Zener Diode Question

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jrcfg
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Zener Diode Question

Post by jrcfg » Fri Jan 07, 2005 11:31 am

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.

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Externet
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Re: Zener Diode Question

Post by Externet » Fri Jan 07, 2005 12:22 pm

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
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Re: Zener Diode Question

Post by rocket scientist » Fri Jan 07, 2005 1:45 pm

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

Post by bridgen » Fri Jan 07, 2005 1:49 pm

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

Post by jrcfg » Fri Jan 07, 2005 2:25 pm

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).

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Externet
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Re: Zener Diode Question

Post by Externet » Fri Jan 07, 2005 4:33 pm

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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Re: Zener Diode Question

Post by rocket scientist » Fri Jan 07, 2005 4:51 pm

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.

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Re: Zener Diode Question

Post by russlk » Fri Jan 07, 2005 5:34 pm

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.

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Re: Zener Diode Question

Post by rocket scientist » Fri Jan 07, 2005 5:42 pm

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

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Re: Zener Diode Question

Post by rshayes » Fri Jan 07, 2005 7:02 pm

To limit the voltage on the regulator to 30 volts will probably require a 24 volt zener. The zener diode may be at the high end of its tolerance, which would be 26.4 volts for a 10% zener diode. During a surge or transient, the current will probably be higher than the test current at which the zener voltage was measured. This will increase the zener diode voltage. The temperature may also be higher, since the zener voltage was probably measured at 25 C. As the diode conducts, self heating will also increase the temperature. At 24 volts, the temperature coefficient of the diode is positive, so heating increases the clipping voltage.<p>Your 5 volt regulator will require about 8 volts on the input to function properly, unless it is a low dropout regulator. The 28 volt supply may be on the low side, possibly as low as 23 volts. A resistor in series with the regulator input can be used to absorb some of the voltage difference between 8 volts and 23 volts. The maximum value will depend on your maximum load current. For a 1 amp load, the resistor can be as large as 15 ohms. Adding this resistor also reduces the regulator dissipation.<p>When the input voltage is at the nominal 28 volts, the zener diode current would be .27 amps when the load current was zero. If the supply is 10% high, or 30.8 volts, and the zener is 10% low, at 21.6 volts, the zener current would be .61 amps and the zener would be dissipating about 13 watts. As the load current increases, the zener diode would dissipate less. The resistor would be dissipating about 9 watts.<p>During a surge, the supply voltage might go as high as 50 or 60 volts. This will only last for a few milliseconds, but the zener dissipation will be considerably higher during the surge, possibly around 55 watts. The resistor dissipation would be about 98 watts. A wirewoud resistor would be a good choice, since it could stand a momentary overload.<p>Transients might be several hundred volts, but they only last for a few microseconds, and are relatively easy to clip.<p>A capacitor from the regulator input to ground would serve several purposes. It will increase the stability of the regulator and give extra filtering of noise and transients. Something greater than 10 uF at 35 volts or higher would be a reasonable choice.<p>If the maximum load current is less than 1 amp, the resistor can be larger, and the zener and resistor dissipations made lower.

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Chris Smith
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Re: Zener Diode Question

Post by Chris Smith » Sat Jan 08, 2005 7:45 pm

The resistors in a zener circuit, is to limit the max current through the Zener to stop it from frying,...but and... while building a crow bar circuit to clamp the voltage, while you need other resistors of other values in the overall circuit to make it work.

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