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Zener Diode

 

 

What is a Zener Diode?: A Zener diode is a special case of  ordinary semiconductor silicon diode.  Specifically, the Zener displays the following property: The direction of current is from positive to negative and it blocks current in the minus to plus direction (Reversed Bias). See sketch for Zener diode symbol and picture of markings on a part itself. If a positive voltage is applied to the anode terminal of a Zener diode and a negative voltage is applied to cathode terminal, the Zener diode will behave as an ordinary semiconductor diode.  However, when a positive voltage is applied to the negative, the Zener diode will breakdown when it is reversed biased by a voltage greater than the breakdown voltage. As a result a Zener voltage (Vz) will appear across the diode terminals, with its cathode being positive and the anode being negative.See Figure below for pictorial and associated symbol:

See example below for more information

In the preceding figure which reflects a zener diode acting as a zener voltage regulator across load resistor R2.

There is a + 12 vdc voltage source.

There is a 9.1 vdc output voltage requirement from the zener

Total Current passing through R1 = I total

The current through reversed biased zener  = I zener

The required load current is passing through  = I load

Important Considerations:

1) Diode Voltage = 9.1 vdc +/- 10%

2) Zener diode voltage must be several volts below the source voltage in order to force the zener into its breakdown region (9.1 vdc).

3) The Zener maximum power rating must never be exceeded. In order to achieve this, R1 must be selected first, in order to limit the current through the Zener. This is calculated with the R2 load disconnected.

Case 1 Details using 1N4739 1 watt Zener diode (reference zener diode data sheet available on Internet):

This Zener voltage which appears across the zener terminals is very stable, hence it is used a relatively inexpensive zener voltage regulator or voltage reference. The Zener voltage is steady over a wide range of input or output load variations.

Given the following:

1) Test Current  = 28ma = Best operating Zener Current (available from zener diode data  sheet for the 1N4739)

2) Load requires 10ma based on device that is across the 9.1 vdc zener

Calculations:

I total = I zener + I load          Kirchoff’s Laws

or

I total = 28 ma + 10 ma = 38 ma.

R1 resistance value = Voltage across R1/I total     Ohm’s Law

or

R1 = 12-9.1 = 2.9 vdc/38 ma = 76.3ohms

9.1 volt Zener has a 1N4739 part number. Data sheet indicates that the maximum power rating = 1 watt.

Maximum Zener Current = Max Power/Zener voltage

o r

1 watt/9.1 vdc = 110 ma  with load R2 disconnected.

With R2  removed, Zener power = 9.1vdc x 28ma = 254 milliwatts

This choice of diode is perfectly safe even with a 10% variation of voltage tolerance and subsequent power change we are still under 1 watt.

Case 2 Details using BZX79-C9V1 500 mw Zener diode (reference zener diode data sheet available on Internet):

Given the following:

1) Diode Voltage = 9.1 vdc +/- 0.1 vdc

2) Test Current  = 5ma = Best operating Zener Current (available from data  sheet for the BZX79-C9V1)

3) Load requires 10ma based on device that is across the 9.1 vdc zener

Calculations:

I total = I zener + I load               Kirchoff’s Law

or

I total = 5 ma + 10 ma = 15 ma.

R1 resistance value = Voltage across R1/I total       Ohm’s Law

or

R1 = 12-9.1 = 2.9 vdc/15 ma = 193.3ohms

9.1 volt Zener has a BZX79-C9V1 part number. Data sheet indicates that the maximum power rating = 500 mw.

Maximum Zener Current = Max Power/Zener voltage

o r

0.5 watt/9.1 vdc = 55 ma  with load R2 disconnected.

With R2  removed, Zener power = 9.1vdc x 5ma = 45.5 milliwatts

This choice of diode is perfectly safe even with a +/- 0.1 vdc variation of voltage tolerance and subsequent power change.


Conclusion: Either the Case 1 or Case 2 diode will meet the max power specification for the selected diode, but the Case 2 diode is much more accurate voltage-wise.

The Battery voltage source to the Zener diode can actually be a variable or fixed voltage power pack  whose DC voltage is three to five volts greater than the Zener’s voltage.

Another Application

A Zener diode can be used as an overvoltage protector for the output of a power supply that experiences an unwanted voltage surges, as long as the zener voltage is less than the power supply output. When the voltage across the power supply spikes up, the zener across the power supply output will absorb the spike and limit the voltage output to equal the zener output.  Further explanation is beyond the scope of this tutorial.

What are Power Zener Diodes? As the name implies, Power Zener Diodes can take alot more power dissipation.  For example these diodes are available in the 22 to 39 vdc range with currents as high as 50 amps. They are obviously used in high powered power supply applications and in automobile alternators as rectifier diodes.  Not much application to model trains but if your interested take a look at Diodes, Inc. at www.diodes.com

A Surface Mounted Zener Diode is one that is mounted directly to a Printed Circuit Board (PCB)  without the use of wire leads attached to the diode which in turn are fed through holes in the circuit board.

Zener Diode Testing

There are two methods.

(1) The first way is using just an ohmeter.  If the diode is in the 2.4 volt to 12 volt range set the meter to the 10kohm scale. Put red probe on cathode and black probe on anode and the meter needle will go towards full scale. If you reverse the probes with red probe to the anode and black probe to the cathode you should measure about 2 to 4 ohms using a lower scale. By the way if you read zero ohms in both directions the zener is shorted or if if both readings are full scale on the 10k scale, then the zener is blown open. Some zeners will read other values other than 2 to 4 ohms, but what is important that you get less than 100 ohms (red/anode to black/cathode) and a much larger reading (greater than 10k red/cathode to black/anode).

(2) The second way is to test the zener in a dynamic fashion by creating a circuit similar to either case 1 or 2 above, with the load resistor removed. However, the zener must be put in series with a current resistor on the order of 5k. Then measure the voltage across the zener and determine adequacy per data sheet.

If you wish to acquire a better understanding of Electronics Theory, I suggest you go to  the following  link: Electronics

Connecting Zeners in Series

Rather straightforward, if two zeners are hooked up (cathode to anode + cathode to anode) the two voltages will add up.

Connecting Zeners in Parallel

Putting Zeners in parallel creates a situation where one will turn on before the other and draw all the current, thereby negating the effect of the second Zener.

In general, the following vendors are recommended for any of the above electronic components:

Radio Shack (Local Store or on-line at  www.radioshack.com)

Futurlec at  www.futurlec.com

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