# zener summing



## geosman (Sep 15, 2007)

Have pity on me for even asking but if I have 2 zeners that sum to the breakdown voltage of an existing one that needs replacing and if the wattage of each meets or exceeds that of the replaced zener, will this work satisfactorily?
Now what if I need to replace a 6.2v zener and I have 3.3v and a 3.0v how much of a problem am I looking at for that 0.1v? I am thinking none since zener values are +/- 5%-10%, proviso the first answer is yes. Is there a limit to how much of a voltage separation there should be for doing this? I'm guessing that 10.0 and 1.0 could be too much?


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## Done_Fishin (Oct 10, 2006)

from what I remember and that may be in association with BZX78C diodes etc .. the zener voltage of diodes is usually +/- 0.3 Volts but you'd have to check that by reading the datasheet of the diode you would be trying to use .. 0.1V was with temperature stabilised diodes .. forget the number since it's been a long time since I was invoilved in that sort f stuff ..

anyway .. if you had to use two zeners instaed of one .. you'd have to check that they both worked on the same zener curve ... ie at 5mA and also see what the reaction might be if they were to carry 50mA ..

assuming the diodes were similar in operation then you still have the worst case scenario to deal with .. that both zeners are either 0.3 Volts positive or negative of the optimum value .. meaning that your 6.2V zener might be anywhere in the region 6.3V +/- 0.6V ie 5.7V ~ 6.9V depending upon characteristics 

personally I would try to find a 5V6 and a 1N4001 and place them in series or any smaller value diode and a series of 1N4001 diodes to make up the difference .. the Zener would be reverse biased as one might expect but the 1N4001's would be connected in forward bias mode. The Zener obviously would have Anode to Ground and the cathode to the cathode of the 1N4001 whilst the Anode of teh 1N4001 would go towards the positive ends of the circuit. Each 1N4001 would have a potential drop of 0.7V so you could figure out for yourself what and how many to use according top the Zener you had to hand.


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## mack1 (Nov 27, 2007)

> Have pity on me for even asking but if I have 2 zeners that sum to the breakdown voltage of an existing one that needs replacing and if the wattage of each meets or exceeds that of the replaced zener, will this work satisfactorily?


Hi geosman,

Like Done_Fishin, it's been a while since I did circuit design work. Zener voltage is at a certain current through the zener. The breakover point for a zener is not sharp. After the breakover is reached, voltage will rise a little as current increases through the zener. Like forward current through a diode, it breaks over at about .5 volts (silicon diode) and will go up to about .6 volts as current is applied. 

Hooking your two zeners in series should work fine as you have considered the wattage of the two. Zeners don't care about the difference if voltage ratings. They will simply add their part to the sum. 

Best regards,
Mack1


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