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Electronic Design

Simple, Low-Cost Capacitor Matcher Replaces Expensive Lab

Matching several capacitors, without needing a really precise value, can be accomplished with a "high-tech" laboratory capacitance meter. But this is cumbersome and overkill, and not everyone has one. This simple circuit matches, compares, and even measures (within limits) capacitors (see the figure). It's very low in cost and small (just 25 by 50 by 100 mm).

The matcher consists mainly of a classic multivibrator based on T2 and T3. Their base resistors are replaced by potentiometer (P) and a current-limiting resistor (R5). The unknown capacitor (CX) is the timing capacitor for T3. The reference capacitor (CREF) is the timing capacitor for T2. Transistors T1 and T4 square the collector signals of T2 and T3 and isolate them from the load represented by galvanometer G. Diodes D1 and D2, together with resistor R4, protect this zero-center galvanometer against excessive voltages and currents.

Alternately, the galvanometer re-ceives positive and negative currents. If both half periods are equal, the galvanometer will stay on zero. (Its inertia mechanically integrates the square wave.) If they're different, the galvanometer will deviate to either side, depending on which half period is longer. The half periods depend only on the ratio of CX to CREF, and the position of P. Placing a graduated scale on the potentiometer allows the determination of this ratio when the galvanometer is on "0." Thus, CX can be measured or compared to CREF.

Component values can be found on the schematic. Actually, they're not critical, and the circuit can almost be built entirely from scrap parts! Ideally, T2/T3 and T1/T4 should be identical—or even better, matched. But any asymmetry can be corrected on the potentiometer's scale. The following values depend on the particular galvanometer used:

  • R4 = (0.6 - VG)/IG, with VG and IG being the galvanometer's full-scale voltage and current. In my case, VG = 60 mV, IG = 0.1 mA. Therefore, R4 = (0.6 - 0.06)/0.0001 = 5.4 k‡ (5.6 k‡ was used).
  • R1 = R8 = VCC/IG. Here, 9/0.0001 = 90 k‡. I arbitrarily used 47 k‡. After all, the galvanometer is protected by R4.

Depending on the values of CREF and CX, resistors R3, R6, and P may require adjustments to obtain oscillation. CREF should be approximately the same value as CX. Instead of a single CREF, this capacitor could be selected from a batch of known values by means of a commutator.

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