## Zener Diode Question

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jrcfg
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### Zener Diode Question

This is a newbie question as I am still learning...if I use a 30V zener diode and the data sheet sez it's max regulator current is 30mA, does that mean I have to use a 1K resistor in series with it to keep the current from exceeding that 30mA? (Data sheet lists absolute max Iz as Pv/Vz = 1W/30V = .033333333mA) Thanks.

Externet
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### Re: Zener Diode Question

Hi.
Last time I checked, the current limiting resistor determination was still sort of empirical.
The resistor value has to consider the current consumption of the circuit that will work at 30V; the supply + voltage; and the resistor is not exactly in series to the zener, but in series to the supply. <p>S+-------R------ZK-------+30V Load<p>S+ = supply positive >30V
R = resistor
ZK = Zener cathode node<p>Miguel
- Abolish the deciBel ! -

rocket scientist
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### Re: Zener Diode Question

jrcfg:<p>No, you did the calculation for a 30 Volt drop across the resistor, which is not the case. <p>Diodes are confusing because they are nonlinear devices that do not follow Ohm's law. To a good approximation the voltage drop across a diode is constant, independent of the current through it. In your case, the voltage drop across the diode is 30 volts. The power disipated in the diode is P = 30*I (voltage times current), which can be no greater than 1 Watt according to the specs. Thus the current limit of .03Amp. <p>The series resistor has to keep the current in the circuit less than this. However, the voltage drop across the resitor is the supply voltage minus 30 volts (the diode drop), not 30 volts. You need a resistor that is at least R = V/I =(Vsupply - 30 volts)/.03 Amp

bridgen
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### Re: Zener Diode Question

Hi jrcfg. <p>Tell us the value of supply voltage which you intend to use.
It will then be easier to give numerical examples.

jrcfg
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### Re: Zener Diode Question

Thanks for the replies...actually, I want to use the 30V zener to protect a +5V regulator...the regulator has a max input of 30V. The regulator (and the circuit it supplies) are to be used in a vehicle (28V system) and I want the zener in place just in case the vehicle's voltage regulator fails high. So...I don't really have a good number for what the input voltage would be in this case. Over volt in a 28V vehicle would be around 30 or 32 volts (a guess).

Externet
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### Re: Zener Diode Question

Hi.
That is another story...
Suggestion; pre-regulate the ~28V source with a 7815 regulator and feed the 7805 with its 15V output.
That asumming those are the types of regulators to be used.
Alternative; a DC to DC converter module can work with less dissipation.
Miguel
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rocket scientist
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### Re: Zener Diode Question

Given that, you have to design for the worst case. 180 Ohms will allow for an input voltage of over just over 35 Volts while keeping the diode current below 30 mA. You may be able to use a much bigger resistor, but keep in mind your load ( the 5V regulator) will 'see' the resistor when the diode voltage drops below 30 Volts.

russlk
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### Re: Zener Diode Question

Peak voltage in a 12 volt vehicle can reach 60 volts due to starter transients. I expect that a 28 volt vehicle could see 150 volts. An L - C filter will protect against those transients.

rocket scientist
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### Re: Zener Diode Question

Yikes! Good point. Do you think an RC lowpass would work? Then he would just have to find a nice big eletrolyte capacitor.

rshayes
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