Fet for turning on high voltage.

[Ioannis]

I think this is what you want.

Maybe you have to check the values of R6, R7 to ensure Vgs of the P-Mosfet is within safe limits.



Also you can replace the N-Mosfet with a BC337 and a 1K base resistor. But I think the proposed schematic is more elegant and fast without gate resistor (direct PIC connection). The P-Mosfet for my application is low Rds and much more Amperes than what you need. So, maybe, you have to choose another one.

My selection on R6, R7 was with speed to consumption priority and with these values they should be at least 1/2W. If you can live with smaller speed, or you only need ON/OFF case, then select higher values, but always make sure you give P-Mosfet gate the correct voltage levels.

Ioannis

[HenrikOlsson]

Hi,
Thank you for posting the schematic Ioannis, that's the circuit I tried to describe earlier.

Looking at the datasheet for the P-channel MOSFET we can see that it has a specified gate threshold of -2 to -4V (ie it starts to conduct at a Vgs of -2 to -4V) and a maximum Vgs of +/-20V. In this case simply grounding the gate of the P-channel thru the N-channel switch would have put 24V between the gate and the source of the P-channel MOSFET which is out of spec for it - the voltage divider takes care of that.

When the N-channel MOSFET is "off" the gate of the P-channel is pulled up to 24V by the 1k resistor, the P-channel MOSFET is off. When the N-channel MOSFET turns on it grounds the lower side of the voltage divider which makes the voltage at the gate of the P-channel MOSFET ~7.5V. 7.5V at the gate minus 24V at the source gives a Vgs of -16V which is well above the threshold yet within the +/-20V maximum.

Obviosuly, using a 74A MOSFET to switch 100mA is overkill but the principle of switching the high side is the still same. Using a solid state relay as suggested or perhaps even an optoisolator capable of supplying the current would of course work nicely too.

/Henrik.

[Ioannis]

Hi Henrik.

You are right about the power mosfet, but the schematic was ready from another project. I mentioned that, so one can of course select a lower amp one.

I just needed the low Rds for my project. P-Mosfets with low Rds, are hard to find.

The idea of optocoupler is great! Vishay SFH619A is 125mA capable and sure there must be stronger devices.

Ioannis

[jmgelba]

Thanks for the responses guys. Good stuff. This is what I tried in simulation and it works. The NFET used will be a STN3NF06.

[HenrikOlsson]

Hi,
If I'm looking at the correct datasheet the SI4401 P-channel MOSFET has an absoulte maximum Vgs rating +-/20V. If wired as in your schematic the MOSFET will see 24V (or 45V if that's the supply voltage) between the gate and the source which is above its rating - in real life it will likely self destruct even though it doesn't in the virtual world.

You need a second resistor between the gate and the collector to form a voltage divider as in Ioannis schematic.

/Henrik.

[jmgelba]

Henrik, Im not using the SI4401 in the schematic, it was a random device picked from a list of available parts.