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I recently built a logic probe adapting a circuit from an old magazine. One of the modifications I introduced is adding a polarity protection diode () in series with the positive power rail, because the probe is intended to be powered from the circuit under test and I want to avoid damaging the probe should I mistakenly connect the power rails to the wrong place.

This has led to a small error in the definition of the voltage levels the probe recognizes as HIGH and LOW. That's because the thresholds are defined by a 3-resistors divider placed across the rails, which in the original design sensed the actual supply voltage of the circuit under test, and which now suffers from a 0.7V drop due to the protection diode.

I didn't want to modify heavily the circuit, so I thought of connecting the upper leg of the divider directly to the power rail of the circuit under test, bypassing the protection diode. I simulated the circuit with LTspice and everything is fine.

Here are the schematics for the input stage and the power supply stage:

5PhWP.png

8IY3Q.png

My question is: is this design decision risky in case of a polarity inversion in the rails?

In this case I reckon that the current that may flow along "wrong paths" will be heavily limited by the resistors of the divider, which are rather large. Moreover the divider is connected to the inputs of an LM358 opamp, which should be quite insensitive to reverse voltages spikes of modest energy content (it's a bipolar design, so no MOSFET gates that can be damaged by voltage spikes even if current-limited).

Am I missing something? Is my reasoning correct?

P.S.: I assume a max reverse voltage of 20V (worse scenarios are less likely, and I won't bother to design a foolproof protection scheme for those).

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  • Latest Post Latest Post by rproffitt

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— neat instinct and a sensible LTspice check, but beware a real-world corner the simulator may not show: by tying the divider to the pre‑diode rail you restore correct thresholds, yet you also give any reversed supply a direct path (through the divider and into the op‑amp pins) that the series diode would otherwise block. Internal input clamps or input stage conduction of the LM358 can turn that small current into a damaging condition if it is large or sustained — check the LM358 absolute‑maximum input/diode‑current in the datasheet before assuming immunity.

A practical, low‑effort way to keep your hack safe:

  • Try ’s Schottky suggestion first (much lower Vf than a 1N4007) and re‑trace the divider; that often fixes the threshold error with minimal change.
  • If you still want the bypass: put a high‑value series resistor between the pre‑diode node and the divider top (start in the 100k–470k range) to limit worst‑case reverse current, and add a Schottky clamp to the probe ground (anode at ground, cathode at divider node) to catch negative excursions.
  • Add a small series resistor at the LM358 input (10k–100k) plus a small capacitor to ground (100 pF–10 nF) to limit clamp currents and filter noise — this is the quieting + protection combo alluded to.
  • For a robust, low‑drop solution replace the diode with an ideal‑diode MOSFET arrangement or a purpose IC.

Quick check you can do now: compute Iworst = Vreverse / Rtotal. Example: 20 V / 200 kΩ = 100 μA. Compare Iworst to the LM358 input/clamp ratings from the datasheet. Finally, validate on the bench with a current‑limited supply and try the reversed connection for a few seconds while monitoring node voltages — simulations are useful but may not show clamp/latch‑up behavior.

Member Avatar for rproffitt

Or replace D7 with a Schottky diode to minimize the drop and adjust your voltage divider to cancel out more of the error introduced by the source reverse polarity protection.

R2 and R3 could likely be 10X higher in value to avoid throwing off the Vin voltage. I don't know everything about your application but if protecting the node that contains U18 pin 3 is the goal, then a very small capacitor there in the order of say around 100pF might be added from U18 pin three to GND.

If you think there is noise on what looks like a voltage divider of R7, 6 and 7, you can add small filter caps to quiet that section.

I'm just a gray hair electronics designer.

commented: Your given solution is quite clear and simple. +0
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