Mosfet touch switch.

[Lee1111]

I wired up my touch switch according to raidy's post here.

How can you even use the smallest switch?
Everyone knows the problem. They want to use small switches, but could not withstand the current. This little circuit for much the same as $ 2 solves the problem. It is so sensitive that even a not too dry enough fingers to turn the atty. Or any small switch. The current on the switch is now less then 1/1000 mA.


I have made this post a separate thread, since the circuit can be used very universally.

I hooked the gate up to a 3a switch I had been using before. This was for testing out the mosfet. At first I had the polarity wrong and it stayed on all the time. Got that fixed but now it seems to have dramatically drained the voltage or something b/c my carto was barely firing. I'm using a buz11 mosfet which seems to be grossly overrated for this(looked over it when odering).

Help? Thanks

 

[Lee1111]

Oh and I'm testing it using a usb cable with 5v (which is mysteriously only 3.4v under load--but I think that's another post?)

 

[Jensb]

why you not use the irl3103 ? i use them and never have problems.

 

[Lee1111]

You think I just ordered the wrong one then? I wish I could make these work. Tight budget :/ Any idea why the atty isn't firing at full steam. Also, another note, when I had the polarity wrong, it was firing on full. Anyone with more help would be great.

 

[stouchon]

Having the polarity backwards may have damaged your mosfet. Other than that I would just suggest to double check your connections.

 

[Lee1111]

I have tried a second one. It DOES work as intended. Just not getting enough voltage or current through to fire the atty fully. works at about 1/3 - 1/4 performance as normal

 

[Jensb]

do you use the irl3103?
we had many dicussion in the german forum and with this fet it works best

 

[o4_srt]

you cannot properly fire an atty using a USB cable, unless said cable is connected to a transformer (wall adapter/power supply) and not your PC.

A PC USB port cannot supply enough current, at 5v, you need around 2 amps, most USB ports can only supply 500 mA. This is why your voltage is dropping under load.

The problem with your MOSFET is that the BUZ11 has a threshold voltage of 2.1-4 volts. Meaning, that in order to allow current to flow from source to drain, the gate needs to see source voltage + threshold voltage. So if your source voltage is 3.7 volts, the gate needs to see 5.8 to 7.7 volts.

The IRL3103 has a threshold voltage of 1 volt minimum, so your gate needs to be 4.7 or higher for a 3.7 volt device.

applying 3.7 volts on the gate of an IRL3103 while using 3.7 volts as your source voltage will only allow the MOSFET to operate in triode mode, or semi-on, making the MOSFET more like a resistor. In order to allow it to run in active (or saturation) mode, it needs to see drain voltage plus 1 volt on the gate. This will allow it to run wide open, allowing everything through, which is what we want.

I recommend using a P channel (enhancement) device instead, as the threshold voltage is usually negative. This allows you to reduce the voltage on the gate below what you want to see at your atty, and is far easier to achieve proper operation with. If you do not put the proper voltage on the gate, the MOSFET will be operating in an in-between state, and heat up, and possibly burn out.

 

[Lee1111]

do you use the irl3103?
we had many dicussion in the german forum and with this fet it works best

you cannot properly fire an atty using a USB cable, unless said cable is connected to a transformer (wall adapter/power supply) and not your PC.

A PC USB port cannot supply enough current, at 5v, you need around 2 amps, most USB ports can only supply 500 mA. This is why your voltage is dropping under load.

The problem with your MOSFET is that the BUZ11 has a threshold voltage of 2.1-4 volts. Meaning, that in order to allow current to flow from drain to source, the gate needs to see drain voltage + threshold voltage. So if your drain voltage is 3.7 volts, the gate needs to see 5.8 to 7.7 volts.

The IRL3103 has a threshold voltage of 1 volt minimum, so your gate needs to be 4.7 or higher for a 3.7 volt device.


I recommend using a P channel device instead, as the threshold voltage is usually negative. This allows you to reduce the voltage on the gate below what you want to see at your atty, and is far easier to achieve proper operation with. If you do not put the proper voltage on the gate, the MOSFET will be operating in an in-between state, and heat up, and possibly burn out.

Aha! Thank you finally for the solid answer, and suggestion for the IRL. I see I am just using the wrong mosfet. I will get some of those ordered. Thanks again. Supplier suggestions for USA?