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Asymmetrical ESD protection with unidirectional TVS?

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jasensio

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Hi everyone!
I have some 24V inputs and I need to protect them against surges. It is not difficult to find an unidirectional 24V TVS to clamp positive spikes, this one for example (using one zener per input):

**broken link removed**

But, would it be suitable (when forward biased) to protect the input against negative signals and avoid voltages below -1V/-2V get into the system?


Regards.
Jose
 

andre_luis

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But, would it be suitable (when forward biased) to protect the input against negative signals and avoid voltages below -1V/-2V get into the system?

It is quite advisable, and depending on the currernt strength of the incoming signals, such protection can be achieved by anti-parallel diodes.
 

jasensio

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Hi Andre.
Do you mean to protect within the whole range or only for negative spikes?
This is what I have.



The TVS's clamp voltages above 24V which is fine.
The problem is that the optocoupler has a maximum reverse voltage of 5V, and more circuitry is connected to that point.
 

andre_luis

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I was rather referring to place a diode reversely biased in paralel to the diode's opto inputs. In the case of reverse polarization coming from field sensors, R1 would protect both the inner IC diode, as well this additional diode.
 
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