I've built a circuit which is meant to provide a 20 millisecond delayed turn on for another device.

It uses an LM555, and a P-channel mosfet (the device I'm driving needs a high side switch). R1 and R2 are load resistors which are unrelated to the delayed turn-on functionality I'm trying to achieve.

Circuit is shown below. The basic schematic is pretty much the same as many examples that I found from google, and seems to match what the LM555 datasheet tells me should work. My thinking is that the circuit should operate as follows:
- when power is applied to the circuit (via "LIN"), the Trigger and Threshold pins should initially be low. In this state, the output should be high, hence the mosfet remains off.
- after 20ms, the threshold voltage should have risen above 2/3 Vcc so the output should go low, hence turning on the mosfet.

I've built the circuit and put a logic analyser on the input LIN and output LIN_SW, but as best my logic analyser can resolve (down to the nanosecond), the output goes high immediately.

Any suggestions for what blinding obvious thing that I've got wrong ???

I get depressed by my inability to make even the most basic circuits work

 

I've built the circuit and put a logic analyser on the input LIN and output LIN_SW, but as best my logic analyser can resolve (down to the nanosecond), the output goes high immediately.

hi Geoff,
When using a PMOS, if the Gate is High, the PMOS is not conducting, so the load LIN_SW is not switched ON.
After the 555 Mono times out the 555 output will go Low turning On the PMOS,,, isn't this want you need for a 20mSec delayed turn On for LIN_SW.??

E

EDIT:
The PMOS symbol your circuit is drawn upside down.!

 

hi Geoff,

This is a LTS sim for your circuit using a PMOS FET [ correct way around].

Its shows a 20mSec delay after LIN power up to LIN_SW switching High.

E

 

hi Geoff,
When using a PMOS, if the Gate is High, the PMOS is not conducting, so the load LIN_SW is not switched ON.
After the 555 Mono times out the 555 output will go Low turning On the PMOS,,, isn't this want you need for a 20mSec delayed turn On for LIN_SW.??

Yep, that's what I was hoping to achieve....

Any suggestions for what blinding obvious thing that I've got wrong ???

The PMOS symbol your circuit is drawn upside down.!

Akkkk. I'm a utter and complete moron. Thanks. I'll try putting the mosfet around the right way and see how that goes.

 

I realise now how I screwed it up. Originally I set out using a low-side N-channel FET, and I guess when I changed over to the P-FET I forgot to switch it around. Arrrggghhhh....

Thanks for your help Eric. This forum (and the wise heads who frequent it) is absolutely invaluable.

 

Success !

I've re-built the circuit, and it now works as planned.

Thanks again for the help Eric.