Test the resistor with a multimeter before reading the instructions, much will become clear. Task simplicity surpasses another class of radio. Traditionally use the resistance measurement mode, or - test diodes. Both are marked on the front panel, respectively, with the Greek omega( Ω) and the symbol taken from the electrical circuits( bold arrow with a cross bar touching the tip).Each of these modes is convenient. If we take, for example, Atlantic multimeters, which filled the counters, then there are no differences in them. Both in the dial tone mode( diode), and when measuring the resistance, the nominal resistance appears on the screen.
Useful for checking resistors modes of the multimeter
Beginners consider: it makes no sense to measure the resistance of the conductor during a dial, it is easier to fix an open circuit, a short circuit. A trivial question, we will answer: a matter of taste or convenience of a situation. Generally speaking, during the dialing of the diode, the voltage drop in the forward direction is known. The denomination formed by the imperfection of the tester plus the known value added by the material( silicon, germanium).At the terminals there is a certain voltage level, ranging from hundreds of millivolts to volt units, using which parameters are measured. With regards to nonlinear elements( diodes, transistors), knowledge of undocumented information will make it possible to find the corresponding point on the current-voltage characteristic, to check whether the empirical( measured) numbers correspond to theoretical( reference) numbers. Performed an audit will assess the health of the diode.
A well-known denomination makes it possible to perform unusual evaluation operations:
- Own capacitance. The impedance of the resistor is not purely active with a few exceptions. The choice of elements of high frequency circuits( megahertz, gigahertz) takes into account the feature. The resistance of the reactive part is directly determined by the circular frequency defined by the formula ω = 2Пf( П = 3.14 - Pi number, f - frequency, Hz).Clearly, it is difficult to do with one multimeter, it forms a constant voltage measurement. The reactive( imaginary) part of the impedance becomes zero, according to the formulas Z = R + i( ωL - 1 / ωC), where L is the own inductance of the resistor, C is the capacitance. The attentive reader will note: at a fixed frequency, the inductive and capacitive components balance each other, the impedance Z becomes purely active. The resonant frequency of the resistor, the product will work better. Thus, there is no rule; the smaller the capacity, the inductance of a radio element, the better, the law of the golden mean. Determining the boundary is not difficult: ω = √LC is a well-known formula.
- Own inductance. The famous MLT resistors, a frequent guest of the equipment, are not applicable at high frequencies. Ceramic base is wound high-resistance residential( constantan, manganin, nichrome).Formed, shaped inductance. The difference is limited to the core material. Moreover, the typical formulas, knowing the number of turns, the inductance of the resistor we calculate, with the help of standard techniques.
We describe the process of work. The first look is a task unsolvable. Many people have no idea: the tester is unable to directly process the parameters of high-frequency circuits. A certain upper limit is fixed, above which the multimeter shamelessly lies. Solving the problem, radio amateurs offer to solder a special circuit formed by several passive elements, through which measurements are taken. The board will act as a bridge between the measured alternating voltage and the probe. Works are carried out on the corresponding voltage range( indicated by a tilde ~ and the letter U).
The circuit is incredibly simple. Let's briefly discuss the issues that are disturbing beginners:
- Why do I need a prefix multimeter. The device will stop lying, embarrassed by the high frequencies. You can work with a wide range of electronics. We are going to conduct a resistor impedance measurement test. You will need an alternating high frequency circuit.
- Where to get the land for this scheme. The horizontal bar icon adorns the front panel of the tester, will answer the question. The scheme requires the presence of red, black probes, pros pass on trivial aspects. Electrically connect the earth. The black multimeter probe is a horizontal dash of the electrical circuit.
- There are no KD522B diodes, replacement options are needed. The cutoff frequency of radio elements is 100 MHz. We select analogs, guided by an obvious consideration: the new element is suitable to be an integral part of impulse circuits. Supply 1N4148( import equivalent).
- Assigning oblique lines crossing resistors. Maximum power dissipation. Two slashes correspond to 0.125 watts. You can simply calculate the parameter - multiply the current of the resistor by the applied voltage. The parameter is unlikely to play a great role, the input resistance of the multimeter is traditional high( 1 MΩ).Compare: the insulation resistance of the circuit is at least 20 MΩ.The current consumption will be low, the power resistors dissipate a little( Joule-Lenz law).
- The principle of the console. The simplest integrator. Will take high-frequency pulses, forming a constant voltage. The values of the resistors form a divider, serving the purpose of matching with the input resistance of the tester. Get ready to pick up by experience. It is easier to find a high-frequency oscillator with an adjustable amplitude by performing a check.
- Units for indicating capacitance values, resistors. By default, capacitors are labeled pF.The prefix includes radio elements 68 pF.Resistors 2 MΩ, 180 kΩ.
- Measurement Process. Read more. ..
Measurement of own inductance, capacitance of resistor
At first we will assume that we have the necessary measuring instruments. Then the procedure is set:
- Take the generator of the first frequency. For example, 15 MHz. Parallel to the resistance, variable capacity( whole battery) is switched on. The values of the capacitors( parasitic resistor, selected by the user) are added. The total capacitance is formed by a variable, its own( resistor).Formed parallel oscillatory circuit.
- Consistently turn on the purely resistive load. Another resistor of similar rating. The performed measure forms a voltage divider. Further regulation will try to get a resonance. To register the fact that the scheme has reached a given state, it is necessary to assemble a divider.
- By selecting the variable capacitance value, we achieve system resonance. Twist back and forth, the tester measure the voltage of the oscillating circuit, inserting the prefix described above. The minimum potential difference indicates the point of resonance.
- Remember the nominal variable capacity. The regulator knob is traditionally present, the scale is absent. View testimony is not possible. Disassemble the circuit, keeping the settings, measure the nominal. The easiest way to use a multimeter is equipped with an appropriate scale( F).Otherwise, a number of indirect measurements will be required. A separate topic.
- We repeat the experience, taking a different frequency. Obtaining a noticeable difference in recorded indications. The magnitude of the discrepancy characterizes the obtained nominal variable capacitance. The numbers must be different( ensuring minimal error).Tried, failed? The conclusion suggests itself: we neglect our own capacitance of the resistor in the specified conditions( very small).The inductance is found using the typical circuit resonance formula: ω2 = 1 / LC.
- We begin the calculation, guided by the following considerations: the square of the oscillator's circular frequency( radio frequency multiplied by two Pi numbers) is inversely proportional to the product of the capacitor own inductance and the sum of the parasitic, variable capacitances. By measuring two different frequencies( say, 15.7 MHz), you can get two results. Important values of variable capacitances. If the formula divides the squares of circular frequencies, we get: the square of the ratio of ordinary frequencies correlates only with the quotient of the capacitances, the inductances will be reduced.
This is how it looks:
( f1 / f2) 2 =( C + C2) /( C + C1);f1, f2 - frequency of experiments( Hz), C - the own capacity of the resistor;C1, C2 - variable capacitances, respectively, of the first and second frequencies of the experiment. Using the formula, work to find your own capacity, going the beaten path, calculate the inductance of the resistor. Please note: it is important to find the minimum voltage. The way to do this is a separate topic of conversation.
Finding the minimum voltage of the resonant circuit
Be your own capacitance, the inductance of the resistor is small, the resonant frequency will come out high. Often, the parameters can be completely neglected. If successful, the variation of the variable capacitance can be observed: the readings of the multimeter decrease, increase. The frequency response in this case shows one hump( or rather failure).We need to move in the direction where the potential of the circuit falls.
As the multimeter is digital, it will soon be clear: we have found the interval of capacities, where the display readings are minimal. It is necessary to record both edges( each measured by a tester, removing the capacitor from the circuit).Then the desired value is found as the arithmetic average between the two( add and divide in half).
Sometimes it is convenient to solder a test circuit. And check the resistor with a multimeter on the board. It is advisable to include there various tumblers, bundles of tanks and all in the same spirit.
Checking the resistor with a multimeter
We said so much about exotic parameters that many people do not understand, probably, how a standard test of a resistor is carried out with a multimeter. Usually it happens like this:
- Estimated resistor rating. Used reading markings. It is much easier to measure the resistance of a resistor with a multimeter if you can select a range in advance. Marking is now mostly color, and on the Internet you can find online calculators that will kindly translate a series of bands into the desired value. It is difficult to confuse the direction, because silver and golden colors, for example, can be only from one edge.
- Then set the desired scale from the ranges marked with the letter Ω, the display reading is read. The polarity of the probes when checking the resistor is not important.
- The resistor accuracy is then determined. It is also color coded. And if the health check shows: the variable resistor is within the allowable range, the element is 100% fit. Otherwise, it is necessary to conduct additional studies like those mentioned above.
It happens, you want to check the resistor with a multimeter, without soldering. In this case, it all depends on the scheme. First, the presence of a short circuit is evaluated, then an open-circuit test is performed. With parallel connection, the active parts of the resistors and inductances are added. Capacities are discontinuities in any case, because the multimeter uses direct current to measure.
Knowing these features, skillfully applying the laws of Ohm and Kirchhoff, in most cases you can check the resistor with a multimeter on the board without soldering.