Brian said:
Robert Monsen said:
Brian wrote:
In a simple voltage regulator circuit, using a resistor and zener diode. The
formula for the resistor in series with the zener diode is:
R = [Vin(max) - Vz] / [Iz(min) + IL(max)]
My question is, how do you calculate or find what the minimum
zener
current
should be? I don't find this in the data sheets for zener diodes.
There is a 'test current' given in the datasheets. That's the best one
to use if you want the zener to be near it's rated value.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of
God.
So in a worst case situation, if say the input voltage varied from 10
volts to 20 volts and the zener volts was 9 volts (with a load current of 20
milliamps), you would use the zener test current for the Iz(min)? I know,
there is a better way to do this than with a zener diode, but for
information sake I would like to know the right way to do this.
Thanks,
Brian
I think your forumla is wrong.
You need to think about what's happening. A zener diode is a shunt
regulator. It will gobble up as much current as it can if the voltage
across it is above it's rated value. Thus, your resistor is there to
prevent a virtual short circuit between Vin and Vz.
One main problem with shunt regulators is that they don't work very well
if the input voltage drops below their design value (I guess this is
true of all regulators)
Because of this, if you want regulation, you want to make sure the
voltage is always above Vz, given your required load current. If the
current is maximum (IL + Iz(min)) then the voltage drop across the
resistor is R * (IL + Iz(min)). Since the zener must have at least it's
rated voltage across it to work, so the input voltage must be *at least*
Vin = Vz + R * (IL + Iz(min)).
What this means is that the formula should really call for Vin(min).
As an example, suppose your input voltage ranges between 10V to 20V, and
you have a 5.1V zener. It's Iz(min) is 10mA, and your load is 20mA.
Then, using your formula,
R = (20 - 5.1)/(30mA) = 496 ohms.
However, if you use that with your 20mA load at 10V, then, even if the
zener stops drawing any current (which it will do), your resistor will
drop 9.93V! This only leaves you 0.07V. So, you aren't regulating.
However, if you use
R = (10 - 5.1)/(30mA) = 163.3 ohms
then at 20V, your zener will still be regulating (although the voltage
across your load will be a bit higher, since the zener voltage will rise
a bit with more current). It will suck up the excess current. If your
load is using 20mA, then it'll take in the extra 70mA.
Thus, using Vin(min) in the forumla, your regulator will be regulating
when input is between 10V and 20V.
One other parameter you need to complete your design of the zener
regulator is the power rating of the zener. You need to ensure that the
power rating of the resistor and zener are within their maximums. And
you need to consider temperature effects on the zener. However, you
didn't ask about that.
Usually, using a linear regulator that has temperature compensation and
current limiting built in is a better bet. However, there are times when
a shunt regulator is the only way to go.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.