From: tombiasi@optonline.net
On 10/22/2013 8:59 AM, jjk wrote:
> On Sunday, October 20, 2013 2:27:14 PM UTC-4, Tom Biasi wrote:
>> On 10/20/2013 2:04 PM, Fred McKenzie wrote:
>>
>>> In article ,
>>
>>> jjk wrote:
>>
>>>
>>
>>>> Zener Diode Dilemma
>>
>>>>
>>
>>>> I purchased 50 1N4728 3.3v Fairchild zener diodes and trying to test them
>>
>>>> before installing in a circuit I'm building.
>>
>>>>
>>
>>>> The diode test function on my meter shows 0.6v forward biased and .9v
>>
>>>> reversed biased. Performing the same tests on a 5.6v zener, I get 0.6v
>>
>>>> forward biased and when reversed biased I see the same reading as not
>>
>>>> connected to the 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
bad
>>
>>>> lot of diodes I purchased a few 3.3v diodes made by NTE. They all too
exhibit
>>
>>>> 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
>>
>>>> scope on the power supply to verify it was stable.
>>
>>>
>>
>>> 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
>>
>>>
>>
>> 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.
>
>
> Thanks to all who replied.
> There is definitely very wrong. I just took delivery of 50 Fairchild
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 approached 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. :)
>
> 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
>
>
Do you have a different power supply to try?
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
