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Are there any drawbacks to placing a diode in the conduction path to ground for reverse polarity protection? A major plus is cost, because you can use multiple low current diodes, instead of one larger diode (or PMOS) on the input.

enter image description here

My thoughts are that having an offset voltage which varies with current is not ideal. I also remember reading that keeping an extremely low impedance path to ground helps to ground high-frequency noise which would reduce EMI. Are there any other issues that I may have overlooked, or does anyone utilize this method successfully?

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    \$\begingroup\$ My thoughts are that having an offset voltage which varies with current is not ideal - not ideal? I would say it's pretty devastating in most cases... \$\endgroup\$ – Eugene Sh. Oct 27 '17 at 15:00
  • \$\begingroup\$ We don't do opinions here, they generate bad answers (and its off topic). Please edit your question and ask a specific one. Also provide some context. Its unclear if you wish to put said diodes on the ground of a digital IC (which would really mess things up), a device (which would also mess things up) or some kind of transistor \$\endgroup\$ – laptop2d Oct 27 '17 at 15:14
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    \$\begingroup\$ @laptop2d: "We don't do opinions here ..." That comes across as being a bit harsh. Clearly the question is asking what the potential problems might be. It's his/her first post and it shows that at least the OP is aware that there could be a problem. \$\endgroup\$ – Transistor Oct 27 '17 at 15:27
  • \$\begingroup\$ @MarcusMuller XY-problem? \$\endgroup\$ – winny Oct 27 '17 at 16:13
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If you want to protect a board against having the wrong polarity power applied, the usual way is to have one adequately rated diode at the positive DC input, and a second one to prevent the positive rail from going more than 0.7V negative:

schematic

simulate this circuit – Schematic created using CircuitLab

The advantage of this is that your 0V reference is not compromised. I've never seen the method you propose used, and it could indeed cause problems - not least that every signal input to the device will now go more negative than the ground pin, which could cause all kinds of odd behaviour, depending on the device.

The proper use of ground reference is pretty important in just about every electronic circuit.

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You are correct in thinking that there could be problems. Obviously, as you've stated, there is an offset voltage on the whole chip internal ground. Any analog inputs or outputs to external ground-referenced devices would require careful consideration.

In the case of the schematic snippet you have posted you would also need to consider what voltage each of the devices would see in the event of reverse polarity.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The "protected" chip may not be all that well protected.

As shown in Figure 1, for example, D1 has prevented current being driven through the GND pin and backwards through the chip. 'IN', however, has no protection and some current may flow. Whether the chip survives that or not depends on the current and the internal structure of the chip.

A major plus is cost, because you can use multiple low current diodes, instead of one larger diode (or PMOS) on the input.

Careful here. You can't use diodes in parallel to increase current handling unless you take precautions (typically a little series resistor for each diode) to balance the currents through them.

My thoughts are that having an offset voltage which varies with current is not ideal.

Correct. If each chip had a separate diode they'd all be at different voltages.

I also remember reading that keeping an extremely low impedance path to ground helps to ground high-frequency noise which would reduce EMI.

Yes. You might get away with adding the capacitor to the chip + and - supply pins and leaving the diode out.

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  • \$\begingroup\$ Thank you for your response, let me clarify I meant that if you have multiple nodes requiring protection you can provide a lower current diode on each (not multiple diodes in parallel for one node). \$\endgroup\$ – user92266 Oct 27 '17 at 17:23
  • \$\begingroup\$ In which case they would all have slightly different ground voltages. \$\endgroup\$ – David Schwartz Oct 27 '17 at 17:26
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There could be some design considerations, and it mostly depends on if you need a really low impedance path in your circuit.

take for instance the example of car stereo amps, the basic reverse voltage protection is a reversed biased diode across the power input after the fuse. If the person installs the amp power leads backwards, the diode shorts and pops the fuse.

The diode btw would just need to be silicone type so it shorts and causes the fuse to go so even a cheap diode (like a 1n1004) will take out a fuse.

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