How to choose an LCR bridge and knowledge of resistors, capacitors, inductors

Resistors, capacitors, inductors are the most commonly used by design engineers passive components, but not many really understand the LCR, you would like to take this to the design, maintenance engineers and other technology of LCR.

An ideal L, C, R:

In other words, the ideal state, the phase angle (θ) under the pure resistance is 0O, in the pure capacitance is-90O, pure inductance was +90 O.

But that's the ideal, the actual L, C, R is not so simple, especially for L, C is concerned. Consider the following analysis.

Second, the low frequency of the L, C equivalent circuit:

Because of the presence of R, so inductance is no longer +90 O, capacitor will not be-90O, the resulting phase angle (θ), to look at trigonometric functions:

Imaginary part of which the inductance or capacitance, while the real part of impedance, because the inductance and capacitance related to frequency, so in a different operating frequency will be different (θ) value.

Understand inductance, capacitance in smaller if the elements of RS (or higher RP), this component more close to the ideal, where we define the quality factor (Q) and Dissipation Factor (D):

Q = 1 / D = the virtual work / real power

In series equivalent circuit point of view, if the constant current I flows

Therefore, if RS = 0, then Q becomes infinite, relative value D = 0.

Parallel equivalent circuit is also so on, so you can get a conclusion:

In terms of the passive components, Q the bigger the better, D value as small as possible.

Third, the general application of capacitance C:

The most common is the electricity coming from (AC) to become DC. Circuit as follows:

Electricity (AC220V) by the transformer, then by the bridge rectifier, then that is a pulsating DC (Do not consider the electrolytic capacitor C).

With electrolytic capacitor C, you can become a direct current, as shown below:

But, after all, C is not ideal capacitor in series equivalent circuit point of view, and with the load RL, the the circuit as shown below:

ES shall ESR (Equivalent Series Resistance, ESR)

When the capacitor charge to the peak time, that will load RL discharge (discharge current IL), this time in the RS on that produces a voltage drop (or power consumption), the waveform becomes:

Vripple (ripple) Thus, the original DC voltage will be superimposed on top of a communication signal, the so-called ripple, and this ripple with the size of the load than with the RL concerned, but also with the size of the ESR If the ESR is too large the ripple becomes larger, the same, consume too much energy, which is in the actual circuit, sometimes touching the electrolytic capacitors will cause fever, if the ESR is too large, then the electrolytic capacitor hot, The danger of explosion! !

Therefore, the proposed design engineers to use LOW ESR capacitors. In other words, at this time ESR measurements become indispensable to engineers design the measurement factor.

The use of high-frequency circuit, usually with the formation of filter inductance to suppress noise on the line or on the sudden surge (Spike) for circuit protection. At this point C the choice of work and its frequency, so in the choice of capacitance measurements, measurements need to be aware of multi-frequency content, usually a small capacitor in the high-frequency lines are mostly employed, so the measurements should pay attention to the choice of frequency.

Another application, described by the following formula:

That is, when the C fixed, the use of constant current source charging the capacitor C can generate ramp, of course, still need to pay attention to C quality. (Q value the bigger the better, or D value as small as possible.) This capacitor is widely used in the A / D converter of the dual slope integration circuit.

Fourth, the general application of inductor L

Inductor is a coil, regardless of this group of coils around the iron core are not on, as is the use of wire wound into a coil, there will be resistance online presence. Is still the series equivalent circuit description:

Into the current I, if the RS too, then consume too much energy, which is why the transformer causes fever. Therefore, RS has become necessary for measurement of a program. But imitation mostly bridge between the current bridge, do not have the DCR function, it can not be measured RS value.

Description as shown below:

ZL = RS + jWLS

The measured value is out of the ZL contains jWLS rather than a RS, then if you can make f = 0 value measured Rs, how to determine the inductance of the good or bad, of course, be as small as possible RS.

Inductor applications, also used ZL = jWL = j2πfL this formula. If f is larger, the higher the inductance can be high-frequency noise (Noise) or transient surge (Spike) to be inhibited.

Integrated L, C, common to the general diagram shown below: This is the reason.

5, measured L, C easy to fall into the error when

a. How to choose parallel mode (Lp, Cp) or tandem mode (Ls, Cs).

Usually in the parallel mode (Lp, Cp) measured the time constant is used, and in tandem mode (Ls, Cs) is measured using constant current mode. (Involving circuit design, this will not go into the details one by one.) Capacitor, generally aimed at small and large inductor is used in parallel mode; large capacitors and small inductors are used in series mode measurements, which are held with the D value of the difference in the conversion formula as follows:

By the above equation, under ideal conditions, D value = 0, Lp = Ls Cp = Cs

b. Why the different frequencies, the measured inductance, capacitance will be different?

Like any components like amplifiers will have a frequency response problem, in other words, if the error of measuring instruments excluded, then measured at a frequency point out the value of the means that the device in the real value of this frequency. In other words, if the engineers want to measure the value of a particular device, it must consider the device in the circuit operating frequency how much, and select Gai frequency or close to measure the frequency would get the the components in the circuit real value. From the practical side to consider, can be attributed to the following conclusions for users.

Small capacitors and small inductors commonly used in high-frequency circuit (measurement frequency should be higher) large capacitors, inductors commonly used in large low-frequency circuit (such as electricity by the full-wave rectified 50Hz 100Hz, the measured frequency selectable at 100HZ)

c. How to choose measuring range?

Measurement range of choices, such as used in the measurement with the iron core of coil (inductance), the core material involving, so the choice of frequency points, is required to select the appropriate range, to be measured.

d. zero correction

When the test line more shall the existence of small capacitors and small inductors to be deducted in order to measure the true value of the device itself. In general, the capacitor is open to zero, the short circuit inductance zero, range of instruments for manual must also follow the small capacitor (CP mode) open-zero, a small inductance (LS mode) the principle of zero short-circuit.

VI Conclusion

In summary, the choice of LCR table, can be based on the use of conditions to determine the user to choose a general type (with LCR and D measurement function) bridge, a good LCR meter is very obvious, that is, In addition to wide measurement range, fast response, high accuracy, the need to have the frequency selection, range selection, and D value (Q value), DCR, θ value, ESR value measurement and other functions, to take full advantage of passive devices Some characteristics of the design circuit, or hope to be able to purchase a professionally measured parameters of passive components such as the DCR, Q, ESR De LCR bridge, addition usually capacitors, inductors smaller, the frequency of its work more high measurement frequency should be higher in order to accurately measure the actual value of the source device. While measuring voltage option is also very important function of size.