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# voltage limitation circuit

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#### explorick

##### Advanced Member level 4
i have an IC working from voltage 0 to 20V. I will shutdown the supply if the voltage exceeds 20V. But at the same time, i want to allow the circuit to be operational for a 40V transient(may be termed as long time transient)occurring for a minute. Can you let me know how to design a input circuit for this IC? If i keep a capacitor the time is too long to maintain the voltage constant which will make the capacitor value huge. Instead if zener diode is used there is a series resistor coming into picture which will have a series drop. Instead if I use varistor or TVS devices the voltage rating will be too high. What can be done to hold such a pulse and maintain the IC running?

Operational at 40V for 1 minute is not really 'transient', it is "operational at 40V" period. And it's difficult to term it a pulse either as far as this
voltage supply is concerned.

So, if an IC is not designed for operation beyond 20V, then you have no choice but to replace with a different device, or to insert a voltage regulator
(and I can see you've considered a zener as a regulator), or to consider some type of shutdown circuit (i.e. not maintain your operational requirement).

yes the 40V value cannot be termed as a transient but it is something that needs to be handled by my circuit such that it should not get propagated to the IC. And for a vlaue of >=40V for more than 1 minute the circuit will shut down the power supply to the IC. I was aware of 0.5sec dropouts of voltages which will be handled by a capacitor based on the current requirement.
The case stated here could be appearing in some of the designs. So I wanted to know if somebody implemented such things in a circuit. The values are not exact 20V or 40V for me. I just want to know how higher(not too higher like ESD pulses where energy of pulse comes into picture) voltages are handled if they occur for a time of say 1min.

I'm afraid I'm not really knowledgeable on failure modes of ICs. Maybe others can help you.
I guess it really depends on the device. You could do a simple test, just apply 40V to the device for less than 1 minute.
Certainly you hear of 5V ICs fry when people connect them to 12V, so possibly many 20V components may fry at 40V.

The IC would damage if the voltage is 40V.So that's why a circuit before the IC needs to be employed to make sure that the voltage is limited to 20V. And I want ideas on how this circuit can be designed

We're going in circles. I did suggest a voltage regulator, in my first post, rather than a TVS/Varistor approach.
If that's what you want, you'd need to investigate a regulator based on your expected voltage range, and your load requirements.

thanks.. Let me see if I can get more ideas from other members of this forum..

It seems like you need a programmable voltage regulator. Well, if you don't like normal voltage regulators, then you may use a micro controller attached with a transistor working as a programmable voltage regulator with the supply to the IC. You can sense the input voltage and according to it's value, you can program the MCU with PWM techniques and finally with filtering, you can get desired output dynamically.
But still, you have to use regulated 5V (and not beyond or your MCU will go boom !!) for the MCU. But as MCUs use very low power, you may simply use a zener here.

Rocket scientist

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