Why do we not see Zener diodes used in series as a voltage dropper? - Page 1
Forum section: electronics / projects

author: silicon junkie
author_rank: Newbie
timestamp: August 17, 2019, 02:06:34 pm
content: Here’ a question: Why do we not see Zener diodes used in series as a voltage dropper?Assuming minimum forward current and max dissipation is observed it should be ok but I don’t think I have ever seen it doneStandard diodes are sometimes used but if you want to drop more than a volt or two then a string of them becomes impractical.My practical use is to reduce the power dissipation (edited: by about 500mW) in a SMT regulator by sharring it with a 3.9v series zener.Any ideas why we never see this done in practice?

author: OM222O
author_rank: Frequent Contributor
timestamp: August 17, 2019, 02:37:00 pm
content: at low currents the power dissipation of an LDO is minimal, so nothing is needed.at moderate currents, usually 1 or 2 volts of drop helps significantly, so normal diodes / resistors are used (resistor needs to be calculated for optimal performance).at high currents, either a tracking pre-regulator is used for efficiency reasons (usually buck converter) or if you don't care about efficiency at all, again a resistor does much better that diodes. zeners are useless for this sort of power applications. let's say you want to drop 2 volts at 1A (which is very conservative, usually the voltage difference or current should be higher to use resistors!), that means 2 watts of power. I'm not sure if zeners with that power rating even exist, and if they do, they will be much much more expensive than even a SMD 5watt resistor.simply put: in non of the conditions a zener makes any sense.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 17, 2019, 02:37:23 pm
content: Scroll down in this article to see zeners wired in series. Probably the reason they aren't used more in series is that it is so easy to make a variable supply using one of the cheap 3-terminal regulators.https://www.electronics-tutorials.ws/diode/diode_7.html

author: hv222
author_rank: Regular Contributor
timestamp: August 17, 2019, 02:47:28 pm
content: Zener diodes are used in series as a voltage dropper. Check out this application note:https://www.analog.com/media/en/technical-documentation/application-notes/an92f.pdf(figure 8 and 15). Zener diodes are used to drop voltage, before current sensing resistor to match maximum input voltage of operational amplifier.

author: SteveyG
author_rank: Unknown
timestamp: August 17, 2019, 02:48:10 pm
content: They are regularly used in this manner. Absolutely nothing wrong with it

author: silicon junkie
author_rank: Newbie
timestamp: August 17, 2019, 03:09:56 pm
content: Thanks for your feedback, much appreciated.

author: coppercone2
author_rank: Super Contributor
timestamp: August 17, 2019, 03:25:07 pm
content: its done for cost/simple reasons

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 17, 2019, 03:40:55 pm
content: Because resistors are cheaper and also provide filtering when used with bypass capacitors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2019, 03:59:54 pm
content: Because regulators are cheap, plentiful and accurate, and many modern applications can't tolerate the loose voltages of zeners?Back in the days of analog circuitry, where multiple voltages were present, and none of them needed to be particularly accurate, and the circuit was full of trimmers to adjust final offsets anyway: they were a common item. Even neon tubes (typical ~65V drop), going back still further.Tim

author: bob91343
author_rank: Super Contributor
timestamp: August 17, 2019, 04:35:53 pm
content: One reason not mentioned is voltage tolerance. If you subtract a stable value from an unregulated source, the result is a much more poorly regulated output. If the input varies from 40 to 42 volts and you subtract 25 volts, you get 15 to 17 output, much less stable in terms of fractional change.Further, due to zener tolerance, the output will vary with load, in addition to its source variation, making the result evern worse. And I didn't even mention temperature effects which compound the issue further.Of course the other comments are valid but this in particular makes the idea a poor one.

author: amyk
author_rank: Super Contributor
timestamp: August 17, 2019, 06:56:29 pm
content: This product contains zeners used in series:

author: krho
author_rank: Regular Contributor
timestamp: August 19, 2019, 07:39:32 pm
content: Well for one you cannot drop the cc pin in USB-c from up to 25V down to 5V with the regulator. So you use 5V1 zener for that.