# Zener diode wattage

Discussion in 'General Electronics Chat' started by Scarr, Feb 22, 2014.

1. ### ScarrMember

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Hi, I have been trying to calculate the wattage for a Zener / Resistor that clamps a input with these parameters

Peak voltage 30v DC (this is what it can get to sometimes)
Output voltage 5v (this is max what I want to on this input)
Output current 30mA (this is the maximum current sourced on this line)

This is to protect a input line.

I suspect because of my lack of knowledge I am getting some large values!

2. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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You don't mention the value of the resistor, nor supplied a diagram of how you've wired it?.

3. ### Miles ProwerMember

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Given these values, your worst case scenario would be:

R(ballast)= (30 - 5)/30E-3= 833R (860R design nominal)

Pr= (30E-3)^2 X 860= 774mW (select 1.0W resistor here)

Pz= 5(30E-3)= 150mW (select a 0.5W Zener here)

Done.

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5. ### ScarrMember

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Miles, your a star thanks.

6. ### crutschowWell-Known MemberMost Helpful Member

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Miles' values are okay if the input is always 30V. What is the minimum input voltage?

7. ### ScarrMember

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Crutschow, this is what confused me when using the online calculators, it said "max input" and "min input", well I want the line to be as close to 5v at all times

Last edited: May 24, 2015
8. ### kubeekWell-Known Member

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Input min/max, not output. From what you say it seems you want the to have 5V output with anything between 5V and 30V at the input, am I right?

9. ### ScarrMember

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OK let me explain what this "line" does, its a 5v TTL RS232 hope this helps you help me

Steve

Last edited: May 24, 2015
10. ### crutschowWell-Known MemberMost Helpful Member

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You will have to place a resistor in series with the line to avoid zapping the zener. The question is, how much voltage drop can you tolerate for the 30mA current. For example if it were 0.5V, then you would use a 17Ω resistor. For that the maximum power dissipated in the 5V zener would be (30v-5v)/17 * 5v = 7.4W so you would need 10W zener. The series resistor would be dissipating 37W. For a momentary short you could probably get by with a 5W zener and a 10W 17Ω resistor.

11. ### ronvWell-Known MemberMost Helpful Member

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I guess I read what you are trying to do a little different. Sounds to me like you are trying to protect the input of you MCU from the 30 volts?
If so, and the MCU has Cmos inputs it won't need much current so you could probably use say a 3.3k in series with the probe input followed by a 5 volt zener to ground. Then everything would get small.

12. ### ScarrMember

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Ronv, yes its to protect a input to a MCU pin.

Last edited: May 24, 2015
13. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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Have you checked the spec on your MCU?, PIC's for example need nothing more than a series resistor as they have internal protection diodes on almost all pins.

14. ### ScarrMember

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Nigel, Its an ATXemga1284AU, I have looked but cannot see it, but maybe I can't see wood for trees any more, anyway would I not need the level shifter for the 5v RS232?

15. ### MrAlWell-Known MemberMost Helpful Member

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Hi,

30ma output current and 1ma zener current means we have a total of 31ma or 0.031 amps. We have to drop 25 volts with 0.031 amps. Ohm's Law says that means we need a resistor of 806 Ohms. If we use a resistor of 800 Ohms we get just a little over 31ma which works ok. The power is about 0.78 watts, so doubling that we get about 1.5 watts, so a 1.5 watt or better resistor is required for good operation.

Note we left 1ma extra for the zener even when the output circuit draws it's maximum current of 30ma, but we may wish to leave more like 5ma for the zener. Then the required resistor is 710 Ohms at 1.8 watts

In both cases the zener can be a 1/2 watt device.

16. ### ScarrMember

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Thanks MrAl but I think we may have passed using a zener at all, from what i can see the level shifter (used to interface between the 3.3v MCU and 5v RS232) will give me the protection I seek but I hope to get confirmation of this from Nigel or anyone else?

Steve

17. ### ScarrMember

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Can anyone confirm that the level shifter will protect against 30v?

Steve

18. ### MrAlWell-Known MemberMost Helpful Member

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Hi,

That part number shows a NPN/PNP transistor array. For that you would need some resistors too. But why complicate matters?
What is wrong with the zener and resistor solution?

What is your input signal level, and i assume your output has to be a 0v to 5v logic signal.

19. ### ScarrMember

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MrAl, Firstly the resistors need it for the level shifting anyway (5v RS232 - 3.3v MCU), so I thought it a good solution? it says "input fwd voltage of 60v" and a "Max DC collector current of 100mA" what would happen

Thanks for all your help so far.

Steve

Last edited: May 24, 2015
20. ### MrAlWell-Known MemberMost Helpful Member

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Hi,

Maybe you could tell me exactly what you are trying to do and i could suggest a simple method.

Are you trying to convert a signal that goes from 0 to 30v to one that goes from 0 to 5v or 0 to 3.3v or something like that?

21. ### ScarrMember

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Sorry, I though I had, OK my project is a RS232 circuit

Thanks

Steve

Last edited: May 24, 2015