really simple ee help needed

[cirthix]

I feel like an idiot posting this

I have a power supply that has an inhibit pin that will shut the supply off if it's shorted to ground and will allow normal operation if it's open-circuited. I want to control this with the ps_on signal from a motherboard so taht hte power supply is on when ps_on is grounded. the caveat: if an external voltage pulls up on the power supply's inhibit pin, it WILL kill the power supply.

basically:
ps_on=5v, inhibit pin shorted to ground
ps_on=0v, inhibit pin open-circuited

how can i do this?

 

[gsethi]

Solution: Buy a New Power Supply.

 

[cirthix]

this isn't a standard atx computer, it's powered from a very small dc/dc converter that's powered from batteries

 

[PottedMeat]

Tri-state buffer?

Hold the input of the buffer at ground, connect the 'enable' pin to ps_on, and hook the buffer output to the inhibit pin.

When the buffer is enabled it will pass the signal from input to output. When it is disabled, the output should be high impedance.

 

[cirthix]

Originally posted by: PottedMeat
Tri-state buffer?

Hold the input of the buffer at ground, connect the 'enable' pin to ps_on, and hook the buffer output to the inhibit pin.

When the buffer is enabled it will pass the signal from input to output. When it is disabled, the output should be high impedance.

I found one of these on an old motherboard :74LVC125AD

Philips Quad buffer/line driver with 5-volt tolerant inputs/outputs 3-State

that'll work, right?

 

[RalphTheCow]

Either call Dilbert's mother or Dogbert's Tech Support Line.

 

[darthsidious]

also, if the inhibit pin doesn't source/sink current, you could just use a mosfet with the source at ground, and the drain connected to the pin. Then just turn the gate high when you want the pin to be at ground, and the gate low when you want the pin to float (this is basically what we call a 'power-down device' in IC design.

Using a tri-state buffer is probably the smarter and safer thing to do though.

 

[So]

Originally posted by: darthsidious
also, if the inhibit pin doesn't source/sink current, you could just use a mosfet with the source at ground, and the drain connected to the pin. Then just turn the gate high when you want the pin to be at ground, and the gate low when you want the pin to float (this is basically what we call a 'power-down device' in IC design.

Using a tri-state buffer is probably the smarter and safer thing to do though.

See, my first thought was a mosfet as well. It's dirt simple.

 

[cirthix]

Originally posted by: darthsidious
also, if the inhibit pin doesn't source/sink current, you could just use a mosfet with the source at ground, and the drain connected to the pin. Then just turn the gate high when you want the pin to be at ground, and the gate low when you want the pin to float (this is basically what we call a 'power-down device' in IC design.

Using a tri-state buffer is probably the smarter and safer thing to do though.

that didn't work, already tried it. i'm going to wire up the tri-state buffer now.

 

[Colt45]

just a plain PNP transistor

collector to inhibit, emitter to ground,

base has a 10k pullup to +5_SB, and and a 10k to PS_ON


plain jane caveman transistor like a 2N3906 will work
total cost = 10 cents, maybe..

wait thats backwards..

ps_on=5v, inhibit pin shorted to ground
ps_on=0v, inhibit pin open-circuited

use NPN then (3904)

collector - inhibit
emitter - ground
base - 4.7k to PS_ON

 

[Analog]

A simple bjt transistor open collector will do the trick. When the base of the bjt sees 5V (via a small dropping resistor), the collector is essentially shorted to ground. When the bjt sees 0V, it is off, and the collector floats (open circuit).

 

[cirthix]

hah, the mosfet idea did work, my problem was that i had the pads on the board switched around since it's on the bottom of the pcb. stupid me!

thanks guys, it's working properly now

I just kept bashing my head and thinking that i didn't understand something, but the problem was a layout one. gah!!

 

[Turin39789]

How did I end up in the psu forum?