Question about Zener Diode lost regulation.

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EDA_hg81

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I am using a Zener Diode to establish a 2.5V reference voltage.
I forget the part number of Zener but it is connected with a 30K resistor.
When I put the sensor on my circuit this Zener diode lost regulation.
The following are the data.
Can I decrease the 30k resistor for generating higher current or have to use another one?
Thanks.
 

Are you sure you have connected the Zener diode in the right direction .. ???
As a diode it has ~0.8V voltage drop in one direction and in the reversed configuration (Zener effect) it should show the Zener voltage, which, in your case, is 2.5V ..
On the other hand, 30k looks pretty big for this job ..
Try to force 1mA current through it, that is 2.2k-to-2.7k resistor, and see how it goes ..

Rgds,
IanP
 

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    EDA_hg81

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I am pretty sure.

before I inserted the sensor, the reference voltage is 2.5v.

after the sensor is inserted, the reference voltage dropped to 0.8V.

after I unplugged the sensor, the reference voltage back to 2.5V again.

Thanks.

Added after 27 minutes:

I have lowered the resistor to 10K.

the reference voltage is fine now.

thank you and have a good day.
 

What is the part number of zener diode and sensor. These are basic data that someone needs to help you.
If we take:

I=Vr/R and
Vr=5V-0.8V=4.2V =>
I=4.2V/30kΩ=140µA.
I - current drawn by sensor
Vr- voltage across series resistor
R - series resistor

To maintain voltage at 2.5V you need to provide current through zener more than 140µA when sensor is not attached so:

Iz>140µA
R=Vr/Iz
Vr=Vp-Vz=5V-2.5V=2.5V
R<2.5V/140µA≈17kΩ

If sensor does not draw more current I would not recommend more than 10kΩ series resistor, but even if you have zener that is only 100mW, current could be set up to 40mA. If we take, let's say that your zener diode voltage was specified @10mA, resistor would be 250Ω. If you don't have power saving requirements that is probably what your series resistor should be. To round it up to some value that you can find very easy, 270Ω.
 

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    EDA_hg81

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Zener diode quiscient current must be at least 3mA or more for voltage regulation. At smaller current diode is below it's knee voltage.
 

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    EDA_hg81

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It is a photodiode XenICs imaging array.

I have to set the reference voltage for Vref_T and Vref_B.

on the datasheet, they didn't provide the load resistanse.

Thanks.
 

As Borber said at 140µA it really doesn't matter much if current is going to be much higher, you should run at least 3mA through Zener diode. If you run 10mA you should be safe for any change in load. Don't forget to use capacitors in parallel with zener. Elko 10µF with 0.1µF ceramic would be great addition.
If you need reference to be more stable, using integrated reference will yield much better performance. TL431 for example is widelly available and is used in same manner as zener diode, but more stable.
 

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    EDA_hg81

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You can buy zener diodes that have their rated voltage when their current is only 50uA. The 1N4681 is 2.4V.

These very low voltage zener diodes are horrible voltage regulators. The voltage changes with temperature and with current.
 

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    EDA_hg81

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There are many precision voltage reference IC's. Sometime it is satisfactory to use simple resistor voltage divider (potentiometer) from 5V supply voltage.
As Siniša mentioned TL431 can be used as it is widely available component and such reference can be adjustable.
 

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    EDA_hg81

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Thank you so much.

one thing I have to consider is the power consumption.

I will keep resistor as 10K.

I planned to use max6122* reference chip, but it is out of stock from digikey.

I have to use zener instead.

I got bad luck.

 

AD680JTZ-ND
LT1460GCZ-2.5#PBF-ND
.... they have a lot of choises and in stock. 10k and zener is not good choise if you need good stability.
 

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    EDA_hg81

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There are plenty of 2.5V shunt references, that consume below 50 uA, e. g. standard LM385-2.5 with 20 uA, it's really not a problem to find a usable device. However, as you already found out, the sensor input current has to be considered. If there's no specification (sounds strange), you can lower the resistance until the reference operates at it's specified voltage and add some extra.
 

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    EDA_hg81

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Just to point out, for reason of low consumption requirement and unknown input current, best choise would be series regulator - reference. Consumption of reference would be small consumption of reference and input current of input and nothing extra like some spare current you need with shunt reference. And you also insure that input has enough current even if it changes because some internal conditions of your sensor.
 

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    EDA_hg81

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EDA_hg81,
In order for the zener to provide its specified output, the current thru the zener must be equal to, or close to the specified test current (It). See the zener data sheet for It.
Let
It = the zener test current
Il = the load current
Vu = the unregulated voltage
Vz = the regulated voltage
Under load, the current Ir thru the series resistor R will equal the Zener test current plus the load current. Then
Rs = (Vu-Vz)/(It + Il)
As the current thru the zener decreases, its equivalent resistance increases and the regulation gets worse.
Regards,
Kral
 

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    EDA_hg81

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Zener diodes less than about 5.6V are lousy voltage regulators. Their voltage drops when their current drops, like a simple resistor. Their voltage also drops when the temperature drops.

Zener diodes from 5.6V to about 6.8V are pretty good voltage regulators. Their voltage remains constant when their current or temperature change.
 

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    EDA_hg81

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