How much can I expect of a Zener diode in terms of voltage correction?

Thread Starter

cstroh

Joined Jan 16, 2019
94
I have a circuit where I am replacing what I believe was a SMD Zener diode. The points for connection are ground and a voltage source of 4.9 volts with the diode not installed. I believe the 4.9 volts needs to come down to 3.3 volts with a half watt 3.3volt zener installed. Is this possible or am I barking up the wrong tree? I installed a 3.3 volt zener but the voltage remained 4.9 volts?
 

MrChips

Joined Oct 2, 2009
26,077
Low voltage zener diodes are very imprecise.

1648588151127.png

This is a typical graph of I vs V of a zener diode.

The zener diode as a voltage regulator is operating in the reversed bias region. You can see that the zener breakdown voltage Vz is not very precise and changes with the current.

If you want a 3.3V supply voltage it would be better to use a 3-terminal linear voltage regulator.
 

upand_at_them

Joined May 15, 2010
896
Zeners have the rated voltage at the test current, which is available in the datasheet. And they're usually rated to be within 5% at that point. They won't be within 5% of the expected voltage outside of the test current. As already mentioned...post your schematic.
 

crutschow

Joined Mar 14, 2008
29,773
If you want an accurate shunt regulator, you can use a programmable reference, such as the TL431 with a couple resistors to replace the Zener.
 

Audioguru again

Joined Oct 21, 2019
4,671
I thought you burnt out the zener diode until I looked at its specs. A 3.3V zener diode has 6 times worse voltage regulation than a 6.8V one. With 4.9V across the 3.3V zener diode without a current-limiting resistor didn't even heat it up much.
 

ScottWang

Joined Aug 23, 2012
7,202
If the new diode needs more current to clamp its voltage then you have to reduce the values of the resistor, so you need to measure the current of the Zener diode first.
 

Thread Starter

cstroh

Joined Jan 16, 2019
94
I have since discovered that my conclusion that I was dealing with a 3.3 volt power supply was wrong. It is really a 5 volt supply and the diode in question was meant to only be an additional trim on the supply. The small diode in question actually is connected in parallel with something called a 2 amp Schottky Barrier Rectifier (MOSPEC SR34).
 

DickCappels

Joined Aug 21, 2008
8,704
To @atferrari 's point, if you provide a schematic, hand-sketched is fine or even a "wiring diagram" showing what is connected to what will put us in a better position to help you, otherwise we are just playing 20 guesses. And there aren't many guesses left.
 

Thread Starter

cstroh

Joined Jan 16, 2019
94
I probably also need to add that I have assumed that D13 was a Zener diode. I cannot tell for certain with these small glass cylinders with no markings. And since I do not have a 5 volt Zener I am contemplating leaving this diode spot open. Anybody see a problem with that?
 

Audioguru again

Joined Oct 21, 2019
4,671
The 3.3V zener diode has such poor voltage regulation that without a series resistor limiting its current then its voltage and current rise up to the 4.9V source.
 
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