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how to calculate the value of resistor?

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Advanced Member level 3
Jun 24, 2012
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I'm using zener diode 1N5239. and my input voltage is 24V. I have to connect the resistor to make current flow through the zener diode. How to calculate the value of resistor and what is the possible current to flow through the zener? Please look into 1N5239 datasheet and tell. Please help!!!

Thats a 9.1v, 500mW zener.

This implies it can handle a max current of ~55mA ( VxI = W ). So anything less than this is OK.
Now, we need a series resistor which must drop (24 - 9.1 =) 14.9v, and ensure that current is less than 55mA....

SO R = V / I = 14.9/ 0.055 ~ 271 ohm.

Any resistor value >= 271 ohm will do the job.
So what will be the minimum current to be flow to activate the zener?

No specific minimum, but zeners work better at high currents than low. e.g. 10mA will be much better than 1mA. The datasheet I looked at used a test current of 20mA.

Maybe just use a 1K 1/2 Watt resistor. That will give about 15mA, which should be fine.

To give an idea of the change in performance with current, the datasheet says Zener impedance = 10 Ohms at 20mA, 600 Ohms at 0.25mA. (lower is better)
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Is it possible to allow uA to zener. I couldn't allow 10mA because i'm designing a battery operated device. Even 1mA is more current in my design.
for example 6V is my input voltage and using 3V1, 500mW zener. so maximum current could be 161mA. So i can use any resistor value greater than 18 Ohms. Is it right?
So if i use 100K resistor, current flow through the path will be 29uA. But i could not get steady 3V. How to achieve ? please help

For low currents, it would be better to use a voltage reference IC such as MAX6103.

Hi,for zener diode,it's mandatory to bias at certain mA result from zener mechanism.If bias uA,a voltage reference is available.

Thanks for the information. I'll look into it.

Also, can i use LM385?

Yes, LM385 looks like a good choice.

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