這是一個(gè)比較復(fù)雜的電路．但當(dāng)你做出來(lái)以后。你就有了一個(gè)可以用來(lái)測(cè)量收音機(jī)中最大容量電容的測(cè)試儀。電容不像電阻把阻值標(biāo)在外殼上，很多電容的容量都沒(méi)有在外殼上標(biāo)出。還有一些舊電容的容量在使用過(guò)程中被擦掉了。此儀表可以測(cè)量出這些電容的容量，對(duì)于電氣技術(shù)人員、老牌收音機(jī)發(fā)燒友和無(wú)線電愛(ài)好者都是好幫手。

常見(jiàn)的555定時(shí)器是此電路的核心部件．它的作用是將被測(cè)量電容(Cx)充電到固定電壓，然后被測(cè)電容通過(guò)回路放電，電流表測(cè)量出流過(guò)47Ω電阻的電流。因?yàn)?55定時(shí)器每秒鐘重復(fù)這個(gè)過(guò)程幾次。使得電流表的指針保持穩(wěn)定。

電流會(huì)因電容容量的大小而不同．即電流表指針會(huì)根據(jù)被測(cè)電容的容量作相應(yīng)的偏轉(zhuǎn)，也就是說(shuō)，電容容量與線路電流的比例是線性的。就像用于測(cè)量電壓和電流的萬(wàn)用表。 ’

此表有五個(gè)擋位，從100pF到1μF，通過(guò)一只兩極五擋開(kāi)關(guān)切換，另外，開(kāi)關(guān)x10用于測(cè)量較大容量的電容，電流分成兩路，測(cè)量100pF、1000pF、0.01μF、0.1μF或1μF以上電容顯示更準(zhǔn)確。

元器件的精確度不是十分完善的。即使是最好的器件，一般九只電阻也考慮有2％的誤差。如果沒(méi)有OA47二極管，可用OA91或OA95鍺二極管代替。將此電路裝入一只塑料盒中．與萬(wàn)用表差不多大。但是稍深一點(diǎn)更好。在財(cái)力許可下，測(cè)試儀的測(cè)量?jī)x表要盡可能大，因?yàn)樗鼪Q定了測(cè)量顯示的精確度。你購(gòu)買(mǎi)的電流表量程為0～50微安，但是刻度要從0到100偏轉(zhuǎn)(即原有的刻度10、20、30、40、50用20、40、60、80、100代替)，你可以用白色修正液或小紙片修改。

將元器件安裝在一塊印刷電路板上，555集成塊用一只插座連接，以備在需要時(shí)方便替換。連接線路要盡可能短，否則線路的雜散電容會(huì)降低測(cè)量的精度。 ’

可以用已知容量的電容器來(lái)校定測(cè)試儀，可找一只已知容量的電容，試測(cè)幾次。如果顯示不正確。很簡(jiǎn)單，只要買(mǎi)幾只相同容量的電容，在其中選一只最接近標(biāo)稱(chēng)容量的用于校正就行了。再用幾只最接近標(biāo)稱(chēng)容量的電容來(lái)準(zhǔn)確校正各個(gè)擋位。

校正時(shí)，將x10斷開(kāi)(即兩只開(kāi)關(guān)都斷開(kāi)不用)，將一只標(biāo)稱(chēng)電容接入回路。將開(kāi)關(guān)打到合適的量程。調(diào)節(jié)47kΩ微調(diào)電阻，直到準(zhǔn)確讀出電容的容量。然后合上開(kāi)關(guān)，此時(shí)讀數(shù)會(huì)有所變化，調(diào)節(jié)10kΩ微調(diào)電阻，使得兩次的讀數(shù)相等，例如，如果你測(cè)量的是一只0.01μF標(biāo)稱(chēng)電容，在0.1μF檔上的讀數(shù)是10，此時(shí)的讀數(shù)就該是20。

如果不行。就換一只與相應(yīng)擋位容量相等的電容(如1000pF、0.01μF、0.1μF等)。調(diào)節(jié)擋位(即1000pF、0.01μF、0.1μF等)，使顯示的數(shù)字為滿量程(100)。合上開(kāi)關(guān)x1O，指針的讀數(shù)會(huì)明顯下降，調(diào)節(jié)微調(diào)電阻470kΩ，使指針的讀數(shù)是10。再下調(diào)一個(gè)擋位(如從0.01μF調(diào)到1000pF)，指針的讀數(shù)應(yīng)該是100。如果不是100，就調(diào)節(jié)470Ω微調(diào)電阻，直到是100為止。此時(shí)電容測(cè)試儀的校正就完成了，你可以試著測(cè)量其他一些電容的容量來(lái)驗(yàn)證校正的精確度。

細(xì)心測(cè)量，多數(shù)量程的精度可達(dá)5％。

開(kāi)關(guān)x10用于測(cè)量10μF及以上大容量的電容；開(kāi)關(guān)×0.5使在測(cè)量較小容量的電容時(shí)，讀數(shù)更準(zhǔn)確。

Capacitance Meter

This project is complex，However，when finished，you will have an instrument capable of measuring all but the largest capacitors used in radio circuits．Unlike variable resistors．most variable capacitors are not marked with their values．As well．the markings of capacitors from salvaged equipment often rub off．By being able to measure these unmarked components，this project will prove useful to the constructor．vintage radio enthusiast or．a(chǎn)ntenna experimenter．

The common 555 timer IC forms the heart of the circuit(Figure Three)．Its function is to charge the unknown capacitor(Cx)to a fixed voltage．The capacitor is then discharged into the meter circuit．The meter the current being drawn through the 47 ohm resistor．The 555 repeats the process several times a second．SO that the meter needle remains steady．

The deflection on the meter is directly proportional to the value of the unknown capacitor．This means that the scale is linear．1ike the voltage and current ranges on an analogue multi-meter．

The meter has five．ranges，from l00pF to luF，selected by a，five position two pole switch．In addition，there is a x10 switch for measuring higher values and a divide——by·—two facility to allow a better indication on the meter where the capacitor being measured is just above 100，1000pF，0.01，0.1 or 1uF．

Component values are critical．For best accuracy，it is desirable that the nine resistors wired to the Range switch have a tolerance．If 0A47 diodes are not available ．try OA91 or OA95 germanium diodes instead．Construct the meter in a plastic box；one that is about the size of your multi-meter but deeper is ideal． The meter movement should as large as your budget allows you will be using it to indicate exact values．The meter you buy will have a scale of 0 to 50 microamperes ．This scale needs to he converted to read 0 to 100 (ie 20，40，60,80，100 instead of 10，20，30，40，50)．use of white correction fluid or small pieces of paper will help here．

The components can be mounted on a piece of printed circuit board．Use a socket for the IC should replacement ever be needed．Keep wires short to minimize stray capacitance：stray capacitance reduces accuracy．

Calibrating the completed meter can be done in conjunction with a ready—built capacitance meter．Failing this，a selection of capacitors of known value，as measured on a laboratory meter could be used．If neither of these options are available，simply buy several capacitors of the same value and use the one which is nearest the average as your standard reference．Use several standards to verify accuracy on all ranges．

To calibrate，disable both the xl0 and divide—by—two functions (ie both switches open)．Then connect one of your reference capacitors and switch to an appropriate range．Vary the setting of the 47k trimpot。until the meter is reading the exact value of the capacitor, Then switch in the divide—by—two function．This should change the reading on the meter．Adjust the 1Ok trimpot so that the needle shows exactly twice the original reading．For example，if you used a 0.01 uF reference，and the meter read 10 on the 0.1 uF range．it should now read 20.Now switch out the divide—by-two function．

If you are not doing so already，change to a reference with a value equal to one of the ranges(eg 1000pF，0.0luF，0.1uF etc)．Switch to the range equal to that value(ie the meter reads full—scale(100)when that capacitor is being measured．Switching in the x10 function should cause the meter indication to drop significantly．Adjust the 470 ohm trimpot so that the meter reads 10.Move down one range(eg from 0.01uF to 1000pF)．The meter should read 100 again．If it does not．vary the 470 ohm trimpot until it does．That completes the calibration of the capacitance meter．Now try measuring other components to confirm that the measurements are reasonable.

With care．a(chǎn)n accuracy of five percent or better should be possible on most ranges．

Use X10 switch to measure up 10uF. Use X0.5 switch for better readings on low values．