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PDC-1

CONVERT ANY AVERAGING WATTMETER TO READ PEP!

   THE UNIVERSAL PEAK DETECTOR CIRCUIT

        FOR ANALOG WATTMETERS IS HERE!

HI-RES COMMUNICATIONS, INC. has introduced the PDC-1 Peak Detector Circuit kit. This universal peak-hold circuit converts any averaging type wattmeter to a peak reading wattmeter with an adjustable hang time for the needle. Meter calibration and accuracy remain unchanged after the insertion of this circuit inside the wattmeter. The circuit has two simple adjustments, the first is the calibration and the second is the time constant (hang time). The needle's hang time can be adjusted from 1/10 sec. to 10 sec. Installation is a breeze! All that is required is a DPDT switch to switch the circuit in and out of the line from the wattmeter's circuitry to the meter itself and a power connection (6.3 VAC for the Collins 312B-4/5 OR 6-12 VDC for other meters).

The PDC-1 was designed by C. J. Hawley (KE9UW) for use in the Collins 312B-4/5 station consoles, but can be matched to any conventional averaging wattmeter. Even expensive, highly accurate meters like the Bird 43 can be fitted with this circuit without any loss of accuracy! Many of the PDC-1s have been installed successfully in Drake, MFJ, Daiwa, Ten Tec, Heathkit and Nye Viking wattmeters (among others!). The PDC-1 measures 2" X 1.5" and fits neatly inside most wattmeters without any modification.

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CIRCUIT DESCRIPTION

This peak reading circuit will quickly go to the peak power level and hold. The decay of the reading is determined by an adjustable time constant. A diode is connected in series with a current proportional to the power and is used to charge a storage capacitor. A very high impedance voltage follower circuit (opamp) is used to read the capacitor voltage. A variable resistor across the capacitor determines the decay time of the peak reading. This is similar to an AGC circuit with a fast attack and slow (adjustable) decay time.

Most wattmeters, including the one in the Collins 312B-4/5, consist of a directional power detector circuit which applies a current proportional to power through a resistor in series with a meter (or a couple of resistors, switchable, for selecting more than one scale). In our modification, we break the connection between the meter and range resistors. The peak circuit, with an input resistance equal to the meter resistance, is installed between the range resistors and the meter. The original calibration of the range resistors is therefore unchanged. The peak circuit may be switched in and out as desired for peak and average reading.

The voltage (on the order of 0.2 volts) across the input resistance of the peak circuit is amplified to take full advantage of the dynamic range of the opamps. The nonlinearity of the diode drop is eliminated by putting the diode inside the feedback loop of the amplifier. A resistance in series with the meter and the output of the peak circuit restores the full scale current to it's original value.

This design can be used with any metering circuit, with modification of parts values depending upon the meter resistance and full scale current through the meter.

SSB and AM operators will find the PDC-1 essential for accurate voice power readings. Utilize your favorite wattmeter to read voice peaks!

QRP enthusiasts find this circuit indispensable for PEP QRP readings! Converts low power wattmeters to actually show QRP power peaks!

The PDC-1 is available directly from HI-RES COMMUNICATIONS, INC. for $28.95 postpaid (USA & Canada), in easy to assemble kit form.

Please add $3.95 postage for OVERSEAS orders. Michigan residents add 6% sales tax.

Bird 43 users can optimize the PDC-1 circuit for the 30 microamp Bird meter movement by changing the values of 3 resistors. These 3 resistors are available from HI-RES for $1 additional to the PDC-1 kit price, just let us know you would like the Bird 43 Resistor Package when ordering.

These 3 resistors are not required to make the PDC-1 work with the Bird 43. There are hundreds of Bird 43s using the stock “universal” PDC-1 kit in use around the world, and they are all working fine with the included standard resistor values. These include Bird 43, serial number 1000 that is in our “back room.” It has one of the original kits in it from close to 20 years ago and it has been functioning perfectly ever since. It is pictured above with the kit mounted on the back of the meter movement. Documentation that comes with the PDC-1 kit also outlines 2 other modifications that may be of interest to Bird 43/PDC-1 users.

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