# Zener and Diode to protect circuit

A

#### Aubrey McIntosh

Jan 1, 1970
0
A circuit that I grabbed off the web, probably from Olimex, has a 100
ohm resistor after the +5 connector, and then it has a 1N4148 reverse
biased, and a 1N3734A 5.6V zenner (of course, reverse biased). I
assume that these are all there to protect things just in case some
Saturday afternoon carelessness gets spilled on the bench.

Right now, I have detailed my BOM with Mouser parts 78-1N4734A and
78-1N4148 parts.

I thought that if the 0 and +5 are connected wrong, that the Zener
would then be forward biased and conduct as a normal diode. So, I
would not have put the 1N4148 into the circuit. I don't see anything
that enlightens me in the data sheet about the forward conducting
current of the Zener.

Why put in two diodes?

I

#### Ian Stirling

Jan 1, 1970
0
Aubrey McIntosh said:
A circuit that I grabbed off the web, probably from Olimex, has a 100
ohm resistor after the +5 connector, and then it has a 1N4148 reverse
biased, and a 1N3734A 5.6V zenner (of course, reverse biased). I
assume that these are all there to protect things just in case some
Saturday afternoon carelessness gets spilled on the bench.

Right now, I have detailed my BOM with Mouser parts 78-1N4734A and
78-1N4148 parts.

I thought that if the 0 and +5 are connected wrong, that the Zener
would then be forward biased and conduct as a normal diode. So, I
would not have put the 1N4148 into the circuit. I don't see anything
that enlightens me in the data sheet about the forward conducting
current of the Zener.

Why put in two diodes?

The supply voltage in the circuit with only one will be -0.6V.
This can be a problem for a very few things.
P

R

#### R.Legg

Jan 1, 1970
0
I thought that if the 0 and +5 are connected wrong, that the Zener
would then be forward biased and conduct as a normal diode. So, I
would not have put the 1N4148 into the circuit. I don't see anything
that enlightens me in the data sheet about the forward conducting
current of the Zener.

Why put in two diodes?

Capital observation.

Zeners in this voltage range tend to be faster than conventional
signal diodes, if anything.

RL