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24-09-2010, 01:23 PM

.doc   ZENER DIODE TESTER.doc (Size: 3.27 MB / Downloads: 99)


The testing of zener diodes requires a variable dc power supply. A typical test circuit can be constructed, as shown in figure 4-19. In this circuit, the variable power supply is used to adjust the input voltage to a suitable value for the zener diode being tested. With the zener diode connected no current will flow until the voltage across the diode is equal to the zener voltage. If the diode is connected in the opposite direction, current will flow at a low voltage, usually less than 1 volt. Current flow at a low voltage in both directions indicates that the zener diode is defective. Before you start to test any zener diode, you must first understand the marking or part number and then look for the voltage ratings. Refer to my article about how to read the zener diode data. Once you know the zener diode voltage from your favorite data book such as the Philip ECG semiconductor master replacement guide then it is easy to check with your meter to see if it leak, open or shorted.

Sometimes a normal signal glass type diode you may think is a zener diode thus you will not get the exact measurement. If you have confirmed that the diode you want to measure is zener diode then you can proceed to use my method to accurately test it. For your knowledge, a zener diode with 2.4 volt to 12 volt should have two readings when test with an analog meter set to times 10K ohm range. But these readings are not shorted reading.when you put your meter probes across the zener diode of 2.4 volt using the times 10 k ohm range, one way will show a full scale reading (red probe to cathode and black probe to anode) which mean the pointer will point towards the 0 ohms scale, if you now connect the probe the other way (black probe to cathode and red probe to anode) the pointer will point to around 2- 4 ohms!

If both ways of testing cause the pointer to point to 0 ohm, then the zener diode is considered shorted. When you measure a 5.1 volt zener diode, as usual one way will point to zero ohms while the other way will show a higher resistance which is in the 20 to 60 ohms. These are the characteristic of a good working zener diode and don’t think that the meter shows two reading means the zener diode is faulty.

seminar girl
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16-08-2012, 04:07 PM


.doc   Zener diode.doc (Size: 49 KB / Downloads: 18)

Zener Diode Voltage Regulator Circuit

Pictured above is a very simple voltage regulator circuit requiring just one zener diode (available from the REUK Shop) and one resistor. As long as the input voltage is a few volts more than the desired output voltage, the voltage across the zener diode will be stable.

As the input voltage increases the current through the Zener diode increases but the voltage drop remains constant - a feature of zener diodes. Therefore since the current in the circuit has increased the voltage drop across the resistor increases by an amount equal to the difference between the input voltage and the zener voltage of the diode.

Zener Diode Voltage Regulator Calculator

Below is a simple to use zener diode voltage regulator calculator. For any given output voltage and current requirement it will calculate the value of power rating of the current limiting resistor and zener diode. Read on to understand how zener diodes and resistors are matched for any particular scenario.

Enter the values for Vout (the desired output voltage - equal to the zener voltage of your diode), Vsource (the maximum power supply voltage - must be at least 2-3 Volts higher than the desired output voltage), and Imax (the maximum current which will be drawn by the load plus a 10% safety buffer).

Matching the Zener Diode and Resistor to the Situation
Here is a hand-worked example which shows how to choose the correct zener diode and resistor for a known load: we have an unstable 12 Volt supply voltage and need a stable output of 8 Volts to power a 100mA device. 12 volts is sufficiently above 8 volts to ensure that any ripples in the supply will not take us below our target voltage.

1. Choose a Zener Diode
Since we need 8 Volts we can choose between a 7.5V or an 8.2V zener diode. 8.2V is close enough to our target voltage so we choose a zener diode with an 8.2 Volt zener voltage.

2. Calculate the Maximum Current in the Circuit
Our load device needs 100mA of current, plus we also need at least 5mA for the zener diode, therefore lets set Imax as 110mA to be safe. If you add 10-20% to the load current, this will give you a safe value for the maximum current in the circuit as long as the input voltage is unlikely to jump much higher.

3. Select the Power Rating of the Zener Diode
Zener diodes are available in a range of difference power ratings. If a large current flows through a small zener diode it will be destroyed, therefore we calculate the power to be lost in the diode and select a diode rated above that value. Here the zener power rating is equal to the zener voltage multiplied by the maximum current (Imax) calculated above which equals 8.2 * 0.110 = 0.9 Watts. Therefore a 1.3 Watt power rated zener diode should be perfect.
We multiply the full maximum current by the zener voltage since when no current is flowing through the load - e.g. when the device is switched off - all of the current will flow through the zener diode.

Select the Resisitor
The voltage dropped across the resistor is equal to the difference between the source voltage and the zener voltage = 12-8 = 4 Volts, and therefore the resistance according to Ohm's Law is the voltage drop divided by Imax = 4/0.110 = 36 Ohms so choose a 39 Ohm resisitor.
If the source voltage is likely to be much over the 12 Volts stated then the voltage dropped across the resisitor will be larger and so a resistor with a larger resistance may be required.

Select the Power Rating of the Resisitor

The power dissipated in the resistor is equal to the voltage drop across the resistor multiplied by Imax. Therefore in this example power = 4 * 0.110 = 0.440 Watts. Using a 0.5 Watt resistor would be cutting it a bit fine - particularly if the source voltage is going to fluctuate higher regularly, therefore a 1 or 2 Watt rated resisitor should be used here despite it costing a few pennies extra.
Renewable Energy System Battery Bank Situation
If the above situation is referring to a renewable energy system battery bank, the 12V source voltage could vary from as little as 10.6 Volts to as much as 15.5 Volts. Therefore we need to check that everything still works correctly at the higher and lower voltage.

If the source voltage rose to 15.5 Volts then we would have 15.5-8.2 = 7.3 Volts dropped across the 39 Ohm resistor: a current of 187mA. If the source voltage fell to 10.6 Volts then we would have just 10.6-8.2 = 2.4 Volts dropped across the 39 Ohm resisitor: a current of 61mA. Therefore in both cases we easily have enough current passing through the zener diode to ensure a stable output voltage will be maintained.


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