How to test input resistance

How to test input resistance

By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts.

Module 7.3

It only takes a minute to sign up. How should I calculate it? EDIT: If ideal op. By rule 2, no current flows into that input. This lets us calculate the equivalent input resistance:.

Your second circuit is drawn incorrectly. You've connected R2 to one of the op amp's power pins, but it should be connected to the output. Here we need rule 1, which tells us that the voltage at the inverting input is equal to the voltage at the non-inverting input -- zero volts ground.

We can work out the math, too:. Ideal op amps have infinite input resistance, and the voltage source is connected to only the input through Rs. This large input resistance is even drastically enlarged due to the feedback effect voltage feedback.

The situation, however, is different for the second circuit inverting amplifier. Now we have current feedback - and the input resistance referred to the opamp input pin is decreased due to the feedback effect decreased by the large loop gain factor and can be neglected if in series with Rs.

Hence, the remaining input resistance as seen by Vs is only Rs. Another explanation: For large values of the open-loop gain Ao usually 1E Hence, we assume that the node voltage at the inv.

how to test input resistance

Rs is probably meant to be the source resistance. It is a property of internal to the voltage generator. Voltage sources are shown as Thevenin equivalent circuits consisting of an ideal voltage source in series with the source's internal resistance, as your diagram shows. The external load therefore is the op amp itself, not including any "Rs" external series resistor because it's not external.

Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered.

Calculation of the input resistance of an op amp circuit Ask Question. Asked 5 years, 2 months ago. Active 5 years, 2 months ago. Viewed 13k times.Every meter impacts the circuit it is measuring to some extent, just as any tire-pressure gauge changes the measured tire pressure slightly as some air is let out to operate the gauge.

While some impact is inevitable, it can be minimized through good meter design. Since voltmeters are always connected in parallel with the component or components under test, any current through the voltmeter will contribute to the overall current in the tested circuit, potentially affecting the voltage being measured.

A perfect voltmeter has infinite resistance, so that it draws no current from the circuit under test. However, perfect voltmeters only exist in the pages of textbooks, not in real life! Take the following voltage divider circuit as an extreme example of how a realistic voltmeter might impact the circuit its measuring:. The lower resistor will now have far less voltage across it than before, and the upper resistor far more.

how to test input resistance

Since the voltmeter is part of that 9. Now, the voltmeter can only indicate the voltage its connected across. Imagine using a tire pressure gauge that took so great a volume of air to operate that it would deflate any tire it was connected to.

The amount of air consumed by the pressure gauge in the act of measurement is analogous to the current taken by the voltmeter movement to move the needle.

The less air a pressure gauge requires to operate, the less it will deflate the tire under test. The less current drawn by a voltmeter to actuate the needle, the less it will burden the circuit under test. This effect is called loadingand it is present to some degree in every instance of voltmeter usage. The scenario shown here is worst-case, with a voltmeter resistance substantially lower than the resistances of the divider resistors.

But there always will be some degree of loading, causing the meter to indicate less than the true voltage with no meter connected. Obviously, the higher the voltmeter resistance, the less loading of the circuit under test, and that is why an ideal voltmeter has infinite internal resistance. Digital voltmeters, on the other hand, often exhibit a constant resistance across their test leads regardless of range setting but not always! The astute observer will notice that the ohms-per-volt rating of any meter is determined by a single factor: the full-scale current of the movement, in this case 1 mA.

To minimize the loading of a voltmeter on any circuit, the designer must seek to minimize the current draw of its movement.

how to test input resistance

This can be accomplished by re-designing the movement itself for maximum sensitivity less current required for full-scale deflectionbut the tradeoff here is typically ruggedness: a more sensitive movement tends to be more fragile. Another approach is to electronically boost the current sent to the movement, so that very little current needs to be drawn from the circuit under test. This special electronic circuit is known as an amplifierand the voltmeter thus constructed is an amplified voltmeter.

The internal workings of an amplifier are too complex to be discussed at this point, but suffice it to say that the circuit allows the measured voltage to control how much battery current is sent to the meter movement. The amplifier still loads the circuit under test to some degree, but generally hundreds or thousands of times less than the meter movement would by itself.Measuring resistances is similar to measuring voltages, with a key difference:.

You must first disconnect all voltage sources from the circuit whose resistance you want to measure. Otherwise, pick the largest range available on your meter. Analog meters must first be calibrated before they can give an accurate resistance measurement. To calibrate an analog meter, touch the two meter leads together. Touch the meter leads to the two points in the circuit for which you wish to measure resistance. For example, to measure the resistance of the resistor, touch the meter leads to the two leads of the resistor.

Some components such as diodes pass current better in one direction than in the other. In that case, the direction of the current does matter. But in circuits where it does, you can use the ohmmeter function of your multimeter to determine the exact value of a particular resistor. Then, you can adjust the rest of your circuit accordingly. How to Measure Resistance on an Electronic Circuit. About the Book Author Doug Lowe still has the electronics experimenter's kit his dad gave him when he was This guide provides an introduction to transformer winding resistance test methods and procedures.

Photo: TestGuy. Winding resistance measurements are an important diagnostic tool for assessing possible damage to transformers resulting from poor design, assembly, handling, unfavorable environments, overloading or poor maintenance.

The main purpose of this test is to check for gross differences between windings and for opens in the connections. Measuring the resistance of transformer windings assures that each circuit is wired properly and that all connections are tight.

how to test input resistance

Winding resistance in transformers will change due to shorted turns, loose connections, or deteriorating contacts in tap changers. Regardless of the configuration, the resistance measurements are normally made phase-to-phase and the readings are compared with each other to determine if they are acceptable. Transformer winding resistance measurements are obtained by passing a known DC current through the winding under test and measuring the voltage drop across each terminal Ohm's Law.

Modern test equipment for this purposes utilizes a Kelvin bridge to achieve results; you might think of a winding resistance test set as a very large low-resistance ohmmeter DLRO. Before conducting a transformer winding resistance test, it is important to observe all safety warnings and take proper precautions.

During the test, it is important not to remove current or voltage leads while current is still flowing through the transformer. This will cause an extremely high voltage to develop across the point where current is broken, which could produce a lethal voltage.

Winding resistance test equipment is available in a variety of styles based on specific applications. A test set used for a power transformer would be much different than one designed for small instrument transformers. No matter what the type, winding resistance testers are always equipped with a current output, voltage measurement, and resistance meter. Photo: Testguy.

Both the primary and secondary terminals of the transformer should be isolated from external connections, and measurements made on each phase of all windings. Test equipment connections should be made in the following order:.

For single-phase and simple Delta-Wye configurations, the following connections can be used. Keep in mind that each transformer configuration is different and your specific setup may not apply to what is shown below, consult the user manual that came with your test kit for more information.

To save time when testing two-winding transformers, both primary and secondary windings can be tested at the same time using the connections shown below:. To reduce core saturation time, the jumper used to connect both windings should be connected to opposite polarities of the transformer.

If the positive lead for the current is connected to the positive terminal of the primary winding, drive test current from the primary winding H2 jumped to the positive terminal of the secondary winding X1. Note: If the resistance between the two windings is greater than a factor of 10, it may be desirable to obtain readings that are more accurate by testing each winding separately.

When measuring winding resistance, the reading should be observed and recorded once the resistance value has stabilized. Resistance values will "drift" at first due to the inductance of the transformer, which is more prevalent in large, delta connected windings.

For small transformers the drift lasts for only a few seconds; for single-phase high voltage transformers the drift may last for less than a minute; for large transformers the drift time required could last a couple minutes or more. Any change in current will cause the resistance value to change.

Many power and distribution transformers are equipped with tap changers to increase or decrease the turn's ratio depending on the supply voltage. Because changing ratio involves a mechanical movement from one position to another, each tap should be checked during a winding resistance test.

During routine maintenance, it may not always be feasible to test each tap due to time constraints or other factors.

Recipe Ratings and Stories

In such cases, it's acceptable to measure the resistance of each winding at only the designated tap position. For "Off-load" taps, the transformer must be discharged between tap changes.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. How to find out what is the internal resistance of a voltmeter? I imagine the higher the better - it'll have less influence on the measured circuit. What is the approximate internal resistance of cheap DMM on the voltmeter setting? Do more expensive multimeters have better higher internal resistance?

Apply a known voltage over a series resistor. This resistor in combination with the internal resistance will form a voltage divider. Using Ohm's Law, calculate the following Example I measured Most DMM's today are 10 Meg Ohms input impedance minimum, even the free one from Harbor Freight so the current measured will be in the micro-amp range.

Therefore you will need a meter that can measure micro-amps And yes, the higher the input impedance the better. However for most uses today, 10 Meg Ohm impedance is all you need. This value should be stated in the manual of the DMM. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered.

Ask Question. Asked 9 years, 3 months ago. Active 5 years, 5 months ago. Viewed 56k times. What is the best method to measure the internal resistance of a voltmeter?The Web This site.

Because Impedance is an AC property it cannot be easily measured like resistance. Connecting an Ohm meter across the input or output of an amplifier only indicates the DC resistance. It is quite possible however to measure input and output impedance at any frequency using a signal generator, an oscilloscope or AC voltmeter and a decade resistance box or a variable resistor.

The set up for measuring input impedance is illustrated in Fig. A variable resistor or a decade resistance box is connected between the signal generator and the amplifier input and its resistance is set to zero Ohms.

An oscilloscope or AC voltmeter is connected across the amplifier load e. The signal generator is set to provide a sine wave output at 1kHz. The amplitude of the input signal should be adjusted so that the display on the oscilloscope is noise free large enough and distortion free not too large. The display on the oscilloscope screen should be as large as is practical and set so that its amplitude and half its amplitude can be easily estimated.

The resistance at the amplifier input should then be increased until the output waveform is exactly half its previously set value. At this setting the signal is shared equally between the test resistance and the input impedance of the amplifier, meaning that the resistance and impedance are equal. After switching off and removing the test resistance, the reading of the decade box settings or measuring the variable resistor with an Ohm meter gives the value equivalent to the input impedance of the amplifier.

The measurement of output impedance uses the same method as for input impedance but with different connections. In this case the amplifier load is replaced with the decade box or variable resistor.

Care must be taken however, to ensure that the resistance connected in place of the load is able to dissipate sufficient power without damage. The amplifier need not be run at full power for this test. Connect the test circuit as shown in Fig. The test resistance is then connected across the output terminals and adjusted for maximum resistance before switching on the amplifier.

The test resistance is reduced in value until the display indicates half the amplitude of that noted with no load. The test resistance is now the same value as the output impedance. Hons All rights reserved. Revision Learn about Electronics - AC Theory. AC Theory Modules 2. Capacitors 3. Inductors 4. DC Transients 5. Phase and Phasors 6. Reactance 7. Impedance 8. LCR Series Circuits LCR Parallel Circuits Module 7: 7. Module 7.

Input impedance

Fig 7. Google Ads.Forgot your password? By Anderton June 20, Is your guitar sounding run down? Dull and anemic? It may not have the flu, but be feeding the wrong kind of input.

A guitar pickup puts out relatively weak signals, and the input it feeds can either coddle those signals or stomp on them. This is one piece of test equipment no guitarist should be without anyway, as you can test anything from whether your stage outlets are really putting out V to whether your cable is shorted.

If theory scares you, skip ahead to the next subhead. If you can, though, stay tuned since impedance crops up a lot if you work with electronic devices. Impedance is a pretty complex subject, but we can just hit the highlights for the purposes of this article.

The lower the resistance to ground, the greater the amount of signal that gets shunted. If you draw an equivalent circuit for these two resistances, it looks suspiciously like the schematic for a volume control Fig.

Conversely, a high guitar output impedance and low amp input impedance creates a lot of loss. Thus, low frequency signals may not be attenuated that much, but high frequencies could get clobbered. Buffer boards and on-board preamps can turn the guitar output into a low impedance output for all frequencies, but many devices are already designed to handle guitars, so adding anything else would be redundant.

Hence, the following test. This test takes advantage of the fact that impedance and resistance are, at least for this application, roughly equivalent.

Wire up the test jig in Fig. Plug in the signal generator and amplifier or other device being testedthen perform the following steps. Test points are marked in blue.

You may later need to switch to a more sensitive range e. Measure the signal generator level by clipping the VOM leads to test points 1 and 2. Try for a signal generator level between 1 and 2 volts AC but be careful not to overload the effect and cause clipping. Set the VOM to measure ohms, then clip the leads to test points 1 and 3.

Real Analog - Circuits1 Labs: Ch10 Vid2: Impedance Measurement

This will essentially equal the input impedance of the device being tested. The range of k to k is acceptable although you may hear some dulling. An input impedance over k means the designer either knows what guitarists want, or got lucky. Note, however, that more is not always better. Input impedances above approximately 1 megohm are often more prone to picking up radio frequency interference and noise, without offering much of a sonic advantage.


Leave a Reply

Your email address will not be published. Required fields are marked *