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Sometimes the power rating of a simple zener diode is not enough for some works, but their precious ability is needed.

What to do?

put two zeners in series? I heard that it will work in theory but in reality the break down voltages of the them will not be equal. so the mixture will not work as desired.

What is the solution?

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    \$\begingroup\$ For which purpose? Zener diodes have different applications. \$\endgroup\$
    – Eugene Sh.
    Oct 15, 2015 at 14:41
  • \$\begingroup\$ @EugeneSh. I want to put it in a circuit to make a steady voltage. to make the voltage smooth. \$\endgroup\$
    – AHB
    Oct 15, 2015 at 14:44
  • \$\begingroup\$ You probably want a proper voltage regulator, or a full DC-DC converter. Bare Zeners are only useful in small circuits. If you want a precise value, special ICs are available for that as well. \$\endgroup\$
    – pjc50
    Oct 15, 2015 at 14:49
  • \$\begingroup\$ yes. but there is no 60V linear regulator for example. but there are 60V zeners. \$\endgroup\$
    – AHB
    Oct 15, 2015 at 14:51
  • \$\begingroup\$ You could build one from discrete components. What exactly is your power requirement? Wasting a lot of energy in Zeners (or even linear regulators) is seldom a good idea. \$\endgroup\$
    – pjc50
    Oct 15, 2015 at 15:33

1 Answer 1

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Two zener diodes in series will work in practice too. But usually that would be used to create a higher voltage zener, rather than a higher power.

You can use a zener diode in conjunction with a transistor to provide a high power shunt:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ What is that resistor and why is it there? and how to calculate the resistance? \$\endgroup\$
    – AHB
    Oct 15, 2015 at 14:45
  • \$\begingroup\$ Calculate the resistor to drop 0.6V for the zener current at the desired voltage. \$\endgroup\$ Oct 15, 2015 at 15:00
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    \$\begingroup\$ That last bit of advice doesn't make any sense. The "max input voltage" is fixed at Vz + Vbe. What you really need to know is the maximum current you want the circuit to handle. Assuming the transistor has a beta of 50, 98% of the current will be passing through the transistor, and 2% will be passing through the zener. You then select the resistor so that it also passes 2% of the current when the voltage across it is Vbe (0.7 volts), which insures that the zener current never varies by more than a 2:1 range. \$\endgroup\$
    – Dave Tweed
    Oct 15, 2015 at 15:30
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    \$\begingroup\$ ... If you want even tighter control, use a smaller resistor, but make sure that the total current through the zener doesn't exceed its ratings. \$\endgroup\$
    – Dave Tweed
    Oct 15, 2015 at 15:30
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    \$\begingroup\$ No, subtract the maximum base current of the transistor from the maximum zener current, then divide 0.7 volts by that number. For example, suppose you have a 400 mW, 12 V zener. The maximum zener current is 400 mW / 12 V = 33.3 mA. Let's say you want the transistor to handle 1A, and its beta is 50, so it needs up to 1 A / 50 = 20 mA of base current. Therefore, the resistor current will be 33.3 mA - 20 mA = 13.3 mA, and its value should be 0.7 V / 13.3 mA = 52.6 ohms. The next larger standard value would be 56 ohms. \$\endgroup\$
    – Dave Tweed
    Oct 15, 2015 at 16:55

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