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 Post subject: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 1:25 am 
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I have a power supply that outputs 300vdc, and I would like to use some zeners diodes to drop the output to 250 @ .06ma. I can use two zener diodes one at 100v, and one at 150v for 250v So my question is what's the best way to go about calculating the current limiting resistor for two different voltage zeners in series? I know how to do it for one diode or two at the same voltage in series, but not quite sure how for two or more with dissimilar voltages.

Dave


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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 1:29 am 
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Hi Dave,

You should use diodes that require the same current through them.

Earlier series of Zeners required different current for each different voltage.
There are modern series that use the same current for all voltages.

- Leigh

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 2:58 am 
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Leigh, Just out of curiosity, could you use two zeners with different voltage and current ratings as long as you used a resistor calculated to limit the current to the lowest diode current rating?


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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 3:00 am 
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madlabs wrote:
Leigh, Just out of curiosity, could you use two zeners with different voltage and current ratings as long as you used a resistor calculated to limit the current to the lowest diode current rating?


That's what I was thinking too.


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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 3:12 am 
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Zeners are spec'd for both minimum and maximum current.

If you use devices with different voltage ratings in series your idle current must fall within that range for BOTH devices. That may significantly reduce the range of input voltage or load current over which the combination will regulate properly. It's not impossible, just picky.

- Leigh

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 3:18 am 
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First off, those are not **ratings**. The Zener voltage is typically specified at a stated current---making both a parameter. Normally, the only rating to be concerned with is power.

So--what hair is being split here??
--A parameter specifies something that is related to the function in a circuit.
--A rating relates to the part surviving.

If a Zener diode is specified with a 10-volt drop at 1mA, that in no way stops you from using it at 0.5mA, or even 2 mA---assuming that you do not exceed the power rating.

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 4:01 am 
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Thanks Leigh and Pixel, good to know.


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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 6:27 am 
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I have used this calculator a couple of times
http://www.calculatoredge.com/electronics/zener.htm


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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 10:17 am 
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pixellany wrote:
If a Zener diode is specified with a 10-volt drop at 1mA, that in no way stops you from using it at 0.5mA, or even 2 mA---assuming that you do not exceed the power rating.
As I said before...

Zeners have both minimum and maximum current specifications.

That defines the range over which the Zener will hold voltage within tolerance.

- Leigh

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 11:26 am 
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If a zener is to function properly, as Leigh stated, it must be used in the Iz-min to Iz-max region of it's characteristic, of course, within it's power rating.


Last edited by Scott on Apr Tue 23, 2019 12:32 pm, edited 1 time in total.

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 12:32 pm 
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Leigh wrote:
pixellany wrote:
If a Zener diode is specified with a 10-volt drop at 1mA, that in no way stops you from using it at 0.5mA, or even 2 mA---assuming that you do not exceed the power rating.
As I said before...

Zeners have both minimum and maximum current specifications.

That defines the range over which the Zener will hold voltage within tolerance.

- Leigh

Understood---the hairsplitting was simply that you can't hurt anything by going outside this range--unless you exceed the power rating.
There is peace in the valley.....:)

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 2:29 pm 
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pixellany wrote:
Understood--the hairsplitting was simply that you can't hurt anything by going outside this range
If you go outside the stated range the regulation will not meet spec.

- Leigh

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 7:24 pm 
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Leigh wrote:
pixellany wrote:
Understood--the hairsplitting was simply that you can't hurt anything by going outside this range
If you go outside the stated range the regulation will not meet spec.

- Leigh

we have no disagreement.......

furthermore, for many zener applications the voltage does not HAVE to meet spec.....

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 10:08 pm 
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So back to the original question it sounds like its best to stick with diodes of the same voltage, instead of mixing different diode voltages in a series type configuration.


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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Tue 23, 2019 10:14 pm 
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Yeti575 wrote:
So back to the original question it sounds like its best to stick with diodes of the same voltage, instead of mixing different diode voltages in a series type configuration.

I would not have come to that conclusion...

To the extent that you are concerned about the current required for the voltage to be in spec, what would count is that you use zeners speced at the same current

but the easiest thing is to just get one zener that has the right specs for the application.....

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Wed 24, 2019 4:19 am 
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First it's been my experience that Zeners are spec'd. at a test current (IZT) such that the power Vz * IZT is roughly equal to 1/4 of the power rating of the Zener and the Zener voltage is spec'd. at that test current. For example, the 1N5378B is a "100v/5watt" part. The "B" in the part number indicates a +/-5% tolerance on the voltage, ie. 95/100/105 volts at a test current of 12mA. Multiplying 100v times 12 mA gives 1.2 watts which is close to 5watts/4.

The 1N5267B is a "75v/500mW" part, again with a 5% voltage tolerance (71.25/75/78.75 volts) at IZT = 1.7mA. That's nominal 127.5mW, again, about a quarter of the 500mW power rating.

Several questions must be answered to enable specifying the Zener series resistor:

1. What is the input voltage to the Zener circuit? (300v, in the OP's case.)
2. What is the desired regulated voltage? (250v.)
3. What is the load current? (60mA.)
4. What is the variation in the load current? (Not given.)

The last will ultimately determine the resistance required and lead to the required power dissipation of the Zener, the range of Zener currents and an estimate of the voltage variation in the desired regulated voltage.

John

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Wed 24, 2019 9:49 am 
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Yeti575 wrote:
So back to the original question it sounds like its best to stick with diodes of the same voltage, instead of mixing different diode voltages in a series type configuration.

If you understand what you are doing then you can have any mix of Zener diode voltages in series you want. The same considerations apply whether you use a single Zeners, two identical Zener or two totally different Zeners. In either case, you have to consider for each diode, the minimum current through the Zener under full load and the maximum current and power dissipation under minimum load, and what is most important is the Zener diode power dissipation when you have NO load at all. Even if your load maybe constant under normal condition, depend on your circuit, at certain times there may be very little load or no load at all. The above applies whether you have a single Zener diode, two identical Zener or totally different Zener voltage diodes.

Lets take a look at the no load condition and the Zener diodes power dissipation. If your load is 60mA and let say we will set the minimum current through the Zeners to 10mA (the actual minimum current will depend on the Zener) for a total of 70mA. So, the total Zener diodes dissipation at no load will be 250V x 0.07A = 17.5W. In your case, using 100V and 150V Zeners, their power dissipation will be 7W and 10.5W respectively. In your example, the only reason I will try to use two identical Zeners is to equal the power dissipation in each Zener and then you can use two 10W Zeners. In either case, you will need (not cheap) very beefy Zener diodes (DO4 case) and fairly large heatsink.

As far as the current limiting resistor calculations, it is exactly the same whether you use a single Zener diode or multiple Zeners in series regardless whether they are identical or totally different Zener diodes.


Last edited by upsss on Apr Wed 24, 2019 6:59 pm, edited 2 times in total.

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Wed 24, 2019 6:30 pm 
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upsss wrote:
Yeti575 wrote:
… Lets take a look at the no load condition and the Zener diodes power dissipation. If your load is 60mA and let say we will set the minimum current through the Zeners to 10mA (the actual minimum current will depend on the Zener) for a total of 70mA. So, the total Zener diodes dissipation at no load will be 250V x 0.7A = 17.5W. In your case, using 100V and 150V Zeners, their power dissipation will be 7W and 10.5W respectively. In your example, the only reason I will try to use two identical Zeners is to equal the power dissipation in each Zener and then you can use two 10W Zeners. I either case, you will need (not cheap) very beefy Zener diodes (DO4 case) and fairly large heatsink.

As far as the current limiting resistor calculations, it is exactly the same whether you use a single Zener diode or multiple Zeners in series regardless whether they are identical or totally different Zener diodes.

You slipped a decimal point. It should be 250v x 0.07A = 17.5W. Big Zeners, indeed!
John

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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Wed 24, 2019 6:34 pm 
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Sorry, you are right, I fixed it.


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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Thu 25, 2019 4:38 pm 
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Most commercial zeners are specified at a single test current value, and may meet tolerance only at that current. However, they do "work" over a wide range of currents. Usually the data sheets give "typical" curves, but no hard specifications for operation at other currents.

Some manufacturer's parts are much "better" than other's, even for the same type number.

The OP said his load is ".06mA", or 60uA, not 60mA. Quite a difference. At this low load current, the zener current can be set quite low to keep heating to a minimum.

Probably the two zeners on hand will work well enough, even operated quite far from their specified test currents. A test will reveal if these particular zeners work well enough for the application.

The required regulation has not been specified. If really good regulation and stability are required, zeners are probably not the best solution.

To answer the original question, a rule of thumb might be to assume a zener current of 5X to 10X the load current, then check the resulting power dissipation for each zener. If the power will be more than 1/4 to 1/3 the zener's power rating, you can reduce the current to reduce the power, but the regulation will not be as good.

A consideration is how the zeners are mounted. Their power rating is with a good heat sink. For parts with leads, this means short leads, each connected to a large sink.

Ted


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