diode question

[somespirit]

hi there- i'm trying to use a schottky diode to control power flow.

my design goal concerns one pin on an external connector. i wish to BLOCK any voltage externally applied to that pin (5-12V), whilst allowing 3.3V to be drawn from that pin as power for an external device.

to be clear- i do not wish to use the 5v as input, i want to block it.

can anyone please advise whether the attached schematic has any problems, or whether it should adequately achieve my goal,

many thanks

 

[ericgibbs]

hi,
The only problem would be the forward voltage drop of the diode, say as a Schottky it was 0.5V, then the 3.3Vout would become 2.8V, is that acceptable.?

 

[somespirit]

thanks, it helps to sanity check what i'm doing, never know when you're missing obvious problems!

it will only be used for the external device to detect the physical connection, so i'll check the datasheet to see whether 3.3v - Vf is enough to light up the pin input.. it should be!

since it is only powering the input capacitor of a microcontroller pin, the forward voltage drop should be pretty bang on the specification, as i understand it?

 

[ericgibbs]

thanks, it helps to sanity check what i'm doing, never know when you're missing obvious problems!

it will only be used for the external device to detect the physical connection, so i'll check the datasheet to see whether 3.3v - Vf is enough to light up the pin input.. it should be!

since it is only powering the input capacitor of a microcontroller pin, the forward voltage drop should be pretty bang on the specification, as i understand it?

The forward drop of the diode falls when current thru the diode is low.
If the mcu pin is very high Z then the cap could charge close to 3V-ish due to leakage.

 

[crutschow]

since it is only powering the input capacitor of a microcontroller pin, the forward voltage drop should be pretty bang on the specification, as i understand it?

Is there some resistance to ground to discharge the microcontroller pin capacitance when the voltage is removed? You need some resistance to ground for that.

 

[somespirit]

thanks eric, carl,

good point about the pull down to ground, i'll make sure i add that (..and make sure the micro controller's internal pull up is off).