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> Low Voltage Zener Diode, 1.8v Zener Diodes
In_Exile
Posted: January 21, 2009 03:38 pm
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I built a very simple zener regulator circuit. A power supply, 1 - 24K resistor and an ON semiconductor 1.8V zener diode. It won't regulate at 1.8V. I built the same circuit with a 12V zener and it worked fine. It regulated at 12V. Perfect! But i cannot for the life of me get the 1.8V to regulate. If i bring the supply over 1.8V the zener voltage keeps rising. Does anyone know of an inherent problem using such a low voltage zener. And yes the zener is reverse biased.
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mankku
Posted: January 21, 2009 03:47 pm
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What's the rated reverse current for the zener voltage? Most zeners I've seen were rated for their Vz at a reverse current of about 5 milliamps. Make sure that you are meeting the reverse current requirement.

Mankku
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In_Exile
Posted: January 21, 2009 04:09 pm
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The data sheet shows an Izt of 50uA. But i just went back and built it for 5mA also. No dice. I'm thinking these low voltage zeners just don't work. The part is an ON semiconductor # MMSZ4678T1.
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mankku
Posted: January 21, 2009 04:25 pm
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I would find it very strange for a part to make it onto the market if it doesn't work, especially if it's ON Semi that's the manufacturer. Have you tried another piece of the same model of diode, just to rule out that the one you're having trouble with is a lemon?

Mankku
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In_Exile
Posted: January 21, 2009 04:36 pm
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I just found this excerp from another electronics forum.

"Why does people keep saying that Zener diode are not available below 2.4V?

If you go at Digikey, you can find Zener diode as low as 1.8V.

Yes, but they are not true 'zeners' and they have a horrible soft kneepoint which makes them a poor choice for regulation.

Right. They are series connected Si diodes. Terrible.

A forward LED has a much better characteristic. Select IR for around 1.3 V a red for 1.6 a green for around 1.8 V.
"

I think these low voltage zeners just don't work for regulation.
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mankku
Posted: January 21, 2009 05:18 pm
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Hmmm... Interesting, I had no idea about that. Actually, I found this datasheet for a series of "low voltage zener diodes" in which the connection diagram on the second page actually seems to verify the fact that there are several diodes connected in series.

However the fact is that for the device to look something like a zener, you have to connect it with the internal diodes forward-biased. The regulation is not constant with forward current as the diode Vf increases with increasing current. Doesn't explain why your MMSZ4678T1 does not work, but anyways, interesting info.

Mankku
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Sch3mat1c
Posted: January 21, 2009 06:05 pm
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Better to use a bandgap reference (e.g. TL431). I forget if they make 1.25V models (if so, you can adjust it to 1.80).

Tim


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In_Exile
Posted: January 21, 2009 06:40 pm
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Yeah i learned my lesson. I'll never use voltage dividers or zeners for regulation again. Just buy a nice little precision 1.25 reference or something.
I did finally talk to an app engineer for ONsemi after multiple emails and voice mail boxes and people who couldn't speak english. He basically ended up telling me that they just don't work for regulation, with tolerances and juction temps they don't regulate very well. I just wanted to make sure it wasn't something i was doing wrong, but with the circuit consisting of the zener and 1 resistor how bad could i screw it up.
I appreciate everyones help. Moral of the story, dont' use low voltage zeners.

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undidly
Posted: February 25, 2009 01:55 am
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QUOTE (In_Exile @ January 21, 2009 06:40 pm)
Yeah i learned my lesson.  I'll never use voltage dividers or zeners for regulation again.  Just buy a nice little precision 1.25 reference or something. 
I did finally talk to an app engineer for ONsemi after multiple emails and voice mail boxes and people who couldn't speak english.  He basically ended up telling me that they just don't work for regulation, with tolerances and juction temps they don't regulate very well.  I just wanted to make sure it wasn't something i was doing wrong, but with the circuit consisting of the zener and 1 resistor how bad could i screw it up. 
I appreciate everyones help.  Moral of the story, dont' use low voltage zeners.

Try one more time with the 1.8 V zener reversed.
If it works then there are forward diodes in it and the polarity marking is telling you
true for a diode but this one works forward,not backward as a normal zener.
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Audioguru
Posted: March 08, 2009 05:14 am
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Simply look at the spec's on the datasheet. A 1.8V zener diode is horrible:
1) It has a very high dynamic resistance. A 5.6V to 6.2V zener diode has a very low dynamic resistance for good regulation.
2) It has a negative tempco. A 5.6V to 6.2V zener diode is stable with temperature change.
3) On a graph its resistance is a diagonal line like a resistor. A 5.6V to 6.2V zener diode makes a straight vertical line because its voltage does not change when the current increases.
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andrew 24
Posted: March 08, 2009 05:44 am
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I guess You could regulate to higher voltage, and drop the excess with serial connected diodes. Supposedly it can work.
Never tried it my self..
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Audioguru
Posted: March 08, 2009 06:23 am
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Series diodes also make a fairly poor voltage regulator that also have a negative tempco. A voltage reference IC or LM317 is excellent.
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captainhannes
Posted: March 08, 2009 07:32 am
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With a LM317 you can have stable output voltages from 1.2V to 37V (depending on the resistor values)
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deltaar
Posted: March 24, 2009 03:10 pm
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One should take into account that some times people refer to low voltage zener diodes. However, some times they are not zener, but actually stabistor. In this case, they work like a normal rectifier diode, but with greater voltage drops, such as 1V4.
See the datasheet. If the desired voltage is stated as VF, it has to be connected as a normal rectifier diode.

Cheers.
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