Zener Diode Dilemma

Zener Diode Dilemma
I purchased 50 1N4728 3.3v Fairchild zener diodes and trying to test them b efore installing in a circuit I'm building.
The diode test function on my meter shows 0.6v forward biased and .9v rever sed biased. Performing the same tests on a 5.6v zener, I get 0.6v forward b iased and when reversed biased I see the same reading as not connected to t he diode at all - which is what I would expect.
Further, I inserted the diodes in a test circuit described below.
vcc -> 560 ohm resistor -> milliamp meter -> zener cathode, zener anode to ground.
When vcc voltage was applied, current was measured as shown below with the diode conducting all the time, and I could never get the voltage across the zener to reach 3.3v without smoking the resistor.
1.5v 0.1mA 2.0v 0.6mA 2.5v 1.1mA 2.7v 1.5mA 2.9v 1.7mA 3.0v 1.8mA
All of the 3.3v parts I tested exhibited the same behavior. Suspecting a ba d lot of diodes I purchased a few 3.3v diodes made by NTE. They all too exh ibit the same behavior - conducting well before their breakdown voltage.
Trying the above with a 5.6v zener shows no current flow until the voltage across the diode reached the device rated breakdown voltage. I also put a s cope on the power supply to verify it was stable.
Any guidance as to what I may be doing wrong would be appreciated.
Thanks.
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On 10/18/2013 8:01 AM, jjk wrote:

First, the diode function on your multimeter will not give you any useful information about zeners. Second, Your test looks strange. Is your supply current limiting perhaps?
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JJK-
Plotting your Voltage vs current data suggests you may be measuring the forward-bias curve of several diode junctions connected in series. (It looks more like a constant-Voltage curve than constant-current.)
The only 3.3 Volt Zener in my collection is a 1N5575A. It checks good as a diode on the diode ranges of a Fluke 8020A multimeter. I did not run your current vs Voltage test, but would expect it to behave as well as your 5.6 Volt test.
I wonder if both your source of the 1N4728 diode and NTE, get their diodes from the same factory? There could have been a batch that were mis-labeled, or they could have fallen victim to counterfeit parts.
Fred
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On 10/20/2013 2:04 PM, Fred McKenzie wrote:

The curve is linear until he gets to 2.7 volts then there is a bump (reading error?). This looks like a forward conducting diode and the readings are across the resistor.
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On Sunday, October 20, 2013 2:27:14 PM UTC-4, Tom Biasi wrote:

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Thanks to all who replied. There is definitely very wrong. I just took delivery of 50 Fairchild replac ement zeners and reran my tests to verify, this time using a 330 ohm resist or. The results are below for both 3.3v and 5.6v parts. The 5.6v part works as expected and doesn't conduct until the avalanche voltage is approached whereas the 3.3v part is still exhibiting the wrong behavior. I had to rais e vcc to 25v to see 3.3v at the cathode, then the magic smoke came out of t he resistor. :)
vcc -> 330 ohm resistor -> milliamp meter -> zener cathode, zener anode to ground. 330 ohm resistor measures 327 ohms
1N4728A 3.3v 1W Zener diode
Iz vcc (mA) Vzener 1.5 0.2 1.42 2.0 0.8 1.72 2.5 1.7 1.91 3.0 2.8 2.05 3.5 4.1 2.16 4.0 5.3 2.24 4.5 6.6 2.3 5.0 8.0 2.37 5.5 9.4 2.43 6.0 10.8 2.48 7.0 13.6 2.56 8.0 16.5 2.62 9.0 19.4 2.69 10.0 22.4 2.73 20.0 55.2 3.1 25.0 71.3 3.3
1N752 5.6v zener diode
Iz vcc (mA) Vzener 1.5 0 1.5 ... ... ... 4.0 0 4.0 4.5 0 4.49 5.0 .1 5.0 5.5 .2 5.43 6.0 1.0 5.67
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Where are you getting these diodes?
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On Tuesday, October 22, 2013 10:50:17 AM UTC-4, JW wrote:

lacement zeners and reran my tests to verify, this time using a 330 ohm res istor. The results are below for both 3.3v and 5.6v parts. The 5.6v part wo rks as expected and doesn't conduct until the avalanche voltage is approach ed whereas the 3.3v part is still exhibiting the wrong behavior. I had to r aise vcc to 25v to see 3.3v at the cathode, then the magic smoke came out o f the resistor. :)

to ground.

I received a package of 50 Fairchild devices from Jameco.com. Then I purchased 5 NTE devices from a local electronics store. All of the ones I have tested from the above failed. I contacted Jameco and they sent 50 replacements and all that I tested fail ed in the same way. I've been in contact with the Jameco technical rep and they are looking into this and trying to obtain devices from another lot fr om their supplier.
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On 10/22/2013 8:59 AM, jjk wrote:

Do you have a different power supply to try?
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On Tuesday, October 22, 2013 11:12:24 AM UTC-4, Tom Biasi wrote:

them

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placement zeners and reran my tests to verify, this time using a 330 ohm re sistor. The results are below for both 3.3v and 5.6v parts. The 5.6v part w orks as expected and doesn't conduct until the avalanche voltage is approac hed whereas the 3.3v part is still exhibiting the wrong behavior. I had to raise vcc to 25v to see 3.3v at the cathode, then the magic smoke came out of the resistor. :)

to ground.

I also tried a 9v battery connected to a 100K pot, wiper connected to a 10k resistor to the zener cathode, and the anode connected to the other end of the pot and to the battery negative post.
+9v | 100K pot ---10K ----cathode | | gnd------------------|
Works fine with all of my 5.x zeners. The 3.3v zeners never get above 1.8v at the cathode when the pot wiper is a at 9.2v.
This feels like a bad dream where you run and not get anywhere. I sure would like to get this behind me and continue with the larger projec t I'm working on.
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JJK-
I did not see how you were using the 3.3 V Zeners. If you are generating 3.3 Volt power, you might consider using the LM-317. They are available in several packages including surface mount, and can be adjusted from 1.2 to 39 Volts using a pair of resistors.
Fred
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On Wednesday, October 23, 2013 8:39:08 PM UTC-4, Fred McKenzie wrote:

Thanks for your comments. The potentiometer circuit I described was for testing an alternate power supply source to rule out a problem with my bench supply. I intend to use the zener for over voltage pin protection on 3.3v devices.
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On 10/23/2013 6:52 PM, jjk wrote:

It is not likely that you are receiving so many defective parts. I suggested a different power supply, now I suggest a different meter. I do this only to eliminate the possible variables. The next is to eliminate you. I mean give them to someone else to test. Please don't be insulted.
Tom
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On Wednesday, October 23, 2013 10:34:04 PM UTC-4, Tom Biasi wrote:

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replacement zeners and reran my tests to verify, this time using a 330 ohm resistor. The results are below for both 3.3v and 5.6v parts. The 5.6v part works as expected and doesn't conduct until the avalanche voltage is appro ached whereas the 3.3v part is still exhibiting the wrong behavior. I had t o raise vcc to 25v to see 3.3v at the cathode, then the magic smoke came ou t of the resistor. :)

de to ground.

10k resistor to the zener cathode, and the anode connected to the other en d of the pot and to the battery negative post.

is a at 9.2v.

oject I'm working on.

Tom, LOL. No insult taken. I got a chuckle from your process of elimination comment.
I agree having that many failures is suspect and is why I posted to this gr oup for possible help. I tried 3 meters and 2 power supplies. In every case all of my 5.* volt zeners work perfectly as expected. All I do is swap out the 5v zener for the 3.3v device and see the problem. I even reversed the direction of the 3.3v zener thinking the cathode may have been marked inco rrectly. The voltage at the anode in this case is an expected 0.6v.
If anyone reading this group would like to test a few of these, please send me an email at jjk439 at gmail dot com and I would be glad to send them to you.
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On Thursday, October 24, 2013 7:40:10 AM UTC-4, jjk wrote:

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d replacement zeners and reran my tests to verify, this time using a 330 oh m resistor. The results are below for both 3.3v and 5.6v parts. The 5.6v pa rt works as expected and doesn't conduct until the avalanche voltage is app roached whereas the 3.3v part is still exhibiting the wrong behavior. I had to raise vcc to 25v to see 3.3v at the cathode, then the magic smoke came out of the resistor. :)

node to ground.

a 10k resistor to the zener cathode, and the anode connected to the other end of the pot and to the battery negative post.

r is a at 9.2v.

project I'm working on.

e

on comment.

group for possible help. I tried 3 meters and 2 power supplies. In every ca se all of my 5.* volt zeners work perfectly as expected. All I do is swap o ut the 5v zener for the 3.3v device and see the problem. I even reversed t he direction of the 3.3v zener thinking the cathode may have been marked in correctly. The voltage at the anode in this case is an expected 0.6v.

nd me an email at jjk439 at gmail dot com and I would be glad to send them to you.
I am now relieved that my sanity on this issue is intact, but still frustra ted at not being able to find any functional zener diodes. I received a ph one call from a tech at Jameco Electronics stating they took delivery of an other 50 3.3v zener diodes, and they all failed their tests the same way th ey failed mine. They concur there is a problem with these devices.
Thanks to all who replied to my post.
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On 10/30/2013 2:04 PM, jjk wrote:

> failed mine.

I guess Jameco doesn't have any incoming QA inspection.
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wrote:

Doesn't Jameco still sell floor sweepings? I sure wouldn't trust any component bought from them.
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On 10/31/2013 8:58 PM, snipped-for-privacy@attt.bizz wrote:

I don't know their practices.
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snipped-for-privacy@attt.bizz writes:

I think you mean Poly-Paks. I had fun with them when I was a youth.
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This exactly matches the datasheet, which says it needs at least 76mA in order to act as a zener voltage reference.
This is a common issue with low voltage zeners - the current they need to operate is high (compared with higher voltage zeners).
If you want a low voltage reference at low current (e.g. for battery operated equipment), an LED can often be used (forward biased) at a much lower operating current (1mA or less), although not quite as stable a reference.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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On Thursday, October 31, 2013 4:42:08 PM UTC-4, Andrew Gabriel wrote:

placement zeners and reran my tests to verify, this time using a 330 ohm re sistor. The results are below for both 3.3v and 5.6v parts. The 5.6v part w orks as expected and doesn't conduct until the avalanche voltage is approac hed whereas the 3.3v part is still exhibiting the wrong behavior. I had to raise vcc to 25v to see 3.3v at the cathode, then the magic smoke came out of the resistor. :)

to ground.

Thanks for your comments Andrew. I'm wondering why the zener would conduct at all at voltages as low as 1.4v ? My goal was not to use the zener as a regulator, but to protect 3.3v device s from potentially external over voltage events (microcontroller driving 5v circuits).
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