hello all,

I have been looking at a lot of zener datasheets, bit none one them mention how long it will take for the Zener voltage (Vz) to appear when the zener is reversed baised. I assume this will not be an instantianous change from the ideal open circuit (for voltages across the zener that is less than Vz), to Vz (for voltages across the zener that are greater than Vz).

I am using a resistor (current limmiting) in series with a 4.7V zener, where the top end of the resistor is connected to +12V and the anode of the zener is connected to ground. the junction of the resistor and the zener is connected to an input pin of a TTL ic. my thinking is that as soon as 12V is applied to the resistor, 4.7V should be present at the pin of the ic. However, if the zener does not switch on instantianously, the zener will be seen as an open ciucuit and the resistor as a pull up resistor to the +12V, hence, for that intant in time before the zener switches on, the TTL IC input pin will be pulled up to 12V which will effectivle blow the chip.

Can anyone help me with the turn-on-time of a typical low power 4.7V zener?

I plan to use a cap in parallel with the zener, hence forming an RC network with the series resistor. the input pin of the IC should see the votlage across the cap untill it exceeds Vz, at which point the zener should switch on, before the votlage across it exceeds Vz by too much. I could use another resistor in series with the the first resistor to form a voltage devider, set at 4.7V, here the zener will be used just as proctection.

Any ideas on which is the best option, or if there are any better ways to do this please let me know.

Thank you for your time and effort
sun.