Figure 10 shows a circuit that can be used to convert a DVM module into a five-range ohmmeter. This circuit actually functions as a multi-range constant-current generator, in which the constant current feeds from Q1 collector into R X , and the resulting R X volt drop which is directly proportional to the R X value is read by the DVM module. Here, Q1 and the op-amp are wired as a compound voltage follower, in which Q1 emitter precisely follows the voltage set on RV1 slider.
The actual DVM module reads full scale when the R X voltage equals mV, and this reading is obtained when R X has a value one-fifth of that of the range resistor, e. An op-amp can easily be used to convert a standard moving coil meter into a sensitive analog voltage, current, or resistance meter, as shown in the practical circuits of Figures 11 to All six circuits operate from dual 9V supplies and are designed around the LF JFET op-amp, which has a very high input impedance and good drift characteristics.
All circuits have an offset nulling facility, to enable the meter readings to be set to precisely zero with zero input, and are designed to operate with a moving coil meter with a basic sensitivity of 1mA fsd.
Figure 11 shows a simple way of converting the 1mA meter into a fixed-range DC millivolt meter with a full-scale sensitivity of 1mV, 10mV, mV, or 1V0.
To set the circuit up initially, short its input terminals together and adjust RV1 to give zero deflection on the meter. The circuit is then ready for use.
The table shows alternative R1 and R2 values for different ranges. Figure 15 shows the circuit of a simple but very useful four-range AC millivoltmeter. The input impedance of the circuit is equal to R1, and varies from 1k0 in the 1mV fsd mode to 1M0 in the 1V fsd mode. The circuit gives a useful performance at frequencies up to about kHz when used in the 1mV to mV fsd modes.
In the 1V fsd mode, the frequency response extends up to a few tens of kHz. This good frequency response is ensured by the LF op-amp, which has very good bandwidth characteristics. Finally, Figure 16 shows the circuit of a five-range linear-scale ohmmeter, which has full-scale sensitivities ranging from 1k0 to 10M. Range resistors R5 to R9 determine the measurement accuracy.
The meter reads full-scale under this condition, since it is calibrated to indicate full-scale when 1V0 nominal appears across the R X terminals. To initially set up the Figure 16 circuit, set SW1 to the 10k position and short the R X terminals together.
Next, remove the short, connect an accurate 10k resistor in the R X position, and adjust RV2 to give precisely full-scale deflection on the meter. The circuit is then ready for use, and should need no further adjustment for several months.
An op-amp can be used as a fixed or variable voltage reference by wiring it as a voltage follower and applying a suitable reference to its input. Variations in output loading cause little change in the output voltage value. Zener diode ZD1 generates a stable 12V, which is applied to the non-inverting input of the op-amp via RV1.
A CA op-amp is used here because its input and output can track signals to within mV of the negative supply rail voltage. The complete circuit is powered from an unregulated single-ended 18V supply.
Figure 18 shows a negative voltage reference that gives an output fully variable from An LF op-amp is used in this design, because its input and output can track signals to within about 0. Note that the op-amps used in these two regulator circuits are wide-band devices, and R2 is used to enhance their circuit stability. The basic circuits in Figures 17 and 18 can be made to act as high-current regulated voltage power supply circuits by wiring current-boosting transistor networks into their outputs.
The circuit can be made to give an output that is variable all the way down to zero volts by connecting pin 4 of the op-amp to a supply that is at least 2V negative. I already explained how your circuit works on one of your other threads or on another website.
Or maybe I explained it to another kid in your class. Did you ask your teacher to explain how it works? Click to expand Last edited: Mar 9, User kewl has asked that this thread be removed because it was in regards to his homework. First it should have been submitted in the homework thread. Second our answers are here for everyone to learn from.
Nuff Said. The circuit is an active precision full-wave rectifier. When it is powered properly it works perfectly. Don't delete this thread because somebody might learn something. Similar threads M. AC to DC conversion using single supply opamp. Replies 37 Views 5K. Mar 9, mayuriv.
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