Let me preface by saying that I'm clueless when it comes to this. I hope this is the correct place to post this.
I'm working on a project from years ago and I don't remember much about the development of it. I handled the actual building of the boards and my friend did the pcb layout.
It was a board for a paintball gun. Voltage was regulated to 5v. When you'd flip a switch, it would fire a solenoid. I've listed some details about the designs below, but the old board built with the 8051 chip would fire any solenoid you hooked to it. The second design used pretty much the same circuit and used the same voltage regs, caps, etc, but for some reason switched to the 355 part. One was an N and one was a P channel. The atmega board with the 355 wouldn't fire the bigger solenoids that the previous design would.
I had a project that used an atmel 8051 chip and this transistor MMBT3906
Due to the 8051 becoming obsolete, we switched to an atmega8 and used this mosfet 355an
I feel like the 355 doesn't allow the same amount of current as the other part, but I honestly have no clue.
I'm hoping someone could point me to a part that would drop into the 355 spot, but possibly provide more power. Like I said, I do not have a clue about this and it might not even be possible.
Transistor & MOSFET driver circuits are "similar" but you can't just drop one in place of the other. And in either case, if the solenoid voltage is higher than the 5V processor/controller voltage you need an NPN transistor or an N-Channel MOSFET. I assume the solenoids are 12 or 24V.
A transistor is "easier" to turn-on because it takes less than 1V and MOSFETs are generally better for high-current. The 3906 is only rated for 200mA so apparently you don't need a lot of current.
You switched from a 40V, 200mA PNP transistor (which appears to pretty much be a surface mount 2N3906 at first glance) to a 30V, 1.7A N channel MOSFET. May I ask why? The 3906 is still very much available; DigiKey quite literally has millions of them in stock. You didn't just drop the MOSFET into the circuit designed for the 3906... did you?
I couldn't tell you. Like I said, my buddy designed the board and I don't know what the reasoning was. It was 20 years ago.
The board operates off a 9v battery and the solenoids are 5v, but some have slightly bigger coils. They're only getting hit with power for like 6-10ms. and I believe others have just fed them 9v instead of regulating the power.
Someone over the years had suggested that this would be a much better option than the 355. Suggested
From comparing the datasheets, it looks like it has higher current and better stats overall. But like I said, I'm pretty clueless.
It's like the difference between gasoline and diesel fuel.
They're both liquids.
They both go in fuel tanks.
Doesn't mean they're interchangeable in any way shape or form. You put diesel into the tank of a car with an engine that runs on gasoline and you're going to be replacing the engine.
You can't just drop an N channel MOSFET into a circuit designed for a PNP transistor. They're completely different components.
No, it wasn't just a parts swap. It was a redesign of the board with a new chip and for some reason the new Fet. Looking back I've got gerbers for the original board, but no schematic.
So the question is, can I find a fet that will drop in and allow better operation like the one I listed above? Or am I better off just leaving them as is with the 355? If I can get better performance with a new Fet, that's what I'm after. I'm not looking to do a board redesign or anything.
It's not worth the effort of putting a lot of time into, but if I could make it perform better with a different drop in fet, then I'd try it..
Generally speaking, in circuit design you need to understand that "normal" BJT transistors are fundamentally different from Mosfets. Transistors amplify based on the current at the base, while mosfets regulate almost exclusively based on the voltage supplied to their gate. You can fully open a Mosfet with a few microamps, provided the voltage is as high as the drain, although it starts to work only at about 3 VDC and has a similar voltage drop.
Also always place the inductive load before an NPN part, but after an PNP. (else it doesn't work at all)
Even tho I'm a rookie, these things helped me a lot.