Author Topic: Zener diode accuracy, not getting expected reference voltage.  (Read 4214 times)

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Offline David97

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Hi everyone. A very noobie question here.

Bought a bunch of zeners from ebay. Just to use for various projects.
http://www.ebay.com/itm/322146150045?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I wanted to start by making a low voltage alarm for may car. When I'm charging rc batteries while flying, I dont want to drain my car battery flat so I cant go home. Idea was to use a zener as a reference voltage, and then scale the batteries voltage through a resistor divider. Use a opamp to compare the voltages, and when the battery voltage fell below the reference, a beeper would sound. Pretty simple.

So I started by wiring a 5.1V zener in reverse bais in series with a 10k resistor to see weather I could make a reference voltage. However using multiple 5.1V zeners I got 4.31, 4.14, 4.24, 4.35, 4.44, 4.39, 4.20, 4.13V None of them were close to the 5.1V that they claimed to be.

Looking more into it, I need to make sure that the zener is getting enough current to fully saturate it before making a stable supply. currentley on the bench, i'm just using a battery pack giving 7.9V. So in series with a 10k resistor gives...
i=v/r = (7.9-4.44)/10000 = 346microamp

Which seems plenty for most zeners. I dont have the datasheet, so cant be sure. But doing some reading tells be most zeners are saturated at 100microamp.

Wondering why I'm not getting a good reference voltage. Any ideas? The zeners are marked 5C1 5T

Any help is appreciated. Thanks.
« Last Edit: August 21, 2016, 06:25:45 am by David97 »
 

Offline Ammar

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #1 on: August 21, 2016, 06:40:59 am »
Do you have a data sheet for them?
 

Offline crazyguy

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Offline rob77

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #3 on: August 21, 2016, 07:38:52 am »
definitely increase the current to few milliamps... most zeners need at least 1mA and some need as high as 5-10mA to behave nicely ;)

but i would suggest something different for your low voltage alarm, use a TL431 shunt regulator instead of a zener diode. the price is in the same ballpark but the results are order of magnitude better.
 

Online T3sl4co1l

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #4 on: August 21, 2016, 05:25:26 pm »
Note that you'll spend a lot of bias current this way, whether it's putting a zener in the sweet spot (where its noise is also manageably low) or powering a hungry TL431 (which will, however, deliver a nice and stable output voltage).

Comparators with internal reference are available, or you can use an external micropower reference device.  These use much less power (~uA) than a TL431 (~1mA) or TLV431 (~100uA), and are only somewhat noisier. :)

Tim
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Offline madires

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #5 on: August 21, 2016, 05:31:52 pm »
Most 0.4W and 0.5W Zener diodes need about 3mA for a stable voltage.
 

Offline w2aew

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #6 on: August 22, 2016, 01:17:32 pm »
Note that you'll spend a lot of bias current this way, whether it's putting a zener in the sweet spot (where its noise is also manageably low) or powering a hungry TL431 (which will, however, deliver a nice and stable output voltage).

Comparators with internal reference are available, or you can use an external micropower reference device.  These use much less power (~uA) than a TL431 (~1mA) or TLV431 (~100uA), and are only somewhat noisier. :)

Tim

My favorite device with internal reference and opamp/comparator is the oldie-but-goodie LM10.  Very flexible device.
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Offline David Hess

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #7 on: August 23, 2016, 05:28:46 am »
The LM10 is an amazing part.  Linear Technology even makes an uprated pin compatible version in the LT1635.  Robert Widlar wrote an application note which describes its operation.
 

Offline Cliff Matthews

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #8 on: August 23, 2016, 11:53:30 pm »
The LM10 is an amazing part.  Linear Technology even makes an uprated pin compatible version in the LT1635.  Robert Widlar wrote an application note which describes its operation.
Thanks! AN-211 pdf link had probs, so another download was here:http://www.ti.com/lit/an/snoa638a/snoa638a.pdf
 

Offline nuno

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #9 on: August 24, 2016, 01:52:50 am »
I can add some real world measurements of some zener diodes (from the interesting quadrant, rotated to be more explicit)



And one from a 3.3V zener, which is... unusable (I don't know why do they even take the trouble of producing them); it's a
BZX85C3V3 (I have characterized one other different 3V3 model and it's also unusable)

 

Online T3sl4co1l

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Re: Zener diode accuracy, not getting expected reference voltage.
« Reply #10 on: August 24, 2016, 01:41:55 pm »
Most zener series go down to 2.4V!  They're practically resistors...

This, by the way, is why you never see TVS diodes below 5V rating -- or, if you do, the peak surge voltage isn't any lower than 5V parts.

If you need a zener at 1.5-3V, consider an LED!  They're not well controlled, but they are fairly stable.  Some even have complementary or negative tempcos relative to a Vbe, so you can make a very stable current source/sink using a, I think, red or yellow LED.

Tim
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