Hi, this is my first post to this forum. I signed up because I have a zener diode problem almost exactly the same as described here. Basically, I am measuring the voltage of a 12V (motorcycle) system and using a micropower i2c A2D converter to do so, but I want to ensure that the input to the converter never exceeds 3.3V. In the attached circuit (the usual one for a zener) I originally used a 10K res for R1 and a 2.7K res for R2 - the intention being that I could measure up to about 17V before the zener clamped the incoming voltage to 3.3V.
However this did not work as expected, and in fact I can increase the input voltage to 30V (in an isolated replication of this circuit, NOT attached to the A2D!) and still not reach 3.3V at the zener anode... it only ever gets to about 2.4V. Graphing the output data shows a distinctly non-linear response.
So, in an attempt to get more current flowing in the circuit which I perceived to be the problem, I exchanged R1 for a 1K and R2 for a 220R. I'm getting different but neverless broadly similar results, in so far as the voltage still does not reach 3.3V at the anode!
It's quite a while since I used zeners, but I don't recall having this problem before, so I am a little confused by these results. By the way I have tried different zeners in case there was a problem with that, no change was observed.
Any ideas anyone? It's driving me nuts!
- CF
With a 30V input, the 10k resistor is providing a current of 2.76mA to the 2.7k resitor and the zener diode. The 2.7k resistor has 2.4V across it so its current is only 0.89mA. Therefore the zener diode has a current of only 1.97ma. A 3.3v zener diode measured at 5ma might work but a 1N4728 that is 3.3V at 76mA might be conducting 1.97mA at 2.4V.I originally used a 10K res for R1 and a 2.7K res for R2 - the intention being that I could measure up to about 17V before the zener clamped the incoming voltage to 3.3V.
However this did not work as expected, and in fact I can increase the input voltage to 30V (in an isolated replication of this circuit, NOT attached to the A2D!) and still not reach 3.3V at the zener anode... it only ever gets to about 2.4V.
The divider is set to drop the ignition volts from 12v to 4v (1/3 the original). There is also a 5.1v zener diode across the output, which is supposed to limit any extraneous voltage spikes.
my output volts is always around 3.15v, even at a steady 12v dc applied across the divider.
Hi Carlos,
You need to increase the value of the 220 ohm resistor, say to 470
or maybe even 680 ohms or something like that. The problem is
that with a 220 resistor on bottom and 1k on top the resistive
divider only puts out a little over 2v, which is not enough to
reach the zener voltage of 3.3 volts. When you want a circuit
like this to work you have to make sure the divider voltage is
higher than the zener voltage, at least.
Nobody says the part number of their zener diode!
They are very different.
A 1N4689 is 5.1V at 50uA. Its current is 10uA max at 3V.
A 1N4684 is 3.3V at 50uA. Its current is 10uA max at 1.5V.
A 1N4733 is 5.1V at 49mA. Its current is 10uA at only 1V.
A 1N4728 is 3.3V at 76ma. Its current is 10uA at only 1V.
Other zener diodes are spec'd at 5mA and 20mA.
With a 30V input, the 10k resistor is providing a current of 2.76mA to the 2.7k resitor and the zener diode. The 2.7k resistor has 2.4V across it so its current is only 0.89mA. Therefore the zener diode has a current of only 1.97ma. A 3.3v zener diode measured at 5ma might work but a 1N4728 that is 3.3V at 76mA might be conducting 1.97mA at 2.4V.
I thought Vout = Vin x (R2 / (R1 + R2))? That would mean an input voltage of approximately 18.5V would produce a voltage of 3.3V at the middle of the divider. Unless I'm wrong about that as well.
HI Carlos,
Well then you better fix your schematic because it shows a 12v input
Also, you can not apply 18.5v either to that circuit and expect it to work.
The reason again being that right around 18.5v or just below it the current
to the zener is too low. Thus, you need to either raise the 220 ohm or
decrease the 1k.
If you need something that tracks from 0 to 18.5v and then clamps when
the input equals 18.5v (3.3v output) then i think you would be better off
with an ACTIVE clamp not a PASSIVE clamp like the zener is.
You might look into the LM431 ic and use it as a shunt regulator,
or another ic that works as a shunt regulator. The idea is to make
the shunt regulator kick in at 3.3v (and do it sharply) and thus limit the
voltage.
Another idea is to use an op amp and clamp the output using a zener,
which will work much better.
If you dont need that much accuracy, perhaps you can get away with
lowering the impedance of the whole network. This will make the
zener appear like a sharper device.
Another idea is to use a higher voltage zener (5v or so) and use a
divider AFTER the zener with a single resistor to the zener.
Many of the higher voltage zeners have a sharper knee so this might
work good enough. I would bet a 12v zener works much better, with
a divider AFTER the zener. You could make the network impedance
lower that way too.
A 3.3V zener diode is a very poor voltage clamp. It starts conducting when its voltage is less than 1V. At 2V then it conducts a lot. It is a poor voltage regulator. Its voltage and current change when the temperature changes.
Yours is 3.3V when it has a current of 20mA. Maybe it is 2V when its current is a couple of mA.
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