Bypassing a polarity protection diode with a part of a circuit

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2017/03/21 02:48:55 (permalink)

Bypassing a polarity protection diode with a part of a circuit

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 (1N4007) 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:

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).

4 Replies Related Threads

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    Re: Bypassing a polarity protection diode with a part of a circuit 2017/03/21 16:03:27 (permalink)
    Looking at the LM358 datasheet it appears the input terminals can tolerate up to 50mA. With 20V and >20k ohms your current will be limited to less than 1mA or so. I would proceed if I were building this.
    Not sure what S1 is so check that as well. It'll have a pathway to Vpow thru R4+R5.
    Since you'd be operating the opamps in a region they're not designed for, it may be best to build this up and test the reverse connection before plugging anything in to see what voltage shows up at the input. We don't know if the opamp input stages will breakdown and pass some of the voltage between its 2 inputs.
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    Re: Bypassing a polarity protection diode with a part of a circuit 2017/03/21 16:07:27 (permalink)
    Your knowledge is far beyond that of the average forum member. I don't think that your design question will be answered here. My experience is with high voltage analogue circuits, so I can't offer any help. I understand what you are trying to do but the circuits you test with that device are usually 5 - 12 volts and I don't see how you could damage the probe as originally designed. With an O-scope I can look for small signals riding on high voltage circuits and require the protection. But the logic probe is just hi/lo and I can't remember using it around high voltage. Perhaps you should just go with the stock design.

    Regards, John 
     I want to make it clear that I am an Eedjit. I have no direct, or indirect, knowledge of business, the music industry, forum threads or the meaning of life. I know about amps.
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    Re: Bypassing a polarity protection diode with a part of a circuit 2017/03/22 01:18:46 (permalink)
    Two things I noticed:
    1. Normally (without D7), diode D1 would safely shunt excess voltage from the Vin terminal into the battery (the battery is a low impedance sink to the overvoltage). This effectively clamps the input to Vcc + 0.7 volts.
    With the protection diode D7 in place there will be no place for excess positive input voltage to go. D7 is blocking the low impedance path to the battery.
    2. Why are you using an LM358 op-amp for a standard "Window Comparator" circuit?
    It could be made to work but there might be instability problems; you are running the op-amp at full open-loop gain.
    It could oscillate depending on the pc board layout.
    A much better way to do the Window Comparator would be to use an LM393 dual comparator IC.
    The LM393 has open-collector outputs and can sink 20mA. You can directly connect an LED with appropriate current limiting resistor to the comparator output. Connect the anode of the led to Vcc. 
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    Re: Bypassing a polarity protection diode with a part of a circuit 2017/04/03 10:58:09 (permalink)
    You could have used a schottky diode, which would have been only 0.3v. However, as someone else said, putting the 4007 in shunt would be better. So to change your circuit, replace the 1n4007 with a fuse and put the 1n4007 in parallel with C2.
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