To: Professor Lord and EE222 Class

From: Doug Hoffman and Stephen Pietrucha

Subject: Nifty Diode Circuit

October 23,1995

Nifty Diode Circuit #5

(Bill Glenn Memorial Limiter)

FIGURE 1: Schematic of Limiter Circuit

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Our circuit consisted of a 1K resistor and two zener diodes in series with an AC voltage source. The diodes were oriented in opposite directions so that they were joined cathode to cathode. Voltage was measured across the two diodes.

We found that our circuit simply acted as a limiter. The output was the same as the input as long as the voltage was not greater than the Nominal Zener Voltage (NZV) of the zener diode. This is apparent on our I-V curve because the curve is linear with a slope of 1. When the applied voltage exceeded the NZV, the output voltage remained at the NZV. This is why we call our circuit a limiter. The output voltage will not exceed the NZV of the diode.

Because the zener diodes have different NZV's we had to find the diode that would work at our input voltage of four volts. To do this we looked at our data sheet on zener diodes. We found that the 1N4736A diode had a NZV of 6.8V. We placed these diod e in our circuit and found that the output voltage was the same as the input voltage from +6.8 to -6.8 volts. This was good for our 4V input. We also tried the 1N4728A diode which has a NZV of only 3.3V. Because the 4V applied voltage exceeded the NZV, this of course did not work. We also tried switching the polarities of both the diodes but this had no effect.

This circuit could be very useful when building other circuits. For example, it could be used like a circuit breaker. We have all toasted op-amps and other components by applying to much voltage to them. By first hooking this applied voltage to the li miting circuit and then the circuit to the op-amp we could stop doing this. By using different zener diodes in the circuit we can change the range of voltages that we can use. By using two different diodes in the circuit we can adjust the range even fur ther.

PLOT 1: I-V curve of the limiter

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From the I-V curve we can observe that the output voltage equals the input voltage until it reaches the nominal zener voltage.

PLOT 2: Comparison of input and output voltages.

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From the plot we can observe both the input and output voltages to be the same when operating below the nominal zener voltage.

PLOT 3: Another comparison of input and output voltages.

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From this plot we can see that "clipping" occurs on the output voltage when the input voltage exceeds the nominal zener voltage. This is the characteristic of a limiter.


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