Trying to switch P channel MOSFET IRF9540PBF-BE3 direct from NE555

Power supply is 12v DC

Tried all? sample circuits from Google, but I just cannot get it to switch on or off

The U15 LED is oscillating away, with 11V and no effect on U21 . But as soon as I connect the NE555 output signal to G1 directly, U15 switches off, and U21 comes on. Something is dragging down the NE555 output.


Any advice or guidance, please.

 

Hi gray,
What voltage do you measure on the Gate?
E

 

Hi Eric

With U15 'ON' measurement at G1 = 11.8v
With U15 'ON' measurement at D2 = 0.062v

With U15 'OFF' measurement at G1 = 10.7v
With U15 'OFF' measurement at D2 = 0.092v

I hope that helps

 

Are you sure U20 is connected correctly?
Readings don't add up.
You say when U15 is OFF the voltage at D2 is 0.092 but you mention U21 is ON.
Is R27 installed?
With U15 OFF the voltage at G1 should be practically zero

 

I'm going to assume you left out the other connections to the 555 for clarity.

 

Check the values and connections for R25 and R27.

 

I didn't look up the specs on your MOSFET but having the one that is responsive to low signal levels is a plus. I have used these MOSFETS to drive stepper motors.

 

Your 555 circuit looks a bit simple. There are some great illustrations in Forrest Mims and Nuts & volts magazine.

 

Please update your schematic with *all* of the circuit components and connections.

ak

 

But as soon as I connect the NE555 output signal to G1 directly, U15 switches off, and U21 comes on.

So just connect it directly.
You don't need R25 and R27.

 

The U15 LED is oscillating away, with 11V and no effect on U21 . But as soon as I connect the NE555 output signal to G1 directly, U15 switches off, and U21 comes on. Something is dragging down the NE555 output.

It would be helpful if you used standard component designators - U is typically used for integrated circuits. LEDs use D, CR, or LED, MOSFETs use Q or M.

And show all connections.

You can avoid the unnecessary colors by printing to B&W PDF and posting a clip from that.

Does your schematic editor allow you to create your own symbols? The one used for the 555 timers is essentially useless.

 

I'd say that R25 = 100Ω not 100k.

 

Just tried a new MOSFET and its the same.

If I disconnect pin3 (output) of NE555 then LED U15 comes 'ON' but U21 still stays 'OFF'

However if I short G1 to ground, I can get U21 LED to come 'ON' and 'OFF'

This gives me the impression that the MOSFET is working, but its something to do with the resistors, or even having U15 in the circuit.

 

I'd say that R25 = 100Ω not 100k.

Changed R25 100k resistor to 100 ohms, removed R27 1k resistor, and replaced it with a link (which meant U15 was then disconnected).
Hey presto the output of U21 is working, with an 'ON' at 11.8 volts and an 'OFF' at 2.3 volts. 100 ohms 0.25 watt resistor is getting VERY HOT.

 

You don't need R25 and R27.

Maybe. The U20 gate threshold voltage could be as low as 2 V. The bipolar 555 datasheet chart for high output voltage indicates that this should not be a problem at low output currents, but some extra margin wouldn't hurt. You could decrease R25 to 10K, and increase R27 to 2.2K.

ak

 

100 ohms 0.25 watt resistor is getting VERY HOT.

That's because it's trying to dissipate 1 watt.
Replace R25 with a 10K resistor and eliminate R27 as mentioned earlier.

 

Replace R25 with a 10K resistor and eliminate R27 as mentioned earlier.

Hi - That works a treat, no overheating of resistors. It's been working on soak test for 2 hrs, and it's working great.

Many thanks for everybodies advice and help.