A Zener Diode Negative Peak Limiter

by Steve Ickes, WB3HUZ 

 Would you like to have more "talk power"? Would you like to not have to worry about over modulation? Would you like to protect your modulation transformer? If you answered yes to any of these questions, then read on.

  The circuit described below is so versatile, it will give your rig more "talk power", keep you from over modulating, and provide protection for the modulation transformer - all at the same time! And best of all, the circuit is simple, easy to build and install.
 Figure 1 is a schematic of the circuit. It consists of a number of zener diodes in series with a silicon diode. This string of diodes is then placed across the secondary of the modulation transformer as shown in Figure 1. That's all there is to it. But, how does it work?
  The key to the circuit is the zener diodes. Zener diodes are a special type of silicon diodes that are designed to operate in the reverse conduction area (reverse biased). In this region, the voltage drop across the diode will remain relatively constant no matter what the current through it.
  Zeners are often used in voltage regulation circuits in a manner similar to the more familiar gas regulator tubes (OA2, OA3, etc.) Zener diodes are specified by voltage and power dissipation. To design a zener regulator for, say, a 5 volt supply, you would use a 5.1 volt zener. As long as the input voltage to the circuit was greater than 5.1 volts, the zener would keep the output at 5.1 volts.
  The power rating would depend on the amount of current the zener would be required carry in its regulation duties. Zeners are commonly available in one and five watt varieties and look like small silicon diodes. Higher power handing types are also available and are generally of the stud mount types.
  Now, back to our circuit. When the audio voltage on the secondary of the modulation transformer has the polarity as shown if Figure 3, the silicon diode is reverse biased and the zeners are effectively disconnected from the secondary. This condition exists during the positive cycle of the modulating signal.
  When the polarity is reversed, during the negative cycle of modulation, the silicon diode is forward biased and the zeners are reversed biased. The zeners now regulate or limit the negative voltage on the secondary of the modulation transformer to slightly less the the B+ value of the final. This regulating action means the B+ to the final is never cut-off, thus, eliminating over modulation on the negative cycle of the modulation.


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  To design a zener limiter for you rig, determine the B+ voltage on the final amplifier. If your rig has an appreciable drop in the B+ under high levels of modulation, determine the lowest value to which B+ voltage drops. The total zener voltage of the zener diode string (the sum of zener voltage of each diode in the string), should equal about 50 volts less than the B+ voltage previously determined. The total power dissipation of the string (the sum of dissipation of each diode in the string), should equal about half the power capability of the modulators.
  Now, the trick is to get the zener string to meet the two conditions above. Since zener diode are commonly available in voltages from a few volts to about about 200 volts, the number used will depend mostly on the power dissipation requirement. For rigs up to about 100 watts output(B+ of 1000 volts or less), 5 watt zeners are the most economical. This type of limiter is not recommended for higher power rigs. Instead, use the Three Diode Negative Peak Limiter.
  Although the zener regulation action will clip the negative audio peak near 100% negative peak modulation, the resulting distortion is minimal. Remember, at the instant the clipping occurs, the voltage on the final tube(s) is only about 5% the normal B+ voltage. This also means the instantaneous final plate current is also only about 5% of normal (or what you read on the transmitters meter which reads the average value). Knowing power is the product of voltage and current (P = EI), then the input power to the final at the instant the negative peak clipping occurs is only 0.25% normal. Thus, the distortion products created by the clipping will be no worse the 0.25% of the input power or unnoticeable. For example, if you are running 100 watts average input, the power input at the instant the negative peak clipping occurs is just 0.25 watts!
  With the circuit in place, the negative peak modulation is absolutely limited to slightly less than 100%. If your rig has sufficient modulator power, you can crank up the audio gain some, achieve higher levels of positive peak modulation, and never produce splatter. Remember, splatter is produced by modulation in excess of 100% on the negative peaks, not the positive peaks. Modulating over 100% on positive peaks will make you audio sound louder at the receive end or give you more "talk power".
  The circuit also provides protection for the modulation transformer. A big killer of modulation transformers is large transients created by modulation in excess of 100% on negative peaks. Since the zener circuit prevents negative peak modulation from exceeding 100%, no damaging transients are produced. Long live your modulation transformer!


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13 January 1998