Electronic – Digital logic input with very wide input voltage range

digital-logichigh voltageindustrialplc

We are developing an industrial controls product that will be used to monitor the presence or absence of voltage ranging from 5V to 480V. Since the unit will be generic and programmable, this input may be used in a variety of unpredictable ways.

The problem I have been wrestling with is how to monitor such a wide range of voltages with a single circuit design. For example, if I drive an optocoupler LED directly, I can't get the 5V to turn it on without the 480V destroying it. Voltage regulators typically operate on much lower voltage than 480V, so I'm in a bit of a dilemma.

The industrial controls solutions I've seen get around this problem by saying, "purchase this other model for high voltage input" or "buy this high voltage input converter and add it on." Is that really the only solution here? Am I trying to do the impossible? Any input would be appreciated, no pun intended!

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Best Answer

You should be able to do this if you think of this input voltage as a analog signal. Attenuate it by 100, and you have something in the 0-5 V range. 5 V in results in only 50 mV, but that is still plenty high enough to detect above any reasonable noise floor.

It might be a good idea to run this into a micro to actually measure the voltage, then have the code decide whether the input voltage is really "present" or not. With such a 100:1 range, I expect it's not as simple as just checking whether it's over 4 V, for example, or not. The code can hopefully check what the expected level is, perhaps see that it is fairly steady, or whatever. Keep in mind that a line that is at 480 V when "on", might have more than 5 V of noise on it when off. I think some logic that does more than just a dumb fixed threshold comparison will be useful.

Due to the high voltage, you want to use a high impedance divider else it will dissipate significant power. 1 MΩ top resistor and 10 kΩ bottom resistor sounds like it might work. That's not quite 1/4 W at 480 V in, and of course much less at lower voltages. It also provides 10 kΩ impedance output to drive the A/D input with.

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