Zener Minimum Current?

In a simple voltage regulator circuit, using a resistor and zener diode. The formula for the resistor in series with the zener diode is:

R = [Vin(max) - Vz] / [Iz(min) + IL(max)]

My question is, how do you calculate or find what the minimum zener current should be? I don't find this in the data sheets for zener diodes.

Thanks,

Brian

Resistor-zener regulators are numerically challenged. In your example, the drop across the input resistor varies over an 11:1 range, and it has to be able to supply the load on the low end, never mind a min zener current. So the worst-case input current will be at least 11x the max load current, or 220 mA. At 20 volts in, the zener dissipates almost 2 watts and the resistor more. Efficiency is insane and things will fry.

Oh, 1 mA is a good min zener current.

John

I think your forumla is wrong.

You need to think about what's happening. A zener diode is a shunt regulator. It will gobble up as much current as it can if the voltage across it is above it's rated value. Thus, your resistor is there to prevent a virtual short circuit between Vin and Vz.

One main problem with shunt regulators is that they don't work very well if the input voltage drops below their design value (I guess this is true of all regulators)

Because of this, if you want regulation, you want to make sure the voltage is always above Vz, given your required load current. If the current is maximum (IL + Iz(min)) then the voltage drop across the resistor is R * (IL + Iz(min)). Since the zener must have at least it's rated voltage across it to work, so the input voltage must be *at least* Vin = Vz + R * (IL + Iz(min)).

What this means is that the formula should really call for Vin(min).

As an example, suppose your input voltage ranges between 10V to 20V, and you have a 5.1V zener. It's Iz(min) is 10mA, and your load is 20mA. Then, using your formula,

R = (20 - 5.1)/(30mA) = 496 ohms.

However, if you use that with your 20mA load at 10V, then, even if the zener stops drawing any current (which it will do), your resistor will drop 9.93V! This only leaves you 0.07V. So, you aren't regulating.

However, if you use

R = (10 - 5.1)/(30mA) = 163.3 ohms

then at 20V, your zener will still be regulating (although the voltage across your load will be a bit higher, since the zener voltage will rise a bit with more current). It will suck up the excess current. If your load is using 20mA, then it'll take in the extra 70mA.

Thus, using Vin(min) in the forumla, your regulator will be regulating when input is between 10V and 20V.

One other parameter you need to complete your design of the zener regulator is the power rating of the zener. You need to ensure that the power rating of the resistor and zener are within their maximums. And you need to consider temperature effects on the zener. However, you didn't ask about that.

Usually, using a linear regulator that has temperature compensation and current limiting built in is a better bet. However, there are times when a shunt regulator is the only way to go.

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So in a worst case situation, if say the input voltage varied from 10 volts to 20 volts and the zener volts was 9 volts (with a load current of 20 milliamps), you would use the zener test current for the Iz(min)? I know, there is a better way to do this than with a zener diode, but for information sake I would like to know the right way to do this. Thanks,

Brian

diode. The

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Good morning, Brian. The minimum zener current is Iz(k), or the zener knee current. It is specified on zener data sheets.

For instance, my Fairchild data sheet for the 1N4740 (10V zener) lists an Iz(k) for the 1N4740 of 0.25mA. If I was using this zener, I would need real justification to go below 1mA minimum current -- zeners just don't work very well near the knee.

Good luck Chris

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The minimum zener current is that current which you want to still flow through the zener when a maximum load current is being taken. Zeners are pretty crap at low currents anyway and work better with at least

1/2 ma through them at all times. It's easiest just to make the minimum current about (say) 10% of the maximum load current. This way, the load can increase a bit without the zener voltage falling over. i.e. Resistor is = [Vin(max) - Vz] / [1.1 x IL(max)] regards john

i normally tried to have the Zener at 50% rated current while the load is on the circuit. there should be a Knee current spec'ed some where, this is the lowest suggested current to operate the diode in. I(k) or something like that. i think it depends on who makes the docs, i have also seen it referred to as valley current.

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Unfortunately, if you want to stay within the voltage tolerance
specified for the Zener in question, there's only _one_ current which
matters; the test current: Izt, and any deviation from that current
could cause the Zener voltage to go out of spec.

You are right about the formula being wrong. I meant to put Vin(min). After seeing all the different answers (which I really appreciate), I did some more digging into data sheets for zener diodes. I did find one data sheet for 5 watt zeners, which had curves for zener volts versus zener currents. On these curves, it showed that there wasn't a lot of change in zener voltage (for zener currents above 1 milliamps), on zener diodes from 6 volts and above. On zener diodes below 6 volts, zener current was very important. On zener diodes below 6 volts, using the test current shown was very important (which I found to be, the zener current required for it to dissipate 1/4 of its' wattage rating). Thanks,

Brian

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One better and more stable way is to set the zener using a current source since when Iz varies Vz also will vary a bit For the best current in a reference.... Usually I aim for a zener 5V1 @ 10mA since these are most stable if set this way

A Zener is a poor choice for this application. You've picked a set of numbers that are a challenge for most simple regulators. IF you can tolerate a minimum of 2V or so between min in and out, use a 3-terminal regulator. Or a more expensive LDO one that will run on 1V differential.

Soapbox alert...

You don't give clues to the application. There are some common issues ignored by newbies.

Zeners have a resistive term. Zeners have a temperature coefficient...sometimes significant. Zeners below 5.1V have a very soft knee, often. Input voltage can have ripple that puts the minimum trough voltage well below what you read on a meter. Sometimes you can reduce the min/max current ratio by using a PTC resistor. An incandescent light bulb makes an interesting PTC. Plot some V/I curves for 12V-28V light bulbs and see if that helps. mike

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First of all, most low voltage 1/2W zeners have a test current of 20ma. Above 10-12V the test current begins to decrease. The value is easily checked out in most catalogs or on line. Since you know the B+ use ohms law to select a resistor value that will allow the test current to flow through the zener. Whammo, you've got it. To verify the stability, test your circuit by varying the voltage across the desired range using a bench power supply. Remember the rule, if you double the voltage across the resistor you have selected, know that the current will also double, and the wattage will increase by four times. Useful rule of thumb to always be aware of.

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