zener diode question, minimum current?

[panfilero]

Hello, I want to use a zener diode as a voltage clamp, and I'm
wondering how to bias it.... I'm getting a small signal TZX3V0A diode,
according to their datasheet, their test current is 5mA.... ok that's
fine, I can get a resistor value to limit my current to that... but,
is that the spec I should follow... the 5mA test current? My concern
is that I don't want to be too low and too close to that.... I don't
know what it's called... knee current, where the zener starts to go
into reverse breakdown... I want to be above that current... not near
it... right? please correct me if my thinking is wrong about this....

thanks

 

[pimpom]

Hello, I want to use a zener diode as a voltage clamp, and I'm
wondering how to bias it.... I'm getting a small signal TZX3V0A diode,
according to their datasheet, their test current is 5mA.... ok that's
fine, I can get a resistor value to limit my current to that... but,
is that the spec I should follow... the 5mA test current? My concern
is that I don't want to be too low and too close to that.... I don't
know what it's called... knee current, where the zener starts to go
into reverse breakdown... I want to be above that current... not near
it... right? please correct me if my thinking is wrong about this....

I don't know about the TZX3V0A, but low-voltage zeners with rated breakdown
voltages well below 5V tend to have very rounded knees - no abrupt breakdown
levels. They have high dynamic resistance and their breakdown voltage varies
considerably with current. If you must use them at all, yes, the current
should be kept as high as is practicable.

 

[panfilero]

I don't know about the TZX3V0A, but low-voltage zeners with rated breakdown
voltages well below 5V tend to have very rounded knees - no abrupt breakdown
levels. They have high dynamic resistance and their breakdown voltage varies
considerably with current. If you must use them at all, yes, the current
should be kept as high as is practicable.

So you think 5mA is too low? this is what I'm looking at
http://www.vishay.com/docs/85614/tzxserie.pdf
I wish it had some kind of reccomended operating point for the
current... I guess that's what I'm looking for... I was assuming since
they tested at 5mA that that was optimal...

thanks

 

[Eeyore]

Hello, I want to use a zener diode as a voltage clamp, and I'm
wondering how to bias it.... I'm getting a small signal TZX3V0A diode,
according to their datasheet, their test current is 5mA.... ok that's
fine, I can get a resistor value to limit my current to that... but,
is that the spec I should follow... the 5mA test current? My concern
is that I don't want to be too low and too close to that.... I don't
know what it's called... knee current, where the zener starts to go
into reverse breakdown... I want to be above that current... not near
it... right? please correct me if my thinking is wrong about this....

Zener 'knees' are 'soft' as hell. I rarely use them for exactly that
reason.

Why do you need a zener ? What's wrong with a bandgap reference ?

Graham

 

[pimpom]

So you think 5mA is too low? this is what I'm looking at
http://www.vishay.com/docs/85614/tzxserie.pdf
I wish it had some kind of reccomended operating point for the
current... I guess that's what I'm looking for... I was assuming since
they tested at 5mA that that was optimal...

Not really. If you look at the rest of the devices on that datasheet, you'll
see that they use 5mA as the test current for all voltages up to 18V. It's
just a convenient reference value.

You have not given details about the intended application or the desired
accuracy of clamp voltage. If Iz changes by 1mA, the high dynamic resistance
of 100 ohms will cause the clamped voltage to change by 0.1V. If the shunt
load current in parallel with the zener varies, that also changes the
current flowing through the diode, and hence the clamp voltage. Dynamic
resistance, and hence change of Vz with change of current, will fall at a
higher current level. In any case, if all you need is a rough approximation
of 3V and does not need to remain steady at the same voltage, the TZX3V0A
may be good enough.

If you need better regulation, you could use a zener-like IC. If 2.5V is
close enough, you can use a fixed voltage type like the LM385-2.5. If you
want 3V, you can use an adjustable type like the LM385-ADJ or an LM431.

 

[Eeyore]

So you think 5mA is too low? this is what I'm looking at
http://www.vishay.com/docs/85614/tzxserie.pdf

What sodding voltage ?

You do know what slope or dynamic resistance means I hope.

Graham

 

[whit3rd]

Hello, I want to use a zener diode as a voltage clamp, and I'm
wondering how to bias it.... I'm getting a small signal TZX3V0A diode,
according to their datasheet, their test current is 5mA.

There are accuracy specifications that change if you alter the
current, but the MOST important reason for using a high test
current is noise. I've seen Zeners (old designs) that literally
showed multiple traces on a curve tester; the low-current
range of diode breakdown has a lot of amplified shot noise.

The noise problem is worst for high voltage devices (which
are avalanche diodes), and shouldn't bother you with a 3V
diode (which is a true Zener diode). As others have noted,
the low-current region of a 3V diode has a rather soft knee,
and that means some voltage uncertainty or high
diode impedance (same thing, just using different words).

 

[ian field]

Hello, I want to use a zener diode as a voltage clamp, and I'm
wondering how to bias it.... I'm getting a small signal TZX3V0A diode,
according to their datasheet, their test current is 5mA.... ok that's
fine, I can get a resistor value to limit my current to that... but,
is that the spec I should follow... the 5mA test current? My concern
is that I don't want to be too low and too close to that.... I don't
know what it's called... knee current, where the zener starts to go
into reverse breakdown... I want to be above that current... not near
it... right? please correct me if my thinking is wrong about this....

thanks

You can use a TL431 down to 2.5V, its often called an "adjustable zener" -
its actually a comparator with its own 2.5V reference built into a TO92
package, you need a couple of resistors to set the voltage but being based
on a comparator the knee is pretty sharp. Without digging out the data
sheet, as far as I can recall its recommended operating range is 1 - 100mA.