Hi all,

I've just designed a PCB for a project I'm working on and noticed that on some designs there's a resistor in series with the polarity protection diode, I'm guessing to protect it in case the diode breaks down and the current starts to flow through it to ground. But how do I calculate the value of this component? The diode is in parallel with the supply accompanied by some smoothing caps as you can see in the image.

Thanks

 

Generally for polarity protection install a diode in series with the 9 volt supply.
What part of the circuit requires polarity protection?

 

Generally for polarity protection install a diode in series with the 9 volt supply.
What part of the circuit requires polarity protection?

Yeah I see diodes in series are more common. The image above was basically how it is on the schematic, but the current is being fed to a tri of common-emitter amps

 

The diode in the schematic is not a polarity protection diode. It is a 9 volt zener diode to limit the voltage to about 9 volts if a higher voltage power supply is connected instead of a 9 volt supply. It will also provide some reverse polarity protection.
Les.

 

No polarity protection, reverse the input polarity and you blow C6.
As mentioned, Zener diode provides overvoltage protection assuming your circuity draws power at Zener (you don't show any power output connections on your schematic.
The resistor is required to drop the overvoltage when the Zener turns on.
BUT, the resistor will also drop the voltage due to the load (current) required by your circuit (not shown to the right).
So the supply voltage to your circuit will be less than 9V (typical) and will go no higher than 9V when input supply goes above 9V.
To calculate R you need to specify maximum current and voltage required by your "circuit"
In other words, you are showing a power supply intended to supply power to a circuit (not shown).

 

Hi all,

I've just designed a PCB for a project I'm working on and noticed that on some designs there's a resistor in series with the polarity protection diode, I'm guessing to protect it in case the diode breaks down and the current starts to flow through it to ground. But how do I calculate the value of this component? The diode is in parallel with the supply accompanied by some smoothing caps as you can see in the image.

Thanks

OK, the resistor is to limit the current thru the zener diode shunt regulator. Selecting the value really requires knowing the load current required by the item using the nine volts, and also knowing what the source voltage will be. The plan is to limit the zener diode current to the specified maximum when the load current is at the anticipated minimum.