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[SOLVED] What is the Minimum Current required for 3.3v Zener diode

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speedEC

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Dear All,

I have used 3.3v zener resistor combination for reference voltage (Vref) on Inverting Pin of LM358 IC battery low/high voltage circuit. I am very much confused on Current which requires zener to run correctly. Some datasheet say minimum current as 76mA and maximum continous current as 276mA (datasheet attached). Someone says minimum current should be 10mA and so. But, I didn't get correct voltage value of zener (3.3v). Any help on this would be highly appreciated.

Thanks
pmk
 

BradtheRad

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The specs for zener diodes (that I've seen) say they're manufactured to a 5% tolerance.

So yours may yield a value anywhere between 3.135 and 3.465 V.

Furthermore when you apply 100uA it will show a few tenths of a volt lower than when you apply 100 mA.
 

FvM

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Low power Z-diodes are often specified for a current of 5 mA. The zener voltage will however have a certain tolerance, so you don't necessarily achieve the nominal voltage at this current. "Minimum current" would in my view refer to a current level, where regulation isn't yet degraded by leakage curents. It should be below 100 µA for industry standard low power diodes. The zener voltage has to be corrected according to the intended current level.
 

speedEC

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Thanks for your response. I forgot to attach the datasheet (selected page) here. Pl see the attachment. What it says that the minimum current as 76mA. Pl advice, if I am doing anything wrong.

Zener_Current.jpg

thanks
pmk
 

speedEC

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I have to set reference voltage as 3.3v to Inverting Pin of LM358. Based on this, I have to set the Voltage on Non-Inverting Pin using "voltage divider" as 3.3 when battery voltage reaches 6.9v (for High Voltage) or 5.5v (for Low Voltage). So that we can compare both Inverting and Non-Inverting pin for low/high voltage. Help pl.

thanks
pmk
 

FvM

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What it says that the minimum current as 76mA.
No it doesn't. Izt is the current for which the nominal zener voltage is specified. Another specification refers to 1 mA, but's it's not said to be a minimum current. So if a minimum current exists, it's below 1 mA.

P.S.: The question raised in post #5 has been discussed in detail in your previous thread. https://www.edaboard.com/post916450.htm
 

BradtheRad

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I believe this kind of comparator circuit can be seen as a spinoff of the wheatstone bridge. Component values are non-critical. It just takes careful adjusting to get the proper range of operation.

So I think you can use practically any value zener up to 5V. Even a 0.6 V diode. Or an LED if you're careful not to overload it.

For the non-inverting input use a potentiometer instead of two fixed resistors. Dial it to the position where you get desired operation. What value pot? Say 2K to 50K.

It's a good bet the designer of the circuit fiddled around with values in a similar manner.
 

speedEC

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For the non-inverting input use a potentiometer instead of two fixed resistors. Dial it to the position where you get desired operation. What value pot? Say 2K to 50K.
I have used 5K pot along with two fixed resistor (voltage divider) for Non-Inverting input. I am struggling only to set 3.3v at Inverting input using 3.3v zener. If I use 620 ohms resistor and 3.3v zener it gives correctly 3.32v on Inverting Pin. If I use 1K then, the zener output is around 3.17v. Thats why I am confusing on setting 3.3v.

thanks
pmk
 

xaccto

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I recall quite some time ago working out some low voltage reference, and obtaining all manner of voltages < 5.1V zeners. 1watt zeners needed at least 12mA for
to operate at their fixed zener voltage, 0.5watt zeners are a little better at 4-6mA....
Your 680R is too high, use 470R
You can improve it by replacing the resistor with a constant current source.....
Anyway plain zener diodes for low voltage reference are a waste of time:wink:, and poor temperature co-efficient.
Use Voltage reference devices such as LM335-2.5V, this will give you an accurate and stable reference and using < 0.5mA.
There's others that use uAmps....
Being battery operated, you do want to consume least amount of current, don't you ?
 

speedEC

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Your 680R is too high, use 470R
If I use 470 ohms, the zener voltage is around 3.42v. I cannot measure exact amps. But around 10mA.

thanks
pmk

---------- Post added at 17:20 ---------- Previous post was at 17:16 ----------

Use Voltage reference devices such as LM335-2.5V
PCB size dramatically grows with the use of ICs. So, I like to use some zener based voltage reference. Let me know your suggestion pl.

thanks
pmk

---------- Post added at 17:22 ---------- Previous post was at 17:20 ----------

Your zener diode is displaying normal behavior.
ok. I'll go with this one. Will this "zener Voltage reference" stable for long period as ICs?

thanks
pmk
 

FvM

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PCB size dramatically grows with the use of ICs.
You can the shunt references in a SOT23 (or TO92 for throughplated PCB) package.
Will this "zener Voltage reference" stable for long period as ICs?
Yes, probably better. Larger initial tolerance and current dependency is their major disadvantage.
 

speedEC

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Dear All,

Thanks to you all that you provided the valuable suggestions and help.

thanks
pmk
 

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