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I'm creating a small assortment for my parts box, and I wonder about Zener currents. For example, a selection of 4.7V 500mW Zeners spans a wide range of Zener current values:

  • BZX79C4V7 = 5 mA Zener current (80 ohms Zener impedance)
  • 1N5230B = 20 mA (19 ohms)
  • 1N750A = 75 mA (19 ohms)

What are the pros and cons of selecting one over the other, in terms of general utility / substitutability? As a rule, is a lower Zener current generally better?

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  • \$\begingroup\$ What do the data sheets tell you about "other things"? \$\endgroup\$ – Andy aka Jul 15 '16 at 20:51
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    \$\begingroup\$ You act like there's a general concensus on what is the "best" component. Maybe I just want the cheapest one. And someone else needs the one with the lowest noise. The third guy want the best availability while the fourth guy only buys form Fairchild. There is no "best" component untill you know all these parameters. But for a hobby box: just get the cheapest one and that will work just fine 99% of the time. And when the day comes when you need a better zener, you will know what parameter needs to be better and you will know what to buy then. \$\endgroup\$ – Bimpelrekkie Jul 15 '16 at 21:10
  • \$\begingroup\$ Nothing that appears really significant but I'm not an expert. When it comes to such kits they seem to either include one of the three lines, but for no clear reason. The two parameters I've noted stand out. The current seemed the most obvious, and I wasn't sure if perhaps lower Zener current was a hallmark of newer device technology (much like 74LS has long since been supplanted by newer technologies). \$\endgroup\$ – MartyMacGyver Jul 15 '16 at 21:12
  • \$\begingroup\$ Sure, the ones with the "better" specs might be newer models. That does not mean you'd always need that. It does not mean that the older (and often cheaper) devices aren't good enough. In general if you need "the best" then you will have an idea why that is so and then you would know what you would be looking for. \$\endgroup\$ – Bimpelrekkie Jul 15 '16 at 21:14
  • \$\begingroup\$ This is all good information. In general I'm trying to avoid obsolete or otherwise deprecated devices, but it wasn't quite clear what an "improvement" would look like when it comes to Zeners. \$\endgroup\$ – MartyMacGyver Jul 15 '16 at 21:34
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As a rule, is a lower Zener current generally better?

Not always. Zener voltage is affected by its drive current, such that if the drive current is too low the zener voltage will be less than the rated clamp voltage. For instance a 3.3 volt 5 watt zener may need 50 mA to reach its 3.3 volt clamp level. When you are shopping for zener diodes pay close attention to the current they are tested at, usually 50% of the diodes maximum safe current which means it is at 25% to 75% of its maximum wattage rating. For long life diodes and semiconductors in general are seldom driven at more than 50% of their capacity (Be it volts/amps/watts). In your list of 3 zener diodes of which are the same clamp voltage but different bias currents, some numbers really stand out.

  1. BZX79C4V7 = 5 mA Zener current (80 ohms Zener impedance). Notice the low bias current but also the high impedance. It will have less of a grip on the clamp voltage but is OK for low-current general purpose use. With such a low current this is more of a voltage reference, to be boosted by op-amps, etc.

  2. 1N5230B = 20 mA (19 ohms). Lower impedance means a more firm grip on the clamp voltage at a somewhat higher bias current. This is a newer product than the last item and one that I would use, though 500mW zeners tend to be voltage references, to be boosted by op-amps, etc.

  3. 1N750A = 75 mA (19 ohms). This is an older outdated zener, hence the higher bias current. It will run warmer and waste more current than options 1 or 2. You may see it at a cheap price in bulk quantities from parts brokers but I would avoid it.

The higher wattage zeners are for clamping surges on analog or digital inputs, and for directly driving a low-current load such as CMOS logic or a switching regulator. Often you see 3 watt and 5 watt zener diodes as actual shunt voltage regulators for circuits that need 100mA or less of current. Also the higher wattages go up to several hundred clamp volts, trading volts for milliamps to maintain the 3 to 5 watt limit. Once again, build the circuit to use only 50% or so of the zeners maximum wattage rating.

When possible run the zener at the lowest current where it can maintain its stated clamp voltage, including any loads. Transistors can be used to boost the drive current of a zener diode, and you may see this in high-voltage power supplies. The following stage maybe a switching regulator or CMOS logic running on 15 volts but the source voltage is 170 volts. This is when zener diodes come in handy.

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  • \$\begingroup\$ That 1N750 is what inspired my question - it's specified in a little circuit I'm building yet likely not for a finely tuned reason that depends on that specific old part. I noticed it was outdated and the questions that followed led me here. Thanks for breaking them down as you have - it helped me make sense of it! \$\endgroup\$ – MartyMacGyver Jul 17 '16 at 18:55
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The reverse current rating of a Zener diode is somewhat analogous to the forward current rating of a regular diode in that the current rating you would need in a particular application will depend upon the application.

In general, the Zener (reverse breakdown) voltage is dependent on the reverse current actually made to flow thru the Zener in a given application. If you depart very far from the current rating, the Zener voltage will vary from the data sheet rating. Thus, the Zener diode will not be as "precise" in terms of reverse voltage if used at 1/10th the rated reverse current as it will if used at 90% of rated current. You must "fit" the Zener to the application. So, it pays to have as large a variety of voltages and current ratings on-hand as is practical and affordable.

For use as voltage regulators and voltage references Zener diodes have largely been supplanted by IC voltage regulators and IC voltage references because the latter are so inexpensive and small compared to their counterparts of past eras. Still, Zeners have their niche in certain applications. E.g. As spike suppressors in certain inductive load situations.

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  • \$\begingroup\$ +1 for mentioning that Zeners aren't used much as voltage regulators nowadays. Suggestion: add a reference (pun intended :-) to the dirty cheap TL431/LM431 "universal Zener IC" and its datasheet. \$\endgroup\$ – Lorenzo Donati Jul 15 '16 at 22:26
  • \$\begingroup\$ There are, however, uses for them - in the present thing I'm working on it's a simple voltage cutoff for the output stage a rather peculiar circuit that would otherwise produce too high a voltage of signal. I wouldn't normally use a Zener for a regulator but there are times and places for them even now. \$\endgroup\$ – MartyMacGyver Jul 17 '16 at 4:17
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Higher zener currents will be more stable for a noisy/low impedance load (since a given draw will be a smaller difference relative to the current though the diode), lower currents (obviously) absorb less power. Generally, there's not going to be a significant difference unless you're doing something specific, that needs particular characteristics. It depends on what sorts of things you expect to need it for. I'd expect that higher would be better in most cases, as it will give you more leeway before blowing out.

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What does a zener do that makes it suitable for this circuit or that circuit - is it the amount of current it draws (or needs) or is it how accurately it produces a constant voltage and how that voltage remains stable against temperature and time?

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  • \$\begingroup\$ I got the answers I needed for the main question. In this case the Zener is being used to set a ceiling on the output voltage from a somewhat peculiar higher voltage circuit. It's not so much about precision (and obviously not fidelity) as it is about limiting the maximum output voltage to logic-safe levels. \$\endgroup\$ – MartyMacGyver Jul 17 '16 at 21:35

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