somespirit
New Member
I need to accomplish two things with a simple circuit--
Derive 4-5V to feed into an MCU pin. (very low current, switch time not important)
On the same wire, I need to tolerate a device which, when attached, will source 5V onto the wire from a 5V 0.5A supply. In this case I need my circuit to do nothing, just tolerate the voltage (and not sink/waste too much, <20mA). I guess it would be a bad idea to allow current to go back into the 5V power supply (if my circuit created, say, 5.5V), or would the 10k resistor stop anything bad happening?
See attached,
To be clear- the switch represents the fact that two devices can be attached (although never at the same time). I can't alter the schematic above the 'switch'.
My beginners maths tell me that the schematic should work fine, my only concern is that I have inserted a resister after the zener diode which shunts the voltage above 4.6 to ground..... I've done this to limit the current wastage when the 5V sourcing device is attached. Will this work, or will this resistor act like a voltage divider with the 10k, thereby reducing the voltage fed to the MCU pin? Would it be better to get rid of this 100R resistor, and have a zener with a Vb of 5V, and trust that the regulated 5V supply and the 5V created by my circuit are close enough to prevent any significant current flow?
any advice would be appreciated!
p.s., all devices share common ground, although this is not shown for clarity.
Derive 4-5V to feed into an MCU pin. (very low current, switch time not important)
On the same wire, I need to tolerate a device which, when attached, will source 5V onto the wire from a 5V 0.5A supply. In this case I need my circuit to do nothing, just tolerate the voltage (and not sink/waste too much, <20mA). I guess it would be a bad idea to allow current to go back into the 5V power supply (if my circuit created, say, 5.5V), or would the 10k resistor stop anything bad happening?
See attached,
To be clear- the switch represents the fact that two devices can be attached (although never at the same time). I can't alter the schematic above the 'switch'.
My beginners maths tell me that the schematic should work fine, my only concern is that I have inserted a resister after the zener diode which shunts the voltage above 4.6 to ground..... I've done this to limit the current wastage when the 5V sourcing device is attached. Will this work, or will this resistor act like a voltage divider with the 10k, thereby reducing the voltage fed to the MCU pin? Would it be better to get rid of this 100R resistor, and have a zener with a Vb of 5V, and trust that the regulated 5V supply and the 5V created by my circuit are close enough to prevent any significant current flow?
any advice would be appreciated!
p.s., all devices share common ground, although this is not shown for clarity.