Extension ntawm Capacitance Measurement Function ntawm Digital Multimeters
Raws li cov yam ntxwv ntawm qhov sib txawv thiab cov khoom siv hluav taws xob sib txawv, kev ntsuas capacitance tuaj yeem hloov mus rau qhov ntsuas hluav taws xob.
Cov tub ntxhais CX / V hloov dua siab tshiab txais yuav ib qho yooj yim active inverting RC sib txawv thiab ib qho kev teeb tsa. Wien choj oscillator tsim ib qho
AC siv lub teeb liab Vr ntawm qhov zaus, uas ua rau CX / V hloov dua siab tshiab thiab ua rau AC tso zis voltage V0(V1) proportional rau lub peev xwm CX. Tom qab lim tawm cov teeb liab cuam tshuam sab nraud ntawm qhov ruaj khov ntawm qhov thib ob-order band-pass lim, lub teeb liab dhau los ntawm AC / DC converter kom tau txais DC tso zis voltage V proportional rau CX.
Thaum lub teeb liab AC Vr tsav lub CX / V Circuit Court, cov zis voltage ntawm inverting integrator yog proportional rau qhov ntsuas capacitance CX. Qhov no paub qhov hloov dua siab tshiab ntawm capacitance CX mus rau qhov hluav taws xob.
Txhawm rau kom phim cov peev txheej yooj yim nrog 2 V voltage ntau ntawm digital multimeter, Wien choj oscillator yog teem rau 400 Hz nrog RMS voltage ntawm 1 V; thiab C1 =0.1 μF. Qhov tsis kam R2 yog hloov tau ntawm thiab , sib haum rau qhov ntsuas qhov ntsuas ntawm 20 μF, 2 μF, 200 nF, 20 nF thiab 2 nF feem.
2 Kev Ntsuas Me Me
Ordinary 3½ tus lej digital multimeters muaj qhov ntsuas qhov ntsuas ntawm 2000 pF txog 20 μF thiab tsis tuaj yeem ntsuas qhov ntsuas me me hauv qab 1 pF. Los ntawm kev siv txoj kev capacitive reactance nrog high-frequency excitation, ultra-me me capacitance tuaj yeem ntsuas. Kev ntsuas hluav taws xob tau pom nyob rau hauv daim duab 2, qhov twg CX yog qhov ntsuas qhov ntsuas thiab Rf yog cov lus tawm tswv yim ntawm qhov inverting input.
Thaum lub teeb liab sinusoidal Vi ntawm zaus f siv, capacitive reactance tshwm thoob plaws CX . Qhov nce ntawm kev ua haujlwm amplifier yog txiav txim siab raws li. Nrog kev ruaj khov nce thiab tawm tswv yim tsis kam, lub teeb liab zaus f yog inversely proportional rau qhov ntsuas capacitance CX . Yog li ntawd, high-frequency signals yog saws los ntsuas qhov tsawg kawg nkaus capacitances.
Daim duab thaiv kev ua haujlwm ntawm lub hauv paus ntsiab lus ntsuas yog qhia hauv daim duab 2(b). Thaum ntsuas: lub teeb liab sinusoidal siab uas tsim los ntawm oscillator yog siv rau CX; lub capacitance hloov mus rau hauv capacitive reactance XC ; C / ACV converter hloov XC rau hauv AC voltage, uas yog amplified thiab cais los ntawm ib tug transformer ua ntej xa mus rau ib theem-sensitive demodulator.
Qhov thib ob input ntawm lub demodulator yog ib lub xwmfab yoj tau los ntawm shaping lub high-frequency sine yoj los ntawm ib tug waveform converter; ob lub input signals qhia tib zaus thiab theem. Lub teeb liab demodulated dhau los ntawm lub lim dej qis los tsim DC voltage proportional rau CX, uas yog tom qab ntawd tso tawm ncaj qha rau ntawm DC voltmeter.
Lub waveform converter muaj ib tug inverting xoom-hla sib piv, hloov tus qauv 1 MHz sine yoj los ntawm Wien oscillator mus rau hauv ib tug ruaj khov inverted square yoj. Txij li thaum lub demodulator tso zis yog ib tug pulsating DC uas muaj high-frequency harmonics, ib tug π-type lim yog siv los suppress ripple Cheebtsam thiab xa ib tug du, khov kho DC voltage mus rau lub 'meter'.
Txhawm rau ua kom lub peev xwm sib txawv nrog lub multimeter's 2 V voltage ntau, lub teeb liab siab zaus tau teeb tsa rau 1 MHz (hloov ntau zaus yuav qhia cov kab mob parasitic) nrog RMS voltage ntawm 1 V. Cov khoom ntawm Circuit Court nce thiab tawm tswv yim tiv thaiv yog teeb tsa kom lub multimeter's 200 mV ponds DC 0 0 corres. sib raug rau 200 pF. Kev ntsuas tag nrho yog 10-4 pF rau 102 pF, nrog rau kev daws teeb meem ntawm 10-4 pF.
