Zeener Diodes in Parallel: 72 Watt Power Supply, May 2014

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EPA III
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Zeener Diodes in Parallel: 72 Watt Power Supply, May 2014

Post by EPA III » Sat May 10, 2014 8:15 pm

The schematic for the module used in this project, Figure 4, shows D2 and D3 as two 2.5V zeener diodes that are connected in parallel. They are in series with a 2.2K resistor for a fairly standard arrangement to provide a reference Voltage for the Voltage comparators a bit further in the circuit.

I have never seen two zeener diodes connected in parallel this way and can not see what, if anything, that it accomplishes. It does not change (increase?) the current through them, that would be controlled by the series resistor. It certainly can not be for a larger power rating as the two in combination only need to dissipate less than 3 mW. So why would there be two zeener diodes connected in parallel? What does it do?
Paul A.

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Re: Zeener Diodes in Parallel: 72 Watt Power Supply, May 201

Post by G8RPI » Thu May 15, 2014 2:19 pm

Hi Paul,
Only the designer can give you the correct answer. There is no obvious reason for this arrangement. Possible reasons are:-
1/ Increase power dissipation. As you noted this should not be an issue with the values used here. There is also the problem that the zener with the lowest voltage will grab all the current. It will get hotter and as zeners below about 5V typically have a negative temperature coefficient, it will stay that way.
2/ Compensate for temperature changes in two locations. Who knows, there is no information on the diode locations.
3/ Average the voltages of two diodes. This would need separate current resistors and a summing resistors (this is used with some high accuracy voltage references).

Your guess is as good as mine :-)

Robert Atkinson G8RPI.

Robert Reed
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Re: Zeener Diodes in Parallel: 72 Watt Power Supply, May 201

Post by Robert Reed » Thu May 15, 2014 7:41 pm

Are these already built into one of the off shelf items or are they reader built added circuitry. I see no mention of them in the parts list. All good arguments here and the only thing I could add is a reduced source impedance of the zener circuit, which like all other suggestions would be out of the question since the circuit would not warrant such a critical parameter. When conditions are permissible, the zener of choice is always a 6.2V version since this is where the zener technology has a zero temperature coefficient. Above or below that voltage the coefficient moves in opposite directions. My experience with typical off the shelf zeners below 3.3V are pure crap and I would not even consider them for new design. At least if there was a part number given for this device, that would sure go a long way to review the data sheet and see if there was some particular application it was manufactured for. If you contact the author, maybe he will shed some light on this. And if successful, be sure to post results because as it stands right now, it does not make any sense.

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