N-Channel with PTN0405 booster touch works

[perlionsmitnick]

Unless someone can tell me why it shouldn't be done, I just tried a IRLU3114ZPBF-ND N-Channel mosfet hooked up to a TI PTN04050CAZ booster and used as a touch switch and it worked fine. I DID NOT use a resistor between gate and source and it latched on and dropped out voltage fine and operated my Scubagen3.

The touch points being the + of the battery and Gate of mosfet,
the source of mosfet to - batt,
and drain going to Grd of booster.

And of course + batt to Vi boost.

After all of the messing around with P channels and P channels with N channels and so forth I had always experienced it finding ground and activating the booster by itself. This test was done on my desk with no case or housing so it may react differently in those circumstances.
In previous test I always had a 10K res between gate and source though.

Thoughts ?

 

[Dalton63841]

I will have to give this a try later. In my experience, using Asnider's schematic with a 10M resistor, it did not work.

 

[nicotime]

@perlion...I have used that mosfet on the output side of the 04050 successfully with a tact switch and resistor and a digital pot...but not on the input end. I dont see why it shouldnt work as long as its inline on the negative line....but I'll let our resident electronics gurus say for sure. Maybe the resistors on the 04050 are pulling the mosfet down instead in effect taking place of the 10k resistor...dunno. I'm still trying to figure out where exactly does positive becomes negative in a simple circuit. lol

 

[nicotime]

Hmmm..I just breadboarded your setup perlion and what I get is a constant but erratic current flow regardless of the gate being switched on or off and with or without a resistor between the gate and source....huh!

 

[perlionsmitnick]

Hmmm..I just breadboarded your setup perlion and what I get is a constant but erratic current flow regardless of the gate being switched on or off and with or without a resistor between the gate and source....huh!

My setup consist of ptn04050cah with tantalum caps and a schotty (sp) diode on Vo that nuck recommended and the mosfet that I listed above. Nothing special and everything was hooked up on my desk in no case. I was checking voltage at Vo and ground on the booster, which is where the atty would be hooked up. If you'd like I can do a diagram later.

I haven't checked the inline current yet, just watched the voltage and it was steady. It did seem that I had to wet my finger the first time but after release I could activate with a dry finger. Also i was dealing with loose touch points (2 wires in open air).

Using this may not be practical because the mosfet may not be fully latching. It would drop out after a few seconds consistently as expected.

 

[nicotime]

Maybe try running the mosfet on the output side of the board...I did it with a tact switch but dunno about a Touch...shouldnt make a difference.

 

[l3oertjie]

It depends on how sweaty your fingers are.....

 

[WillyB]

Maybe try running the mosfet on the output side of the board...I did it with a tact switch but dunno about a Touch...shouldnt make a difference.

I don't think that's a good way to go, the booster will always be 'on' when the batteries are installed.

Well if you look at Nuck's schematic he used that same N-Mosfet.

It obviously worked. Maybe some minor revision to the newer boosters? Am I missing something here?

 

[nicotime]

Doh!!...no its me..I didnt move the ground to the atty...it was still picking up power from the battery without going thru the mosfet...thanks Willy! My mosfet gets damn hot fast like this though...so I must have something still messed up.

Wonder how much power is being lost the other way at idle?

 

[CraigHB]

It should work as long as there's nothing connected to battery negative other than the source pin of the N-channel MOSFET and the gate drive. The issue with using N-Channel MOSFETS as switches is they switch the low side (negative side) of the circuit. Since negative is conventionally common, you have to go out of your way to make sure no ground (negative) connections are bypassing the switch. Failing to do that can create ground offsets which cause operational problems. Avoiding issues with low side switches can be tricky with more complicated circuits and sometimes even with simple ones. The convention is to always put the switch on the high side (positive side) to avoid those complications. High side switching requires a P-Channel MOSFET.

 

[jrm850]

I could be wrong here, but that 10k pulldown seems low for a touch sensor. The gate charge is so low and switching speeds aren't important so going higher than the impedance of your finger should improve saturation. Without full saturation your on state resistance is going to be higher and you will waste battery by heating your transistor. I checked my dry finger with a multimeter and read 2.2Mohms.

 

[WillyB]

Doh!!...no its me..I didnt move the ground to the atty...it was still picking up power from the battery without going thru the mosfet...thanks Willy! My mosfet gets damn hot fast like this though...so I must have something still messed up.

Wonder how much power is being lost the other way at idle?

Well the thing about mosfets they are not simple on/off devices. They have to ramp up to full saturation. The in-between, semi-on, triode region has high channel resistance. This is the region where the mosfet begins to conduct. While in this area they are basically functioning like resistors (hence the heat). You want to get it out of there as soon as possible to a fully saturated state.

Folks usually just talk about the threshold voltage, there appears to be more to it than that.

I don't really understand them, luckily I have no use for them.

What value resistor are you using?

 

[nicotime]

It should work as long as there's nothing connected to battery negative other than the source pin of the N-channel MOSFET and the gate drive. The issue with using N-Channel MOSFETS as switches is they switch the low side (negative side) of the circuit. Since negative is conventionally common, you have to go out of your way to make sure no ground (negative) connections are bypassing the switch. Failing to do that can create ground offsets which cause operational problems. Avoiding issues with low side switches can be tricky with more complicated circuits and sometimes even with simple ones. The convention is to always put the switch on the high side (positive side) to avoid those complications. High side switching requires a P-Channel MOSFET.

Interesting...I thought N-channel was the most common mosfet. This booster is one I have been playing around with on my board for some time now..it may even be fried...and forgetting to move that ground probably didnt help it much either. Good info..thanks!

I could be wrong here, but that 10k pulldown seems low for a touch sensor. The gate charge is so low and switching speeds aren't important so going higher than the impedance of your finger should improve saturation. Without full saturation your on state resistance is going to be higher and you will waste battery by heating your transistor. I checked my dry finger with a multimeter and read 2.2Mohms.

If this was directed at me...no I was just using the normal tact switch to trigger it...not a TS setup. I'll have to take all this junk off my board and start clean...even though it looks right..something is lighting this mosfet up bigtime...actually burnt my finger in a matter of seconds. I switched it for a Z34 and it does the same thing. If anything I may get a good fireworks show if I keep it up! I know this isnt helping perlion much though...I was just trying to learn some of this myself.

 

[nicotime]

Well the thing about mosfets they are not simple on/off devices. They have to ramp up to full saturation. The in-between, semi-on, triode region has high channel resistance. This is the region where the mosfet begins to conduct. While in this area they are basically functioning like resistors (hence the heat). You want to get it out of there as soon as possible to a fully saturated state.

Folks usually just talk about the threshold voltage, there appears to be more to it than that.

I don't really understand them, luckily I have no use for them.

What value resistor are you using?

I had a 10k on a tact. Yeah I usually just use a simple circuit for a tact...but I have these toys laying around Willy..I cant help myself!!

 

[Dalton63841]

I wonder has anyone tried a simple NPN, or even PNP, transistor for the touch switch. I know a Darlington pair usually makes a pretty great touch switch. Just curious why I have never seen it used here.

Well the thing about mosfets they are not simple on/off devices. They have to ramp up to full saturation. The in-between, semi-on, triode region has high channel resistance. This is the region where the mosfet begins to conduct. While in this area they are basically functioning like resistors (hence the heat). You want to get it out of there as soon as possible to a fully saturated state.

Folks usually just talk about the threshold voltage, there appears to be more to it than that.

I don't really understand them, luckily I have no use for them.

What value resistor are you using?

 

[CraigHB]

I could be wrong here, but that 10k pulldown seems low for a touch sensor.

Yes, that would be low since your finger is about 2Meg. You're creating a divider with the gate pull up/down resistor and your finger so you need the resistor to be some amount greater than your finger to drive the gate hard enough. However, too high and you run into other issues. Though, even with the higher resistances, like you said, I don't see where gate charge is going to become an issue.

Well the thing about mosfets they are not simple on/off devices. They have to ramp up to full saturation. The in-between, semi-on, triode region has high channel resistance. This is the region where the mosfet begins to conduct. While in this area they are basically functioning like resistors (hence the heat). You want to get it out of there as soon as possible to a fully saturated state.

Folks usually just talk about the threshold voltage, there appears to be more to it than that.

Well put actually. There really isn't much more to it than that so no need to overcomplicate it. Threshold voltage is just that, the threshold of conductance. There are lots of factors that make things really complicated when looking at AC characteristics, but we really don't need to concern ourselves for an on/off switch.

Interesting...I thought N-channel was the most common mosfet.

Actually no. Both types have extensive applications in circuit design. They're often used in a complementary configuration with a N-chanel and P-channel. It's the depletion mode FETs that are much less common. They're like a normally closed switch. The ones we use are enhancement mode, like a normally open switch.

I wonder has anyone tried a simple NPN, or even PNP, transistor for the touch switch. I know a Darlington pair usually makes a pretty great touch switch. Just curious why I have never seen it used here.

Bipolar transistors work fine as switches, however, for a touch switch, we're working with very small currents. In that case, the extremely low current draw of a MOSFET gate is just the ticket. A bipolar transistor requires a much higher bias current to fully turn on. Elimination of the bias current was the reason field effect transistors were invented.

 

[TomCatt]

I had a 10k on a tact. Yeah I usually just use a simple circuit for a tact...but I have these toys laying around Willy..I cant help myself!!

What 'simple circuit' do you use for a tact switch?

ETA: Just a resistor inline?

 

[nicotime]

What 'simple circuit' do you use for a tact switch?

ETA: Just a resistor inline?

Like this one TC....
View attachment 52992

 

[TomCatt]

Oh, OK...Thanks Nico.

 

[perlionsmitnick]

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 ?