of .60 v and mosfet is too hot to touch.

Back to the P channel.

Any recommendation for a robust P-chan for touch that is not a microscopic TI son8 ?

You can use a pull-up resisistor on an N channel if you wanted to run it normally closed. I don't think it will help you for this application.
Have you tried other Mosfets or just the one? Any chance you let the smoke out? Mosfet seem to short more often than not when they fail.

If you are looking for TO220 package P channel, the Infineon spp15p10plg will work. I only mentioned this one because I have some sitting in front of me for another project. 2vgth, .2ohms on state resistance.

 

[CraigHB]

If you can deal with an SOIC8 package, which is not terribly small, the Fairchild FDS6679AZ is a good one. It has very low "on resistance" and protection diodes built-in which are good to have for a touch switch. The threshold voltage is a little higher than the Infineon part already mentioned, but should be fine, especially for a dual batt setup.

To answer your question about N and P, for both of them, the voltage differential between the gate and source pin is what determines the resistance to current flow. Current flows from drain to source for an N-channel and from source to drain for a P-channel. To turn the transistor on, the voltage differential needs to be positive (gate voltage higher than source voltage) for an N-channel and negative (gate voltage lower than source voltage) for a P-channel.

 

[nicotime]

more testing. Yes the mosfet gets much hotter faster when used as touch switch compared to a regular switch. It must be hitting that region that other were talking about, the not fully on state.

I cooled it down with a wet sponge and did the test both ways and it gets hotter in touch mode.

? for mosfet gurus. Do N channels only use the gate to source res for pull down or drop out as opposed to pull up for P channels? And can a pull up resistor be used with an N channel?

OK TEST CONCLUDED: Just checked and with my 'genius setup' lol, with an atty attached to the circuit I have a continuous voltage draw of .60 v and mosfet is too hot to touch.

Back to the P channel.

Any recommendation for a robust P-chan for touch that is not a microscopic TI son8 ?

Maybe you need to put a piece of that wet sponge in each ear and cross your legs while touching that switch!!

Maybe its something with the booster because on another touch mod I have that is just used as a simple switch..no boosters or regulators...the same mosfet didnt get hot. One thing that did cause it to get hot would be if I touched the mosfet while firing...then I guess the gate voltage went haywire by the small voltage from my hand to the heatsink of the mosfet (drain) to the TS contacts.

I did try a IRF9Z24NL P-channel and it didnt work for me....but I didnt know if it would work or not..just a cheap ebay grab while buying other parts.

@jrm....I did switch out the 3114 with a IRLZ34 that has a huge heatsink compared to the 3114 and it got hot also.....dunno.

 

[perlionsmitnick]

Thanks Craig, jrm and nico. Lot of good info here. I will try the sponge in the ears thing. It sounds promising and if it doesn't work, at least I'll look ridiculous. :d

I'll try the ones listed here when I get my hands on them . Thanks.

BTW, I do have a working touch with a P channel. I call it my ship in a bottle. Pics to come soon.

 

[breaktru]

I tried the IRLU3114ZPBF-ND all by itself w/ No regulator and No resistors. It turned on okay but would not drop out. Adding a 1 meg across Gate and source worked but when I introduced it to the 04050c, it would not even turn on. I substituted the 1 meg for a 10 meg and it turned on and off fully.
This worked okay for a plastic box mod where you can isolate all the negative wires but for a metal tube mod, the P-channel is a must.

 

[perlionsmitnick]

I tried the IRLU3114ZPBF-ND all by itself w/ No regulator and No resistors. It turned on okay but would not drop out. Adding a 1 meg across Gate and source worked but when I introduced it to the 04050c, it would not even turn on. I substituted the 1 meg for a 10 meg and it turned on and off fully.
This worked okay for a plastic box mod where you can isolate all the negative wires but for a metal tube mod, the P-channel is a must.

Thanks for the diagram. It worked ok except that I had to wet my finger each time. Any way to improve on that?

 

[executivul]

Simple solution for gate-source/gate-drain resistor: take a good multimeter and set it to the mega ohms scale (20M on mine), take the red lead in your left hand touching the metal, take the black lead in your right hand also touching the metal, get a reading of your body resistance. Mine is between 3 and 4 MegaOhms, depending on the moist of my hands. Now take both leads with lyour left hand without touching the metal and put your right hand's trigger finger or big finger (the one used in vaping) across the metal leads, get the reading. Mine is also 3-4 Mohms depending on the time of the day, moist on my hands, etc.
Now think of the voltage divider used to trigger the gate, if I have my hand at 4Mohms and the GS resistor at 8M then the gate gets 2/3 Vcc, that might not be enough, but if I have 4M hand and 40M resistor then the gate gets almost all the Vcc (40/44).
Considering battery voltage in the lowest case before protection kicks in at 3.0V just search for a mosfet which opens enough at 40/44*3V, get that from the Vgs/Ids diagram.

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[CraigHB]

One problem you can run into if you go too high with the pull down resistor (N-channel) or pull up resistor (P-channel) is you can actually start floating the gate. If the resistance is too high, the mosfet can turn on just from picking up voltage through the normal emf noise in the environment. A lot of it comes from the power grid in and around your home. I've seen MOSFETs with floating gates activate on several occasions playing with circuits on a breadboard. Even 10Meg is a very high pull up/down resistance for a gate pin. 100Meg would probably give you trouble.

 

[breaktru]

Good info Craig.
I pick my resistors (increment gradually) to the actual full turn on and full turn off points under actual conditions. Anything lower or higher won't work for me. I've had others copy some of my circuits and couldn't get them working with the same values. Perhaps tolerances in their parts played a roll.

 

[CraigHB]

Yea, we're working in that sketchy area close to the edge of tolerance. That's when things start looking like FM (.......ing magic). Just the length of a wire or manufacturing tolerance of a part from one to the next can make a difference. That kind of thing happens all the time when you're playing with RF stuff. This isn't RF, but the idea is the same.

 

[perlionsmitnick]

So what is the problem with npn or pnp transistors for 3.7 volts?

 

[executivul]

@perlion: normal BJT transistors require a current through the base to open, much larger than a mosfet which require almost no current at all (quick explanation). Current by Ohm's law is voltage/resistance (I=U/R), resistance is your body and mine is 3MegaOhm that is 3 million ohms, now if you need a current of let's say 10mA (0.01A) you should have 3,000,000*0.01=30,000Volts to open that transistor, a bit much for a LiIon.
Somebody correct me if I'm wrong please, a little tired by this hour.


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