Substituting a MOSFET with a NPN or PNP?

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Hi,

Please see the attached schematic (part of) that has T2, T3, and T4 using a MOSFET for amplifying the signal -- that is my guess, as I am not familiar with MOSFET at all. Ok, I can read on it, but my question is how would I substitute the MOSFETs, the IRLML2402 MOSFETs, with NPN or PNP transistors? Greatly appreciate for the sample wiring diagram.

I noticed that TWO pins are used for each MOSFET.

Thanks!

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Found the below

http://talkingelectronics.com/pr...

Helps explaining on the equivalent circuit to MOSFET.

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There's not enough detail in the schematic, so take what I am writing with a large dose of skepticism .

I am guessing you are attempting to interface a 3.3 volt AVR to a Commodore floppy drive bus, which used a synchronous serial line similar to the I2C bus.

On each bus line, there is a pull-up resistor, or possibly a current source, which keeps the line at 5 volts when not being driven.

PA4 thru PA7 operate as outputs driving the serial bus via the MOSFETs. When the avr output pin is driven high, the transistor turns on and pulls the bus line low.

PA0 thru PA3 are inputs, sensing the bus lines via voltage dividers (10K and 22K ?) which drop the 5 volt bus signal down to 3.3 volts.

You can substitute NPN transistors for the MOSFETs pretty simply in the following manner...

Connect the collector to the bus line.
Connect the emitter directly to ground.
Connect the base to the avr output (PA4 thru PA7) via a resistor. 680 ohms might be a good guess to start with, but this depends greatly on the chosen transistor.

You can leave the voltage dividers feeding PA0 thru PA3 as they are.

For safety, it might be a good idea to connect the collector to the bus line via a PTC fuse, or a limit resistor . This is in case of accidental connection directly to 5 volts.

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mikericetga wrote:
There's not enough detail in the schematic, so take what I am writing with a large dose of skepticism .

I am guessing you are attempting to interface a 3.3 volt AVR to a Commodore floppy drive bus, which used a synchronous serial line similar to the I2C bus.

On each bus line, there is a pull-up resistor, or possibly a current source, which keeps the line at 5 volts when not being driven.

PA4 thru PA7 operate as outputs driving the serial bus via the MOSFETs. When the avr output pin is driven high, the transistor turns on and pulls the bus line low.

PA0 thru PA3 are inputs, sensing the bus lines via voltage dividers (10K and 22K ?) which drop the 5 volt bus signal down to 3.3 volts.

You can substitute NPN transistors for the MOSFETs pretty simply in the following manner...

Connect the collector to the bus line.
Connect the emitter directly to ground.
Connect the base to the avr output (PA4 thru PA7) via a resistor. 680 ohms might be a good guess to start with, but this depends greatly on the chosen transistor.

You can leave the voltage dividers feeding PA0 thru PA3 as they are.

For safety, it might be a good idea to connect the collector to the bus line via a PTC fuse, or a limit resistor . This is in case of accidental connection directly to 5 volts.

@mikericetga: You are absolutely right! It's for the commodore 64/16/128 with IEC serial bus.

Yeah, I think the limit resistor is a good thing to have when connecting the collector to the bus line or Vcc; at least, that's what I have been using with NPN and/or see schematics on the net.

Attached is the full schematic.

I will try your suggestion, which is pretty close but more details than the link I posted earlier!

Thanks!

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