P channel mosfet is always on

Hi dear

I have a project with arduino. I want arduino gpio open and close 12-14v changable external suply (car electric) with this schematic without zener.
Ekran-g-r-nt-s-2021-02-16-221314 — ImgBB

BC547 works well. Collector voltage changing when arduino pin turns HIGH or LOW. But mosfet is always open.
I checked gate - gnd voltage When gpio Hıgh - LOW 0.18v - 9v so there is a voltage drop from 13.5.
I cant raise gate - gnd voltage near suply voltage. Then I make shortcut gate and suply for a second withoot R10 res. MOSFET closed as I want. But this short cut heated BC547.

I tried some resistors different value but. I cant raise gate voltage. I tried lower r10 res then gate voltage not gone 0v it was 6v-9v wierd. I guess suplly current beat BC547

How can solve this.


Use cct. F1

What is VGS(max) ?

What is your load ?

IRLML5203 datasheet

FYI, the positive source voltage should be at the top and the load at the bottom so the mosfet symbol is
basically upside down, being that it is a HIGH side switch, convention dictates it should appear on the high
side of the load , rather than the way it is shown. Also, the zener voltage is not given.

Also, post title says "mosfet always on' but post says 'mosfet is always OPEN'.
'OPEN' means no conduction, or OFF, not ON.

Then I make shortcut gate and suply for a second withoot R10 res. MOSFET closed as I want. But this short cut heated BC547.

Was the transistor on when you shorted the gate to +12V ?

Try checking with your volt meter, gate to source. If it is about negative 5V or greater If it is a P channel mosfet it should be on and the the drain pulled to . If it is less then that it will be open and there will be no conduction. These are approximate values but they are on the data sheet. Figure 11 on the data sheet will give you the values. What is D5 for? If it is for inductive spike protection it needs to be either across the solenoid or drain source of the MOSFET. I would suggest you redraw the schematic to conventional standards.

Problem is almost solved. I turned revers 547. Then now gate source voltage almost same source-gnd. Mosfet can on and off.

bottom 12v is source. 12vbesleme is load.
zener also for gate safety 15v but I dont attached for now
one last problem maybe not. when Mosfet is off there is a small voltage on drain-gnd about 1v. Maybe I put a 10k drain -gnd

d5 for only safety. already I shorted it :slight_smile:

Yes I check figure. Now bc547 open goes "gate to gnd"...and "Gate - source" 13v than mosfet on.
bc547 closed than gate pulled up source so gate - source almost 0v than mosfet off
İts working :slight_smile:

Glad you found the problem. Your schematic is still
wrong in that the mosfet should be rotsted vertically 180 degrees.(The power source should be at the top and the load at the bottom because it is a high side switch , drawn upside down with the switch shown on the low side.) electrically it is correct but the schematic does not conform to standard convention.

That circuit is rather borderline - the MOSFET only has a gate-source voltage maximum of +/-20V, yet
the supply is a vehicle 12V, which are renowned for having heavy spikes of interference, and generally
being unclean supplies.

I'd have chosen an automotive grade MOSFET, these tend to have much high voltage ratings (like
55V for the drain, +/-30V for the gate).

I think its worth adding some decoupling to ground on the 12V supply as it feeds the MOSFET source,
to reduce the risk of rogue voltage spikes reaching the device.

The protection zener on the gate is a great idea, alas that circuit is missing the series resistor it needs,
which would go between NPN collector and the MOSFET gate (perhaps here 100 ohms would be suitable).

MarkT:
That circuit is rather borderline - the MOSFET only has a gate-source voltage maximum of +/-20V, yet
the supply is a vehicle 12V, which are renowned for having heavy spikes of interference, and generally
being unclean supplies.

I'd have chosen an automotive grade MOSFET, these tend to have much high voltage ratings (like
55V for the drain, +/-30V for the gate). ---> Great part what are some of the part numbers?

I think its worth adding some decoupling to ground on the 12V supply as it feeds the MOSFET source,
to reduce the risk of rogue voltage spikes reaching the device.

The protection zener on the gate is a great idea, alas that circuit is missing the series resistor it needs,
which would go between NPN collector and the MOSFET gate (perhaps here 100 ohms would be suitable).

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