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Zener diode basics help.
Zener diode basics help.
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Old 3rd January 2018, 04:41 AM   #1
diy didi is offline diy didi  South Africa
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Default Zener diode basics help.

This is a back to basics question. More specifically how to calculate the series resistor to for the zener diode if one is for example making a series pass regulator with a transistor or a CCS.
Experimenting with zeners led me to the conclusion that a resistor value calculated for a 15V 1W zener will differ to one calculated for a 15V 400mW zener.
The 1W needs a higher min Iz to keep regulation. But what is the min Iz needed? I know datasheets sometimes state this, but sometimes one has an unknown brand zener. What then?
Anyone have a quick fail safe method?
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Old 3rd January 2018, 05:20 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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The max current through zener is zener wattage divided by zener voltage.
Running zener at max current all the time isn't good as it will get hot.

I usually run zeners at half the specified wattage or less.
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Old 3rd January 2018, 09:17 AM   #3
AndrewT is offline AndrewT  Scotland
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I work with min Iz at ~ 5% to 10% of the maximum rated current.
That usually gets on to the slope above the knee, but unfortunately not on to the flattest slope where the dynamic impedance is lowest.

If you run Iz @ 99% of rated current you will probably be at the lowest dynamic impedance, but as Nigel says it will run VERY hot and when mains goes to the highest tolerance allowed, may burn out.

You can measure the Vz at a range of currents (Iz) and plot the curve. You do not need much in the way of equipment, since Tj varies as Iz varies and you don't have to maintain constant Tj.
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Old 3rd January 2018, 02:44 PM   #4
infinia is offline infinia  United States
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Quote:
The 1W needs a higher min Iz to keep regulation. But what is the min Iz needed? I know datasheets sometimes state this, but sometimes one has an unknown brand zener. What then?
Anyone have a quick fail safe method?
It all depends on your definition of regulation.
Shunt regulator design using say 5-10% zeners feeding a emitter follower pass transistor have a wide range of voltages and tolerances to consider making your Q of Iz vs Vz trivial in my thinking.
Design it for wide range of Vin and the worse case beta and you will see.
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Old 3rd January 2018, 07:13 PM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by diy didi View Post
....what is the min Iz needed? I know datasheets sometimes state this...
There's no actual "minimum", only how soft you can tolerate.

If the supply was steady and the load was steady, you would not need a Zener. You need a Zener to cover supply and load variations. So there is no "target current". You must ponder a range of currents according to the line/load variations you need to suck-up.

And if you need an EXACT voltage, the Zener needs additional circuitry to keep its current rock-solid, and to adjust-out its 5% slop. There are better ways (good voltage regulators, precision voltage references).

Attached is Vishay's "1W" part spec, excerpted. For the 15V part, 61mA is near the max, and leaves little leeway for raw supply or load variation. It is also HOT, not a good thing.

OTOH you never want Zener current to go to zero, at minimum line and minimum load. You don't want to go "too low" either. "Too low" is not well defined in data because it will still be Zenering at quite low currents, but the terminal voltage will be less than nominal.

The 15V part is tested at 17mA (0.25W). It is also speced for Zz at 0.25mA. Voltage not given but say 13V. 13V/0.25mA is 52K static impedance. But the dynamic impedance @0.25mA is 700 Ohms. Voltage is 52K/0.7K or 74X better than no Zener. If you aim for around 17mA, Zz is 14 Ohms. 15V/17mA is 882 Ohms. Rejection is 16X. Note that lower current is somewhat better rejection.

Let's normalize these against the sheet-top "1300mW" rating:
61mA = 70% of show-off "1300W" rating
17mA = 18% of show-off "1300W" rating
0.25mA = 0.3% of show-off "1300W" rating

Since 1W parts are now common, I would tend to buy large and then work it below 25% of rating but above 1% of rating. If I feared I'd got cheap leaky rejects, I might move the lower goal to 5% or 10%.
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Old 3rd January 2018, 07:48 PM   #6
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by diy didi View Post
This is a back to basics question. More specifically how to calculate the series resistor to for the zener diode if one is for example making a series pass regulator with a transistor or a CCS.
Experimenting with zeners led me to the conclusion that a resistor value calculated for a 15V 1W zener will differ to one calculated for a 15V 400mW zener.
The 1W needs a higher min Iz to keep regulation. But what is the min Iz needed? I know datasheets sometimes state this, but sometimes one has an unknown brand zener. What then?
Anyone have a quick fail safe method?
I usually look at the datasheet of a Zener with equal power rating from a known brand and hope they behave similarly.
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Old 3rd January 2018, 08:09 PM   #7
audiofan is offline audiofan  Canada
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What you can do is:

Calculate the resistor with the maximum voltage of the supply, with the lowest current of the load for the maximum dissipation of the zener. ( 90 % of maximum dissipation of zener from spec sheet )

Then calculate the resistor for the lowest voltage of the supply, with the maximum current of the load for the minimum dissipation of the zener. ( 10 % of maximum dissipation of zener from spec sheet )

After you select resistor halfway betwen these 2 values .
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