Zener diode accuracy, not getting expected reference voltage.  - Page 1
Forum section: electronics / beginners

author: David97
author_rank: Contributor
timestamp: August 21, 2016, 06:22:18 am
content: 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%3AITI 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 = 346microampWhich 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 5TAny help is appreciated. Thanks.

author: Ammar
author_rank: Regular Contributor
timestamp: August 21, 2016, 06:40:59 am
content: Do you have a data sheet for them?

author: crazyguy
author_rank: Regular Contributor
timestamp: August 21, 2016, 06:44:22 am
content: you may try increase the test current (Iz) to 5mAhttp://www.electronics-tutorials.ws/diode/diode_7.htmlBZX55-Serieshttp://www.vishay.com.hk/docs/85604/bzx55-se.pdf

author: rob77
author_rank: Super Contributor
timestamp: August 21, 2016, 07:38:52 am
content: definitely increase the current to few milliamps... most zeners need at least 1mA and some need as high as 5-10mA to behave nicelybut 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2016, 05:25:26 pm
content: 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

author: madires
author_rank: Super Contributor
timestamp: August 21, 2016, 05:31:52 pm
content: Most 0.4W and 0.5W Zener diodes need about 3mA for a stable voltage.

author: w2aew
author_rank: Super Contributor
timestamp: August 22, 2016, 01:17:32 pm
content: Quote from: T3sl4co1l on August 21, 2016, 05:25:26 pmMy favorite device with internal reference and opamp/comparator is the oldie-but-goodie LM10. Very flexible device.

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2016, 05:28:46 am
content: The LM10 is an amazing part. Linear Technology even makes an uprated pin compatible version in the LT1635. Robert Widlar wrote anapplication notewhich describes its operation.

author: Cliff Matthews
author_rank: Unknown
timestamp: August 23, 2016, 11:53:30 pm
content: Quote from: David Hess on August 23, 2016, 05:28:46 amThanks! AN-211 pdf link had probs, so another download was here:http://www.ti.com/lit/an/snoa638a/snoa638a.pdf

author: nuno
author_rank: Frequent Contributor
timestamp: August 24, 2016, 01:52:50 am
content: 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 aBZX85C3V3 (I have characterized one other different 3V3 model and it's also unusable)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2016, 01:41:55 pm
content: 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