So the reason the zener is from Vcc to Gate is the fact that it's a P-channel mosfet ?
If it was an N-channel, could you then put the zener from Gate to GND ?
raschemmel:
So the reason the zener is from Vcc to Gate is the fact that it's a P-channel mosfet ?
If it was an N-channel, could you then put the zener from Gate to GND ?
Correct.
Thanks. I've never used a zener with a P-channel mosfet so that was a new one for me. (not to mention that I neglected to look at the parts list so I didn't even realize it was a P-channel. I worked with transistors long enough that I can't look at one on a schematic without instantly recognizing it as NPN or PNP but I haven't got to that point with mosfets. I tend to completely ignore the direction of the arrow in the symbol until something comes up that requires a closer look. (like I need to know the specs and have to pull up the data sheet and see "P-channel" on the first page). I'll have to pay more attention to that.
Wawa:
I wonder why you went from a simple grounded emitter level translater (post#1) to a more complex Arduino voltage dependent constant current driver (post#5).
The first circuit can drive any mosfet fully "on", while in the second circuit you loose 4.5volt (with a 5volt Arduino).
I think the circuit from post#1 is more suitable for 12volt supplies, while the circuit from post#5 is better for 24volt supplies.
Leo..
Well because I had a comment that my simpler circuit (I presume you mean the one in the actual post) wouldn't work for higher voltages (say, 24 V) because of Vgs(max).
So I'm leaning towards keeping the simpler circuit for the circumstances that permit it, and getting the more complex circuit right if you need to switch higher voltages. I am currently testing that on real components.
Tests appear to indicate that the schematic in reply #5 works OK. Given:
- Supply voltage of 25V
- R1 = 1 k
- R2 = 330 Ω
Voltage drop of 10 V for VGS:
Turn-on time of around 50 ns:
Turn-off time of around 70 ns:
The blue trace was at the gate of the (hypothetical) MOSFET - so we are turning it on by driving it lower. I didn't actually have the MOSFET there, I wanted to just test the transistor switching.
Predicted current through R1: (5 - 0.7) / 330 = 13 mA
Measured with no supply voltage: 12.4 mA
Measured with 25 V supply: 12.8 mA
So that seems OK, yes?
So that is without the mosfet. Just the transistor/resistors/zener.
Two things to test. DC behaviour and AC behaviour.
DC looks good.
I think you can simm the mosfet by adding a 1n cap across the zener.
Leo..
I might actually grab a MOSFET from my parts drawer and try the real thing. Tomorrow, I think. 
If only I had something that worked from 24 V that would be great (like a 24 V light globe).
Use a 230volt heater, hotplate, iron, kettle etc..
Leo..