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I'm trying to figure the best way to protect the external circuit from a short if a user mistakenly connects the sensor in the wrong polarity. I use a green object in the diagram to represent the second circuit.

The simplest polarity protection is a single diode on the positive side, but i'm considering what would happen if they connect the positive side to the ground on my circuit? Both circuits will share common ground. I'm not sure if what I have below would protect against anything or even work. Then again i'd have to contend with diode voltage drop (maybe use schottkey? i dunno). What do you think?

enter image description here

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  • \$\begingroup\$ Use a polarized connector and move on with your life. \$\endgroup\$ – mkeith May 21 '18 at 2:26
  • \$\begingroup\$ I've decided to use a thermister. \$\endgroup\$ – DonP May 22 '18 at 13:36
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The diode is a common way to go. 95% of times you would be fine with it.. But most of the people here, inncluding me, dont know the internals of the device. So, for your case to go, as a researcher, is to find an internal schematics for the device and proceed with its research. Sometimes (and is more often in nowdays) EE researchers finishes up with circuit protection schematics. Sometimes not. You have to research it by yourself or to pay specialized lab with some required hardware to do that

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First, give this a read - FETs work great for reverse protection, since the circuit just doesn't power on.

http://www.ti.com/lit/an/slva139/slva139.pdf

Second, I would current-limit or shunt your protection for things like sensors. Using diodes will not only give a voltage drop, likely skewing your readings, but when someone actually tests them it can still cause damage. In this circuit, the sensor wouldn't power up due to the diode on the ground side. Instead, using a zener to shunt any voltage beyond what your device can handle gives you a chance to size the protection (diode wattage, etc.) based on the environment you're working with (and doesn't affect the reading)

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You are suffering from the paralysis of analysis. You simply cannot account for every possible bad scenario. In fact you have done the most basic step already. There are a hand-full that work well with very few parts.

A series diode. Yes, a Schottky diode would cause less voltage drop, and I am not sure why you have a diode on the ground side. D1 and D3 protect against reverse hook-up of the sensor, so D2 and D4 are redundant.

The only additional protection that actually works is a series resistor of 1 K or so before D1 and D3, and a 5.0 to 5.1 volt 1 watt Zener diode to ground after D1 and D3. Now you have over-voltage and transient protection as well.

U1 likely has high-impedance digital inputs so a 1 K resistor is more of a transient current limiter. If the inputs seem sensitive to noise add a 10 K resistor from the input pin to ground, and a 1 nF capacitor across those resistors. Now it is much more immune to local EMI noise such as hand drills, CCFL or florescent tube lighting, etc.

Per @mkeith's comment below, make sure you use polarized connectors.

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    \$\begingroup\$ Best way is to just use a polarized connector. \$\endgroup\$ – mkeith May 21 '18 at 2:27
  • \$\begingroup\$ Thanks for the reply. My reason for having D2 and D4 is that when I ground both devices together, a signal input incorrectly sent to the ground connector on the second circuit will damage (as happened today) the sensor on the first circuit. A polarized connector is a good suggestion but for this application where probing will be done, a standard connector is not an option. That is, the user will be probing the sensor of an automotive circuit and has the freedom to switch the connectors signal/ground with each other. \$\endgroup\$ – DonP May 21 '18 at 3:17
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I've ended up using a resetable fuse resistor. 20 ma. If the sensor signal is shorting to ground it will break.

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