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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Tact switch and mosfet transistor questions
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Offline Dznutz

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Tact switch and mosfet transistor questions
« on: July 13, 2012, 12:29:26 AM »
SO my question is how do you come up with the formula to choose what mosfet to use with a tact switch?

Should it support 0-5? 0-10?

I have a 12v 50ma and looking to fire the evercool and ptn04050c mods.

Sorry I have been searching and reading and all I run into is you should use one but the question is which one?


Thanks in advance all you electronics guru's.

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #1 on: July 13, 2012, 06:43:15 AM »
Generally, you want low RDS(on) and with a single battery, low VGS(th).  That's "on resistance" and "gate-source threshold voltage".  I wouldn't go below a VGSS of 12V and VDSS 20V though you could get away with lower (those are maximums for gate-source and drain-source voltages).  The ones with lower maximums are more susceptable to damage.  MOSFETs can get damaged easily by over-voltage.  Just touching one without grounding yourself first can fry it.  You can get ones that are ESD protected, but selection is limited.

Here's the one I like to use.  It's a P-channel which means it's a high side switch;

http://www.fairchildsemi.com/pf/FD/FDS6576.html

You can use an N-channel, but you have to be careful with ground offsets since it's a low side switch (all grounds have to be connected to the MOSFET, not the battery).  I don't have an N-channel recommendation off-hand since I tend to use high side switches for this kind of thing.  High side switches are just easier.

Offline jester

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Re: Tact switch and mosfet transistor questions
« Reply #2 on: July 13, 2012, 10:02:25 AM »
for a n chanel irlu3114 is what i use and i think breaktru does aswell cheers jimmy

Offline Dznutz

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Re: Tact switch and mosfet transistor questions
« Reply #3 on: July 13, 2012, 07:43:59 PM »
Thank you guys for all the info =)

Offline Dznutz

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Re: Tact switch and mosfet transistor questions
« Reply #4 on: July 14, 2012, 09:58:39 PM »
Ok one more question

Are the one listed above ok for just a standard 3.7 mod?

That should b the ? on this and thank you in advance.

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #5 on: July 15, 2012, 02:58:53 AM »
The P-channel I linked to is good for a single cell.  For the N-channel mentioned, VGS(th) is a bit high for a single cell.  This would probably be a better one.

Offline Dznutz

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Re: Tact switch and mosfet transistor questions
« Reply #6 on: July 15, 2012, 01:10:56 PM »
Thank you Craig

Offline Dznutz

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Re: Tact switch and mosfet transistor questions
« Reply #7 on: July 17, 2012, 10:49:10 PM »
Hi Craig or any one else  :D

My next question is from the information found on fairchild's site they say to use a 4.7? resistor on the gate for ringing purposes. Is this true? I have seen several schematics for mosfets where people are talking about using 10k from gate to ground to prevent damage. again is that true?
Is a resistor from gate to source needed as well? if so how do I determine which resistor is needed? Sorry to be a pain just really trying to learn with out being more confused when I started  :yes"


Thank you in advance
« Last Edit: July 17, 2012, 11:20:56 PM by Dznutz »

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #8 on: July 18, 2012, 01:46:06 AM »
The "ringing" only happens at high frequency like when using the MOSFET in a switching regulator.  In that case, a 2 to 5 Ohm resistor in series with the gate dampens the ringing to keep if from causing undesirable effects.   The need for a damping resistor depends on various design and component characteristics.   With a good driver and a MOSFET with low gate charge, it's usually not necessary.  It's called ringing because the voltage curve appears on an oscilloscope just like the sound waves of a bell ringing.   A damping resistor is never required for a user power switch.

For an N-channel switch, you need a "pull down " resistor on the gate.  Pull down only means the resistor connects the gate to ground.  For a P-channel switch, you need a "pull up" resistor that connects the gate to input voltage.  This resistor is always necessary for a power switch and good values range from 4.7K to 47K Ohms.  10K is a typical value.

For ESD protection (damage from static), Zener diodes are used, not resistors. 

Offline Dznutz

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Re: Tact switch and mosfet transistor questions
« Reply #9 on: July 18, 2012, 03:23:35 AM »
For an N-channel switch, you need a "pull down " resistor on the gate.  Pull down only means the resistor connects the gate to ground.  For a P-channel switch, you need a "pull up" resistor that connects the gate to input voltage.  This resistor is always necessary for a power switch and good values range from 4.7K to 47K Ohms.  10K is a typical value.

Ok I am getting it now. How do I determine the pull down/up resistor value? and esd in not needed to power a switch correct?

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #10 on: July 18, 2012, 02:52:09 PM »
There's no formula required for that.  10k is a good value for a pull up or pull down resistor.  It's just a matter of how much current you want to flow.  More current provides better resistance to noise and less current provides lower power consumption.  It can also be depend on minimums and maximums for the tactile switch. 

ESD protection is only required if you are using the MOSFET for a touch switch, but it's still not a bad idea to add it even with a tactile switch.  There are MOSFETs available with protection built-in, but selection is limited.  You can simply add a Zener diode with a reverse breakdown voltage lower than the maximum gate-source voltage of the MOSFET.  Connect it from gate to source with the polarity bar on the high side.  That would be on the gate for an N-channel and on the source for a P-channel.

Offline Dznutz

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Re: Tact switch and mosfet transistor questions
« Reply #11 on: November 14, 2012, 09:19:21 PM »



Does this work for the mosfet with a tact switch?

Online Breaktru

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Re: Tact switch and mosfet transistor questions
« Reply #12 on: November 15, 2012, 11:00:04 AM »
Does this work for the mosfet with a tact switch?

DZ, it's not correct. The 3101 is an N-Channel. There are several schematics in the modding forums. Just substitute the finger for a tact.

Online Breaktru

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Re: Tact switch and mosfet transistor questions
« Reply #13 on: November 15, 2012, 11:53:16 AM »
This is how it should be wired. Note the part numbers


Offline PoppaVic60

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Re: Tact switch and mosfet transistor questions
« Reply #14 on: November 15, 2012, 01:03:42 PM »
Nice and clear, Dave - nice!

The pics of p & n together is very useful.

I think one of the most irritating "parts" to selecting the components is: we read the forums and take notes - parts and values - and then wander over to digikey & mouser and discover they don't list the same info in the same way. In fact, their parametric/search stuff often ignores things we care about! (Having to DL every bloody datasheet to look for several comparable values is a royal pita ;-)

I was going to mention that many "finger" posts were for touch-switches, (and added that 10-100Mohm resistor).

Again, Nice!

Offline Dznutz

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Re: Tact switch and mosfet transistor questions
« Reply #15 on: November 15, 2012, 03:04:14 PM »
Yes thank you Break, I did wire it the other way, just wondered if the way I posted would work. From tiral and heat , I figured no it wont.

Thank you for confirming it.

Thank you also for the P channel post as well.

Offline poorboy

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Re: Tact switch and mosfet transistor questions
« Reply #16 on: November 29, 2012, 09:12:24 AM »
This is how it should be wired. Note the part numbers

Got a couple of irl3103 should i just remove the tact switch and follow the diagram if i want to use it as a touch sensor switch? Or i have to wire it differently?

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Re: Tact switch and mosfet transistor questions
« Reply #17 on: November 29, 2012, 09:47:13 AM »
Got a couple of irl3103 should i just remove the tact switch and follow the diagram if i want to use it as a touch sensor switch? Or i have to wire it differently?

Same wiring. Just bump up the resistor between G and S. Start at 1M. I have gone as high as 100M ohm.

Offline fsors

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Re: Tact switch and mosfet transistor questions
« Reply #18 on: November 30, 2012, 12:39:57 AM »
This is how it should be wired. Note the part numbers
yeah thanks Dave very nice and clear You Rock! :beer-toast:

Offline poorboy

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Re: Tact switch and mosfet transistor questions
« Reply #19 on: November 30, 2012, 09:15:21 AM »
Same wiring. Just bump up the resistor between G and S. Start at 1M. I have gone as high as 100M ohm.

Yeah seems like the 2M is not working for me. I'll try to buy some higher value resistors. :)

Btw, i have to put a 1k resistor in G to touch pad yeah?

« Last Edit: November 30, 2012, 09:19:37 AM by poorboy »

Offline tintin

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Re: Tact switch and mosfet transistor questions
« Reply #20 on: January 18, 2013, 03:54:19 PM »
sorry i can't start new thread...

Iam searching for a fuse smd are these ok ? http://www.ebay.com/itm/10Pcs-Littelfuse-SMD-0603-5A-5-Amp-Fast-Fuse-467-0467005-NRHF-/221010323199?pt=LH_DefaultDomain_0&hash=item33753de6ff
which ressetable fuse has the lower resistance ?
Also iam searching for a current sensor smd , a charge pump for high side n-mosfet switch and an inductor smd for step circuit do you know something good ?
What extra hardware except from an mcu i need to drived an oled like this ?
http://us112401600.trustpass.alibaba.com/product/120752718-103326398/Santek_0_69_OLED.html

for n-mosfet i want something to step the gate voltage higher that drain voltage to use n-mosfet as high side switch

thanks.

« Last Edit: January 18, 2013, 04:00:20 PM by tintin »

Offline Brian9523

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Re: Tact switch and mosfet transistor questions
« Reply #21 on: January 18, 2013, 06:11:36 PM »
sorry i can't start new thread...

Iam searching for a fuse smd are these ok ? http://www.ebay.com/itm/10Pcs-Littelfuse-SMD-0603-5A-5-Amp-Fast-Fuse-467-0467005-NRHF-/221010323199?pt=LH_DefaultDomain_0&hash=item33753de6ff
which ressetable fuse has the lower resistance ?
Also iam searching for a current sensor smd , a charge pump for high side n-mosfet switch and an inductor smd for step circuit do you know something good ?
What extra hardware except from an mcu i need to drived an oled like this ?
http://us112401600.trustpass.alibaba.com/product/120752718-103326398/Santek_0_69_OLED.html

for n-mosfet i want something to step the gate voltage higher that drain voltage to use n-mosfet as high side switch

thanks.

Your questions are above my pay grade but when I am looking for parts like the smd fuses, I'll go to a parts distributer site like Mouser, Digikey, etc. You have the option to search for a part. The results are shown with the parts specifications. You can refine the search by selecting say the lowest resistance, smd, or what ever... like here: Mouser

Offline tintin

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Re: Tact switch and mosfet transistor questions
« Reply #22 on: January 18, 2013, 06:45:39 PM »
thank you i know mouser but iam searching for tested components...


Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #23 on: January 19, 2013, 02:41:44 AM »
Mouser probably has the best parametric search for finding parts.  It's the first place I look when putting together a new design.  They even show parts they have on the books but dont stock.  If you find something you like, you can try findchips.com or another vendor to locate stock.

The best SMD fuses are the Littelfuse LoRho.  They have by far the lowest resistance and they're small.  I ran two the 1210L350SL in parallel for an incredible 4m Ohms insertion resistance, that's measured.  Though you should use the bigger 1812s if you have room.  The smaller ones wear out faster.  If they trip too many times, the resistance goes up.  The bigger ones don't have that problem.

There's only one single chip current sensor I've seen that will work well for an e-cig mod.  Dave uses it, maybe he can give you a link.  I use a current sense resistor and differential op-amp circuit myself.  It's kind of a pain though, has to be calibrated for each build.  It's good becuase I can set the upper limit to get the best resolution.  Current range is hard wired with the single chip solution.

I can't help you much with a charge pump.  I always use MOSFETs with low enough VGS(th) that I don't need to use one.  It's kind of an archaic way to go with the wide range of logic level P-channel MOSFETs now.  In any case, you should be able to find one that suits your needs.  It's a pretty standard part.

That OLED looks like a run of the mill COG unit.  All you need is an MCU to drive it.  Probably just has an I2C interface, power, ground, and reset connection.


« Last Edit: January 19, 2013, 03:29:14 AM by CraigHB »

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Re: Tact switch and mosfet transistor questions
« Reply #24 on: January 19, 2013, 06:53:42 AM »
i am searching for a current sensor

See: Current Sensor
and check the 2 posts below it.

Offline tintin

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Re: Tact switch and mosfet transistor questions
« Reply #25 on: January 21, 2013, 03:41:41 AM »
I can't help you much with a charge pump.  I always use MOSFETs with low enough VGS(th) that I don't need to use one.  It's kind of an archaic way to go with the wide range of logic level P-channel MOSFETs now.  In any case, you should be able to find one that suits your needs.  It's a pretty standard part.

low VGS(th) doesn't help if you want to use n-mosfet as high side switch for low side helps.
To use it as high side Vg must be higher than Vd to force n-mosfet go to saturation.
Because i have some mosfets with ultra low RDSon and low VGS (2.5V) i want something like charge pump to use as high side switch.
For low side works fine without extra components but i want high side switch.
For mcu i have some tqfp atmega chips (its easy to solder this chips without expensive equipment) i will program it in c with avr-gcc compiler.

Here i have found some p-mosfets with very low RDSon http://www.aosmd.com/products/mosfets/p-channel
« Last Edit: January 21, 2013, 03:46:56 AM by tintin »

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #26 on: January 21, 2013, 02:58:47 PM »
There can be good reasons to use an N-channel for a high side switch, for example if you have a lot of stock already of a certain part or if you need absolute minimum on resistance, but it's not something people do typically anymore.  There was a time where P-channel MOSFETs were considerably more expensive than N-channel MOSFETs and did not have the performance specs to operate efficiently with lower voltages.  That's just not the case anymore.  So given the option, there's really no reason to increase your part count using an N-channel.

Offline pnguin

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Re: Tact switch and mosfet transistor questions
« Reply #27 on: February 20, 2013, 04:18:11 AM »
The only MOSFET I could get locally has negative voltage values on the data sheet. Would the switch work with a NPN 3904 transistor instead of the MOSFET? Or is 2.5 amps (my max vaping amperage) too much for the transistor.

Offline utak3r

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Re: Tact switch and mosfet transistor questions
« Reply #28 on: February 20, 2013, 10:26:38 AM »
Note, that you have to choose your VGS(th) as low as possible. I already had few transistors which theoretically should do the job - but couldn't fully open the gate. Make it around 2V. Some 3V do work, too.

As for the N-channel transistor, see my old circuit:



From globally available ones you can get for example IRLR8743PbF.
« Last Edit: February 20, 2013, 10:31:54 AM by utak3r »

Offline Dasen22

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Re: Tact switch and mosfet transistor questions
« Reply #29 on: February 20, 2013, 07:05:23 PM »
The only MOSFET I could get locally has negative voltage values on the data sheet. Would the switch work with a NPN 3904 transistor instead of the MOSFET? Or is 2.5 amps (my max vaping amperage) too much for the transistor.

A 2N-3904 has a Continuous Collector Current of only 200ma

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Re: Tact switch and mosfet transistor questions
« Reply #30 on: February 20, 2013, 08:17:16 PM »
You can use a bipolar transistor if you want, but MOSFETs make better switches since bipolar transistors tend to be more lossy.  For example, a bipolar transistor with a VSAT of 200mV will lose 1/2 Watt at 2.5 Amps.  A MOSFET with an RDS(on) of 15 mOhms will lose 1/10 Watt at 2.5 Amps.  Also, you need a bias resistor for a bipolar transistor and it has to be properly selected to ensure the transistor is fully saturated under all conditions without overloading the base-emitter junction.

Offline pnguin

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Re: Tact switch and mosfet transistor questions
« Reply #31 on: February 21, 2013, 03:54:45 PM »
The mosfet the guy at the electronics store recommended is the IRFU9110, data sheet here    http://html.alldatasheet.com/html-pdf/181384/IRF/IRFU9110PBF/58/1/IRFU9110PBF.html     VGSth min -2.0. I am a little confused with the negative values on the sheet, all the other ones I looked at had positive values. Looking at the diagram from earlier posts, the resistor goes from gate to source? Thanks for all the help.

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Re: Tact switch and mosfet transistor questions
« Reply #32 on: February 21, 2013, 06:25:44 PM »
The mosfet the guy at the electronics store recommended is the IRFU9110, data sheet here    http://html.alldatasheet.com/html-pdf/181384/IRF/IRFU9110PBF/58/1/IRFU9110PBF.html     VGSth min -2.0. I am a little confused with the negative values on the sheet, all the other ones I looked at had positive values. Looking at the diagram from earlier posts, the resistor goes from gate to source? Thanks for all the help.

That mosfet is a P-Channel. It has a high RDS(on) of 1.2 ohms.
Yes the resistor goes from Gate to Source like: See Circuits
Not sure why neg V.  Maybe Craig could answer that?

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Re: Tact switch and mosfet transistor questions
« Reply #33 on: February 22, 2013, 01:21:55 AM »
P-channels use negative values because N-channels are the positive convention.  To turn the transistor on, Vgate is less than Vsource so VGS is negative.

Also when used as a switch, P-channels are wired differently than N-channels.  A P-channel switches from the high side.  An N-channel switches from the low side.  High side switches are easier to implement, but N-channel MOSFETs are generally cheaper and perform better, specifically, they have lower gate charge (they're faster) and they have lower RDS(on) (not as lossy).


« Last Edit: February 22, 2013, 01:35:10 AM by CraigHB »

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Re: Tact switch and mosfet transistor questions
« Reply #34 on: February 22, 2013, 07:20:31 AM »
Thank you Craig for your expertise  :thumbsup:
I was puzzled w/ the Neg V because I looked at some P-Ch datasheets and some showed Neg and some Pos  :Thinking:

Offline pnguin

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Re: Tact switch and mosfet transistor questions
« Reply #35 on: February 28, 2013, 05:42:49 AM »
Thanks for the help. I have ordered the IRL 3103 as pictured in the diagram. That way I know it will work correctly. You guys are the best.

Offline methos

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Re: Tact switch and mosfet transistor questions
« Reply #36 on: March 01, 2014, 02:08:40 PM »
Ok I have a strange one today. I went to wire in a STD30PF03LT4 & tact switch into a 3.7mod and its always firing. No biggie grabbed another one from the tape and same thing?!?!  After 5 of them I'm baffled. I have used some from the same tape and they worked fine. Wired exactly as break's post earlier.
Any ideas what could be causing this?

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Re: Tact switch and mosfet transistor questions
« Reply #37 on: March 01, 2014, 02:22:01 PM »
Ok I have a strange one today. I went to wire in a STD30PF03LT4 & tact switch into a 3.7mod and its always firing. No biggie grabbed another one from the tape and same thing?!?!  After 5 of them I'm baffled. I have used some from the same tape and they worked fine. Wired exactly as break's post earlier.
Any ideas what could be causing this?

What size resistor did you use.
Too low won't turn on or won't turn on fully
Too high won't turn off

The resistor I showed was for using finger touch. Lower ohms is required for using a tact switch

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Re: Tact switch and mosfet transistor questions
« Reply #38 on: March 01, 2014, 02:28:31 PM »
I used 15k. Same as I always use. I have tried 20k and no res and its always on.

I checked and the ohm @ 14.7

Online Breaktru

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Re: Tact switch and mosfet transistor questions
« Reply #39 on: March 01, 2014, 03:21:27 PM »
This may sound stupid but are you using a N/O tact sw?

Another thing... did you try it with a load (atty)

Are you powering a PTN04050c with it and breaking the positive to board.
I know that you said you have done this before with this Mosfet but did you double check your wiring?

Offline methos

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Re: Tact switch and mosfet transistor questions
« Reply #40 on: March 01, 2014, 03:25:24 PM »
This may sound stupid but are you using a N/O tact sw?

Another thing... did you try it with a load (atty)

Are you powering a PTN04050c with it and breaking the positive to board.
I know that you said you have done this before with this Mosfet but did you double check your wiring?

Yes N/O switch, yes on the load (carto 1.7) and no it's just a 3.7 mod. Last week I did a 4050 with a mosfit from the same tape. That's what has me baffled

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #41 on: March 01, 2014, 04:31:38 PM »
There's not much to go wrong there.  The only thing I can think of is if you're using N-channel wiring for that P-channel MOSFET or if you're confusing the terminals. 

A rather remote possibility is ESD damage.  That one has no protection so it's going to be vulnerable until it's installed in a circuit.  You should use a ground strap when handling unprotected MOSFETs.  I've burned a few that way myself.  They're really delicate, doesn't even take a static shock you can feel.  Though, I doubt you would burn five of them in a row handling them unless the whole tape got zapped somehow.

Offline rhay80

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Re: Tact switch and mosfet transistor questions
« Reply #42 on: March 31, 2014, 04:29:37 PM »
when using a tactile switch as the power switch on a 04050 chip, is it essential to use a mosfet, or can you get away with out using one at all?

Offline Brian9523

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Re: Tact switch and mosfet transistor questions
« Reply #43 on: March 31, 2014, 04:51:09 PM »
when using a tactile switch as the power switch on a 04050 chip, is it essential to use a mosfet, or can you get away with out using one at all?

Look at the amperage rating on the tact switch and then look at the amperage of the 04050 input... This should answer your question

Offline rhay80

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Re: Tact switch and mosfet transistor questions
« Reply #44 on: March 31, 2014, 05:10:39 PM »
Look at the amperage rating on the tact switch and then look at the amperage of the 04050 input... This should answer your question

found the amperage rating for the switch, but reading the pdf for the 04050, i must be missing it but cant find a minimum input amperage, i am such a noob when it comes to this lol :)

Offline Mandro

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Re: Tact switch and mosfet transistor questions
« Reply #45 on: March 31, 2014, 05:33:21 PM »
      
@rhay, if you go back a few posts in the ".44 mini Booster Box Mod Variable Voltage" thread, wreck did a calculation for fuses using this chip.


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Re: Tact switch and mosfet transistor questions
« Reply #46 on: March 31, 2014, 05:43:33 PM »
thanks, found it, i guess i will have to use a mosfet with the tactile switch. :)

Offline Brian9523

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Re: Tact switch and mosfet transistor questions
« Reply #47 on: March 31, 2014, 05:49:59 PM »
thanks, found it, i guess i will have to use a mosfet with the tactile switch. :)

Exactly  :laughing:

I'm not trying to be a smart ass. I'm just trying to point out that reading datasheets or spec listing can help us immensely.

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #48 on: April 01, 2014, 11:26:15 AM »
Always read the data sheets.  You can be surprised at the level of general application information in them sometimes.  For more complex parts, they often give you a typical application you can use as a Chinese blueprint.

Offline CharlieC

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Re: Tact switch and mosfet transistor questions
« Reply #49 on: April 21, 2014, 02:53:23 PM »
Hi guys

Thanks for all the info in this thread.

Question about using a p channel

I want to make an unregulated parallel 18650 "mech" box

Can someone point me to a MOSFET that can handle high currents and work for 4v?

I don't know where to start and I would love to build something that can sub ohm with a tact switch.

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