Download Zener Diode Exercise Number 2

Zener Diode Exercise Number 2
Hypothetical problem. Zener diode is shorted out and took the load resistor,
burned to a crisp, with it.
Most of the time a Zener Diode is used to regulate a
low voltage at some low current. The basic configuration is
a series circuit with a resistor and a Zener diode. In the
circuit Drawing #1 the resistor R4 represents some circuit
that requires 4.70 Volts and draws from 1 ma to 2 mA as it
operates.
In order to regulate that 1 mA to 2 mA we need a
Zener current that is at least five times that value. A current
through the Zener Diode of at least 10 mA is required. With 4.7 Volts across the Zener
and a current of 10 mA our wattage for the Zener is only around 50 mW, so even a 500
mW Zener Diode will handle the load for us. We shop for a 4.7 Volt, 500 mW Zener
diode and come up with a 1N5230B.
Looking at the chart for a 1N5230B we find that with 10 mA through it we hit
right at 4.70 Volts. So the 1N5230B must be our best choice. Note that we can tailor the
voltage by making adjustments to the current through the Zener. The 1N523x series
Zener diodes are 5% tolerance. This means that at normal operating conditions we will
get 4.7 Volts + or - 5%. We can tweak the current to obtain any voltage within that range
for more precise operation.
Imagine just for the exercise that we have a 6 Volt source we can draw from. We
need to find the proper size for the ballast resistor. The voltage across the resistor will be
6 V - 4.7 V, or about 1.3 Volts. The current through the ballast resistor will be the Zener
diode current plus the load current, or about 12 mA (10 mA through the Zener and 2 mA
through the load). Ohm's Law serves us yet again. 1.3 V divided by 12 mA = 108 Ohms.
Since there is no 108-Ohm resistor we go with the next closest standard value, or 110
Ohms.
So much for theory. Now lets grab some real world parts and see if reality agrees
with theory. V In actually measures 6.37 Volts instead of 6.0. That is a reality we must
consider. Making adjustments for this reality we recalculate our Ballast resistor and come
up with 139 Ohms. We have two choices of real world resistors here 130 or 150.
We grab a 1N5230B out of the drawer and plug it into our circuit. And make
some measurements. We add various resistors to simulate a changing load current and see
how much out voltage changes as our simulated load changes.
130-Ohm Ballast
Zener Diode Voltage
Load
4.78 V
4.77 V
4.76 V
4.75 V
4.75 V
4.74 V
No load
5K1 Ohm (0.937 mA)
3K6 Ohm (1.32 mA)
3K0 Ohm (1.58 mA)
2K7 Ohm (1.74 mA)
2K0 Ohm (2.37 mA)
Covering our range of regulated voltage at currents from 1 mA to 2 mA we get
4.74 to 4.77 Volts. That voltage is a little high. Let's see if we can improve. We change
the 130-Ohm to 150 Ohm and retry.
150-Ohm Ballast
Zener Diode Voltage
Load
4.75 V
4.74 V
4.73 V
4.72 V
4.72 V
4.71 V
No load
5K1 Ohm (0.931 mA)
3K6 Ohm (1.31 mA)
3K0 Ohm (1.57 mA)
2K7 Ohm (1.78 mA)
2K0 Ohm (2.35 mA)
Considering all the alternatives let's change the 150 to 180 and see what results
we get.
180-Ohm Ballast
Zener Diode Voltage
Load
4.75 V
4.70 V
4.69 V
4.68 V
4.67 V
4.65 V
No load
5K1 Ohm (0.921 mA)
3K6 Ohm (1.30 mA)
3K0 Ohm (1.56 mA)
2K7 Ohm (1.73 mA)
2K0 Ohm (2.32 mA)
With the Zener diode we chose we find that reality did not agree with theory.
What calculates out that a 110-Ohm resistor really ends up being a 150-Ohm works
better. The results would vary depending on the Zener diode we chose.
We see that we can tweak the precise voltage to any voltage within the range of
the 1N5230B and well beyond the 5% range. If you do the Zener Diode exercise #1 you
will find that we can change the current through the Zener from 1 mA to 100 mA and get
a Zener Voltage point of 3.96 V to 5.04 Volts. Below 1 mA it gets unstable due to noise.
Above 100 mA we push the Wattage capability of the device (100 mA x 4.7 V = 470 mW
for a device rated at 500 mW) and we get unstable do to thermal problems.
The lesson to be learned
When replacing a Zener diode in a circuit you may have to make adjustments to
the circuit, changing the ballast resistor, to get the circuit to operate within specifications.
The schematics will probably say what voltage is expected. The manufacturer may match
resistor to Zener diode as the board is being manufactured. Usually such tweaks are
shown in the parts list. Not always.
Herschel