# Voltage to Current Converter-Passive Voltage, Floating load, Applications

Hey friends, Welcome to the Kohiki.com ALL ABOUT ELECTRONICS. So, in this article, we will understand how to design the voltage to current converter using the op-amp.

## Why Voltage to Current Conversion?

Now, before we understand that, first of all, let’s understand what is the need for this voltage for current conversion and why these types of circuits are used. Now, these type of circuits is particularly useful in the industrial application as well as in instrumentation.

So, let’s say you are measuring some parameters like temperature using the sensor. And here assume that the sensor is giving output in terms of the voltage. Now, many times in industrial applications is quite possible that the location where these sensors are measuring the particular parameter and the location where the value of this sensor is getting displayed is quite far away from each other.

So, the voltage which is generated by the sensor needs to be transported to the other end. So, if you measure the voltage at the other end then you will find that there will be a drop in the voltage. Because during the course of transportation of this voltage there will be a voltage drop across the cable. And moreover that it is quite possible that the noise will get superimposed over this voltage. So, this is the basic problem in industrial applications when you are dealing with voltages.

So, now suppose if we convert this voltage into the current then this current will remain constant throughout the length of the cable. And moreover that this current is less prone to external noise. So, this is the basic advantage of converting the voltage into the current in industrial applications. And particularly, when the two locations are quite far away from each other. So, apart from that, in lab experiments also for testing the different circuit components as well as driving the LEDs, these voltage to current converter circuits are useful.

So, in such applications somehow you need to convert this voltage source into the current source. Or basically, we require a voltage-controlled current source. So, just by changing the voltage, we should be able to control the current. Now, the one way we can design this voltage to current converter is by using the op-amp. But before we see that circuit, first of all, let’s see how we can design this voltage to current converter using the passive circuit components.

## Passive Voltage to Current Converter and its limitation

So, here, just by connecting the resistor in series with this voltage source we can convert this voltage into the current. And the relation can be given by this expression. So, now if you connect the load in series with this resistor R, then ideally the same current should also flow through that resistor. Because here we are assuming that there is no voltage drop across this load resistor.

But actually, if you see, there will be some voltage drop across this load and because of that, the current that is flowing through the load will also change. Now, ideally in the voltage to current converter, the value of the current should only depend upon the input voltage. And it should be independent of the load resistance. But here in this passive converter, the value of the current will also depend upon the load resistance. So, this problem can be avoided by using the active converter.

## Floating load Voltage to Current Converter using Op-amp

So, here is one of the ways by which we can design this active voltage to current converter. Now, this type of converter is also known as the floating load converter. Because here the load is floating. So, now let’s understand how this circuit will act as a voltage to current converter. So, if you see this circuit, here the input is applied at the non-inverting terminal. And because of the negative feedback, there will be a virtual short between the non-inverting and the inverting terminal.

So, at this node also, the voltage will be equal to the input voltage. Now here, we are assuming that the current that is flowing through this resistor R is equal to me and the current that is flowing through this resistor RL is equal to IL. So, if we apply the KCL then we can write, this current I will be equal to this load current IL. And this current I, will be equal to this input voltage Vin divided by this resistor R. And that will be equal to this load current.

So, here we can say that this load current IL is directly proportional to the input voltage. So, once we set the value of this resistor R, then this load current IL will be proportional to the input voltage. So, in this way, this circuit will act as a Voltage to Current Converter

## Voltage to Current Converter Applications

Now, as I said before, this type of circuit can be used to drive the LEDs. Because here once you set the value of this resistor R and the input voltage Vin, then the current that is flowing through this LEDwill be constant. So, now if you change the value of this input voltage, then the current that is flowing through this LED will change. And you can use this circuit to test the brightness of the different LEDs for the constant current.

Similarly, this type of converter is useful in finding the matched pair of diodes or matched pair of Zener Diodes. For example, if you connect the diode in such a fashion in this circuit, then the output voltage V out will be equal to the input voltage Vin plus the forward voltage that is developed across this diode. So, measuring the output voltage at the output terminal, we can find the two exact matched pair of diodes which has the same value of this Vd. And similarly, we can also find the matched pair of Zener diodes. So, these are some of the applications in which this voltage to current converter is useful.

## Grounded Load Voltage to Current Converter using Op-amp

So, nowhere is another circuit using which we can convert the voltage into the current. But here now if you observe the load is grounded. So, this type of converter is known as the grounded load voltage to current converter. So, now let’s understand how this circuit will act as a voltage to current converter. So, now here let’s assume that this node is node A. And the voltage at this nose is equal to V1.

Now, here let’s assume that the current that is flowing through this resistor RL is equal to IL. And I1 and I2 are the currents that are entering into this node A. So, now if we apply the KCLat to this node A, then we can write I1 plus I2 that is equal to IL. Now, I1 will be equal to this voltage Vinminus V1 divided by this resistor R. And similarly, this current I2 will be equal to, this voltage Vout minus V1 divided by this resistor R.And that will be equal to the load current IL.

So, now if we simplify it then we can write, Vin plus Vo minus 2*V1, which is equal to IL*R. Now, if you observe over here this voltageV1 is applied at the non-inverting terminal. So, for the non-inverting terminal, we know that the output voltage Vout can be given by the expression, 1 plus Rf divided by theR1 times the voltage that is present at the non-inverting terminal. Now, here the feedback resistor Rf and resistorR1 are equal to R.

So, we can say that output voltage Vout will be equal to, 1 plus R divided by R times the input voltageV1. That is equal to Vo will be equal to 2*V1. So, now let’s put this value in this expression, and let’s simplify this expression.

So, if we put this value of Vo, then we can write this expression as Vin plus 2*V1 minus @&amp; V1, which is equal to IL*RAnd if we simplify it then we can say that the load current IL will be equal to the input voltage Vin divided by the value of this resistor R.So, now as you can see over here, once we set the value of this resistor R, then this load current IL is directly proportional to the input voltage Vin. And it is independent of the value of this load resistor RL.