The most direct method of calibration is to measure a reference capacitor whose value is about 0.7 ~LF. A precision capacitor will be very expensive, so if you have access to a precision (0. 1 % or better) capacitance bridge, measure the value of a good-quality Mylar capacitor on it. If the capacitor is used at approximately the same temperature as the bridge environment, it will be a suitable reference component.

The 0.7-RF capacitor will be used as a reference for both the nF and ~LF switch positions. Setting one point for each position is all that is required, as absolute linearity is provided by the project circuitry. The reference oscillator's mean output frequency is designed to be slightly high when only C18 and C19 are included in the circuit. If trimmer potentiorneters Rl and R3 cannot be adjusted to bring the display reading into

agreement with the value of the reference component, install C20 and/or C21. Calibration is now a matter of merely connecting the reference capacitor to the CX terminals, placing Sl in the RF

position, and adjusting R3 until the display matches the value of the reference component. Then, Sl should be placed in the nF position and RI adjusted for the same displayed capacitance.

Using the Meter. Apply power to the project by placing Sl in the nF position. Zero the display by slowly rotating the shaft of R10 counterclockwise until the display reads, ".001," advancing the control slightly more until a ".000" reading is obtained. Once zeroed, no further adjustments are necessary. The ~LF POsition does not require zeroing.

Connect the capacitor to be measured across the Cx terminals. Polarized capacitors must be oriented positive to positive, negative to "negative. Do not connect charged capacitors to the project. Although the input circuitry is protected with clamping diodes and a fuse, charged capacitors might damage the project. Capacitance is displayed in either nF or ~XF, depending on the setting of Sl. Values greater than 1000 nF should be read in the ~LF position.

Capacitance greater than 1000 VLF is determined by observing the overranger LED's to the left of the display. Because these Awo LED's cycle every 2/1 second, they are easily observed. If the top LED glows, 1000 VF iS indicated; if the bottom LED glows, 2000 ~LF; if both, 3000 VLF.

This sequence will then repeat, with two dark LED's representing 4000 VtF; the top LED glowing, 5000 ~LF; the bottom LED, 6000 ~tF; both on, 7000 ~&; both dark, 8000 liF; and so on until the cycling stops. Values up to several thousand microfarads can be measured. The upper limit is determined mainly by capacitor leakage, and to a lesser extent by your patience! Capacitors, with high leakage will never charge tO VREF, and thus will not trigger the discharge cycle.

When using the capacitance meter with S 1 in the nF position, treat the reading as if it were in picofarads if the deck mal point is to the left. That is, ".084" should be read as 84 pF, and ".003" as 3 pF. With a little experience, you will quickly become familiar with the autorange function and the behavior of the overrange LED's. 0