op – amp amplifier voltmeter:
An IC operational amplifier can be used to amplify low voltages to measurable level by any deflection instrument. Input voltage E is applied to ap-amp non-inverting input, the output voltage is divided across resistors R3, R4 and Vr3 is fed back to op-amp inverting input terminal.
Vo = E (R3 + R4 ) / R3
The circuit is known as non-inverting amplifier because its output is positive when a positive input voltage is applied and negative when input is a negative quantity. The non-inverting amplifier has a very high input resistance, very output resistance
While when meter and its resistance are connected in parallel then the circuit is used to measure voltage. Two major currents are flowing in this circuit first is I4 and second is IB never forget that IB is very small consider as negligible but I4 is 1000 times of IB.
voltage to current converter:
As we stated that above circuit use for voltage measurement so only one change can make it to measure current. If we connect meter and its resistance instead of R4. How it actually look like
For this circuit meter current is calculated by
Im = IR3 = E / R3
and again is IR3 greater than IB
ohm-meter function in electronic instruments:
You can see that this circuit have two main parts standard resistors and range switch while this circuit is used to find unknown resistance by comparing it with other resistors How we can do that? Lets start this by comparing it with min. resistance if Rx is in kilo ohms then min. resistance is not suitable for us by moving the pointer to R1 we got our desired reading. We use a constant battery in this circuit to provide potential of 1.5V we can calculate E by VDR
E = EB (RX) / (RX + R1)
In this case
E = 1.5 × (1k) / (1k+1k)
E = 0.75V
So the meter indicates one half of full scale and the center of resistance scale is marked 1.
In the above circuit we DC battery of 1.5V which is not good choice because after some time battery power starts decreasing which cause effect on our reading. To recover this thing we use a shunt resistance by using this we can control input voltage
E = EB (R2 || RX) / (R1 + R2 || RX)
current measurement with electronic instruments:
There are two reasons of introducing electronic meters in voltmeters
1- To produce high input resistance but this case does not apply in case of current measurement while ammeters should have lowest possible resistance because the maximum current need to pass.
2- To amplify small voltages to measurable levels and it can apply in case of low level currents.
The basic circuit for current measurement is
Now cut this picture into two parts and then understand there functioning
We already discussed about first part in above topic voltage to current converter but working of second part is very important the small voltage drop across Rs is amplified before being applied to the deflection instrument. This approach is just applicable to the measurement of low level alternating currents as it is to direct current measurements. For medium and high current measurement there is no need to use electronic amplifiers. Many electronic multi-range instruments do not have current measuring facilities. Those that do measure current generally have very low level current ranges and some have relatively high resistances when operating as ammeters.