# Zener diode question

Discussion in 'General Electronics Chat' started by somespirit, Jul 22, 2010.

1. ### somespiritNew Member

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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?

p.s., all devices share common ground, although this is not shown for clarity.

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2. ### MikeMlWell-Known MemberMost Helpful Member

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How much current can the 10-17V signal source?

Is the PIC supposed to interpret 10V as a logic low, while 17V is to be interpreted as a logic high?

3. ### somespiritNew Member

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It's meant to always read logic high. Simply put, I only have the 10-17V available and need to trigger the logic high however possible.

The 10-17V can source ~4A.

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5. ### MikeMlWell-Known MemberMost Helpful Member

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Try this circuit

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6. ### somespiritNew Member

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Okay, that makes sense and the output along the 0-17v curve looks great, thanks! My only question then is- would that circuit be okay if 5V was supplied at the MCU pin? It just needs to not break, and not sink/waste unreasonable current.

7. ### MikeMlWell-Known MemberMost Helpful Member

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I guess I don't understand what you are trying to do?

If the PIC is powered from 5V, then the PicPin's input buffer does not draw excessive supply current if the pin is >3.5V, or <2V. If the purpose of this network is to sense that the supply is >10V or missing altogether, then why are you trying to force the PicPin to 5V?

The thing you absolutely want to avoid is to force the PicPin more than 0.5V higher than the Pic's supply voltage. Having the PicPin voltage a bit lower matters not.

Last edited: Jul 23, 2010
8. ### somespiritNew Member

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The pic can be replaced by a device which puts 5V onto the same pin. I need to "ignore" that device.