Pull up to 5V on a wire that may occasionally see 60V

Question

I have a wire that either floats - that's when I want to pull it up to 5V - or it may see up/down to +/-60V applied to it - and that's when I really don't want it to be connected to the 5V rail. Problem is that I can't figure out how to make a switch that will stay off when large voltages are applied to the wire. There are all these different combinations of voltages that can be applied to these wires - I'm only showing the extreme cases in the diagram.

^{simulate this circuit – Schematic created using CircuitLab}

I want the switch to be closed only when the wire is floating (leftmost case on the diagram) and open in all the other cases. All my attempts were along the lines of connecting a BJT between the 5V rail and the wires - but all of that fails when the wire goes negative and the transistor is in the reverse active region.

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I'm just curious at this point if this can even be done with discrete components, but if there's an IC that can do what I want that's fine by me. Thank you and I really appreciate all the help and advice.

Edit 1: These two wires are inputs to a differential amplifier whose output is fed to an ADC. Input voltages range from -60V to +60V, but the two wires are never more than 60V apart. That means the top wire can't be at 60V and the bottom wire at -60V.

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Sometimes the input is "dry" in a sense that it needs a connection to the power rails to produce a meaningful input. For example, the output of an open collector tachometer without a pull up will "float".

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5V is a power rail that powers a lot of things in the circuit - that's why I want to protect it as much as possible. Also, +/-60V signal is an input to the op-amp and I'm worried putting a very large resistor will affect the accuracy of the measurements.

Edit 2: Can the following circuit possibly work? I ran a sim and it checks out.

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Answer 1

To avoid current going "into" the 5V rail when there is 60V on the wire, put a diode between the 5V and the wire. (Anode on 5V, cathode on wire.) You may want to lift the 5V source to 5.4V or so to compensate for forward voltage loss of the diode at zero current. Also make sure you choose a diode with > 65V rating.

To avoid too much current going out of 5V when the wire is "at" -60V (whatever that means,) add a current limiting resistor. 1 kOhm might be enough; 10 kOhm should be plenty in most cases.

The design question you need to solve, and haven't answered, though, is: What is the source impedance of the +/- 60V? And how important are the 60V? If the 60V is "some other signaling protocol" then perhaps the source impedance there is high, and you need to make your 5V source current limiting resistor be high enough to not interfere with signaling.

Also, how important is the 5V? Is it for signaling, or will it power anything? What is the load impedance that will "see" that 5V?

Answer 2

You have not explained your requirements very well.

To summarize what you have explained:

1. You have a wire that can be floating. One assumes this means it is NOT being driven by a voltage source or connected to ground. Only under this condition do you want to connect a pullup to +5V.
2. The wire can be connected to Gnd.
3. The wire can be connected to +60V.
4. The wire can be connected to -60V

A circuit that achieves this could be as follows (though I'm sure it's not what you want):

^{simulate this circuit – Schematic created using CircuitLab}

Notice that the wire is never 'floating' with this solution since the Relay coil is always connected to the wire. But it does achieve your goal in that when no other source (+/-60 or Gnd) is connected to the wire it will be held at about 5V.
The relay could be a SPDT Reed relay which is low current and supports a wide voltage range.