Mosfet circuit for micro switches....just 2 parts needed.

[Rocketman]

V(GS) Threshold is the Gate to Source voltage where the Mosfet just starts to turn on. A 3 volt threshold, conducting maybe 100 microamps, would probably conduct enough to vape at a GS of 4 volts, maybe.
If operated with a large load in that range the "almost" on resistance would drop voltage and generate heat.
Typically, a threshold a volt or so less than the expected minimum Vin would work. It's best to look at transconductance curves to see if the on resistance is low enough at the minimum operating conditions.

 

[CraigHB]

Ideally, you want V_GS (gate-source voltage) well above V_GSTH (gate-source threshold voltage) during normal operation. MOSFETs are more like valves than switches. The difference in voltage between the gate pin and source pin opens the valve. You want the valve all the way open.

Typically, you're limited by battery voltage so you need to select a MOSFET with a low threshold voltage. That ensures the valve is mostly open when battery voltage is applied. Threshold voltage is less of a concern with a dual cell system since battery voltage is double that of a single cell system.

As already mentioned, you can use the curves provided in the data sheet to find R_DSON (drain-source resistance) at a particular V_GS. From that data, you can determine if loss is acceptable (I²R). You'll never be able to achieve zero power loss, but you're usually good if you can keep it under a tenth of a watt. Losses less than half that are usually obtainable for our application.

 

[Rocketman]

Hopefully as good as a plain old switch, right?

 

[CraigHB]

I'm not sure on the resistance of a mechanical switch, but I think it's only a few mΩ. I'm sure it varies from switch to switch. The switching MOSFET I use for input power on my last mod measured out about 8mΩ R_DSON at 4V_GS. It's a .....en part, FDS6576. I'm using it in the single cell mod I'm currently working on as well.

 

[Rocketman]

A good switch in pristine condition (a good switch) will have about 5mohm. More as it gets old.

A typical e-cig mod switch (cheapo) has about 10 to start off. A tact switch about 50mohm.

A mosfet can be better, and if not abused, and won't get worse with age.

Switch is simple, mosfet can be better. The one you picked is as you say "a .....en part"

Full on hard at less than 2.5. Good choice.

I use this one (bigger package) IPD90P03P4L-04INCT-ND for switching HID lights, and it turns on earlier than you would think.

A few amps I(D) and R(On) is less than 10mohm @ 2.5 volts V(GS). Kind of hard to interpret that from the specs because it's a 90 amp Mosfet


The bigger package is easier for me. Is there an equiv of the FDS6576 in a TO-253 or 263 package?

 

[CraigHB]

The bigger package is easier for me. Is there an equiv of the FDS6576 in a TO-253 or 263 package?

Yea, I can understand that. I haven't reviewed all of the Fairchild MOSFETs, I'm mainly interested in the SOIC packages. They seem to make the best ones, at least it seems that way to me. Here's their MOSFET product page if that's any help.

 

[Rocketman]

spent a few hours there

Sure is a lot easier now that it's online.

Their catalog is a bear to read (with a magnifier of course)

 

[CraigHB]

Argh, paper catalogs, I abhor those things. I get so mad when vendors send me telephone book sized catalogs in the mail. I just imagine all the trees that got chopped down for that waste of paper when I can just go online and get the information 10 times faster without killing any trees.

 

[Rocketman]

YUP, You ain't lived until the UPS guy delivers the Thomas Set

with the DVD

 

[perlionsmitnick]

Argh, paper catalogs, I abhor those things. I get so mad when vendors send me telephone book sized catalogs in the mail. I just imagine all the trees that got chopped down for that waste of paper when I can just go online and get the information 10 times faster without killing any trees.

I'm not a tree hugger but I feel the same way about phone books. Waste of paper and fuel.

 

[TomCatt]

Thanks for the answers . Was just curious, not bein' so much a noob as 'electronically challenged', ya know.

 

[iuseaspork]

Just ordered one of these. I think I may not have chosen wisely as Vgs is 1.95 as opposed to the IRL3103's 1. Hoping it still works well.

PSMN012-25YLC,115

 

[CraigHB]

NXP makes good stuff. I use parts from them a lot.

Not sure on your application, but that's going to fine with a dual cell mod. Look at figure 7 on page 7 of the data sheet. You'll see the on-state resistance is plenty low for anything over 4V. Using it with a single cell would be sketchy though. You can see on resistance starts increasing exponentially around 3V. That would be cutting it a little close, but it still may work okay with a single cell.

 

[iuseaspork]

OK... I think this is the best one I could find that I can get in minimum orders of 1:

FDD3706

Wondering if this would work as a touch switch too.

Edit: (for a single cell mod)

 

[CraigHB]

That one should work nicely. Notice the graph in figure 4 of the data sheet. It's shifted to the left a whole volt over the other one. It also has very low on resistance. Keep in mind it's an N-channel so the switch has to be on the low side (as opposed to a P-channel on the high side).

 

[iuseaspork]

Yup. Thanks a lot. Good thing I found this post, or it would have been a lot of trial and error.

 

[pailpoe]

Hello,

I use on my orion switch (V1.1) this mosfet in SO8 package (not use if you have PWM) : IRF6201PbF

 

[icgns]

Hello,

I use on my orion switch (V1.1) this mosfet in SO8 package (not use if you have PWM) : IRF6201PbF

Hi pailpoe,
Any chance to see the schematics of your orion switch (V1.1)? Thank you.

 

[jhonutz]

This button works on a few my e-cigarettes successfully.

replace 10K resistor with 1M or 10 M and have a sensitive switch

Also if you use a step down regulator IC, and this IC have enable pin you may command with mosfet this pin. (works with lower current).In this case you may use a fet channel P with Vgsts= 1 - 3V (-1 -3V)

PS: It`s good if an moderator make this thread sticky, many people search for mosfet touch switch solution