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High Power Zener Diode
Rod Elliott (ESP)

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High Power Zeners
While high power zener diodes are made, they are usually not readily available. They also tend to be rather expensive, and are often stud-mounted types. These are not always easy to install on a heatsink, and the mounting hardware (insulating bush and washer) seems to be all but unobtainable.

Provided you (or your application) can tolerate a slightly higher voltage than may have been specified, a high power zener can be made using an additional transistor and a resistor.

Using a Zener & Transistor
The method described is not new, and has been used in at least two of the projects described on the ESP website, as well as many commercial products. By using the zener to supply base current to a power transistor, the power rating is limited only by the transistor, with a likely additional limitation imposed by the device current gain at the design current. While zeners generally allow peak (momentary) currents that are much higher than their rated current, the transistor assisted version may not - again, this depends on the transistor.

Figure 1
Figure 1 - High Power Transistor Assisted Zener Diode

The transistor needs to be selected based on the maximum voltage and current expected. If the zener is used only for protection of more sensitive systems on the same power supply bus, the transistor may not even need a heatsink. This depends on the application, so you need to be careful before deciding not to use a heatsink.

The circuit shown is simply an example, and Q1 can be any transistor that is suitable for your needs. In most cases, a TIP3055 or similar will be more than adequate unless very high voltage or power is needed. Make sure that you check the safe operating area of the intended transistor!

The maximum current through a zener diode is determined by ...

I = P / V   where I = current, P = zener power rating, and V = zener voltage rating.
For example, a 27V 1W zener can carry a maximum continuous current of ...
I = 1 / 27 = 0.037A = 37mA
For optimum zener operation, it is best to keep the current to a maximum of 0.7 of the claimed maximum, so the 27V zener should not be run at more than about 26mA. This becomes the base current for the power transistor, and assuming a current gain of 25, that means the total 'composite zener' current is ...
26 x 25 = 650mA
The voltage is increased slightly (to about 27.7V), and the power rating is now ...
P = V * I   = 18W (minimum)
A darlington transistor can also be used for higher current, but will add around 1.5V to the zener voltage. Whether this will cause a problem or not depends on the circuit itself, and is not something that can be predicted in advance.

Construction is not critical, but a heatsink will almost certainly be needed for Q1. Using a clip to attach D1 to the heatsink will allow a higher dissipation, and will allow you to operate the zener at its maximum operating current. Select Q1 to suit the application - in many cases, a raid on the junk box will almost certainly provide something usable. R1 can be 0.25 or 0.5W.


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Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.
Page Created and Copyright © Rod Elliott 02 Jun 2005