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Comments on Isolated Digital Input - Overvoltage protection

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Isolated Digital Input - Overvoltage protection

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I am trying to build an active-low digital isolated switch capable of withstanding 32VDC voltage without creating any damage to the system.

This is the snapshot of the circuit not fulfilling the requirement: Circuit

"ISO_PIN_PROG_1" will be connected to the switch which will tie to the ground and causing the base of the Q29 to go below 0.7V hence cutting off the OC7.

"PIN_PROG_1" will be connected to the MCU input.

If there is a 32V applied by the accident on "ISO_PIN_PROG_1" this would cause problems with the circuit as is. How could I properly solve it?

  • Step 1: Since 3V3_ISO would be LDO driven I would probably add a diode with the anode connected to the LDO output and a cathode connected with the LDO input to prevent the LDO to have output higher than the input. Based on my experience they tend not to love that. I haven't post that because I haven't yet designed the power stage.

  • Step 2: Add Zener regulator at i.e. 12V with a properly dimensioned resistor and add one more resistor in series with Q29 base to limit the current - does it need to be limited because there is a limit how high Ic of the Q29 could be in full saturation? I believe it should because it won't be in the forward active region anymore.

I may be stuck having multiple unknowns and not being really secure whether this is the right topology for the task and whether I am addressing the problems correctly so your help would be kindly appreciated! Thanks as always!

Further clarifications ——————————————————————————

When input is left floating opto’s diode is forward biased as seen on the schematic. When input is low, the opto should be off therefore “PIN_PROG_1” should be high. That is really all in terms of usual usage. However, there is a chance that someone would connect 32V max on the input and without adding any support the circuit shown would be damaged. Proper operation during the event is not needed - so when between 3V3 and 32V on the Q29 base circuit and PSU should not be damaged, output of it is less important. Snap on action is not required. Esd protection is important. Input impedance should be high.

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General comments (3 comments)
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Skipping 1 deleted comment.

Olin Lathrop‭ wrote about 3 years ago · edited about 3 years ago

What do you mean by "active low". Should the opto be on when the input is low? You need to specify the full input range (you seem to have only specified the maximum), and what the function of input to output is. What threshold voltage? What input impedance is required? Is a gray area allowed, or should the result be snap-action? The answers to these questions will significantly effect the recommended circuit.

Olin Lathrop‭ wrote about 3 years ago

"When input is low, the opto should be off therefore “PIN_PROG_1” should be high" This makes no sense. When the opto is off, PIN_PROG_1 will be low because it will be pulled down by R73. You can always flip polarity by how the opto output is connected. The opto should be off for the input polarity that is most common. For example, if the input is usually floating, then sometimes pulled low, the opto should be on when the input is low. Is this really the case?

2kind‭ wrote about 3 years ago

It was a typo. Although I intended to have an opto on when input left floating because of no particular reason