There are

many types of voltage-to-current and current-to-voltage converter circuits, and

most of them use a combination of op amps and transistors to achieve a high

level of accuracy. But when high accuracy isn’t

necessary, a simple converter of this type can be made using just one or two

resistors.

many types of voltage-to-current and current-to-voltage converter circuits, and

most of them use a combination of op amps and transistors to achieve a high

level of accuracy. But when high accuracy isn’t

necessary, a simple converter of this type can be made using just one or two

resistors.

Any

resistor R that is connected across a

power supply V can be considered a voltage to

current converter, since the current depends on the voltage via Ohm's law - the

formula for which is I = V / R.

If one end of the resistor is disconnected, and another component D

is connected to the disconnected power supply terminal and resistor so that R

and D are in series across the power supply, the circuit still

behaves like a voltage to current converter if the voltage drop across the

component D is very small or relatively constant. This

component could be a diode, LED, or zener diode, or even a low-value resistor.

The diagram below shows these possible combinations.

resistor R that is connected across a

power supply V can be considered a voltage to

current converter, since the current depends on the voltage via Ohm's law - the

formula for which is I = V / R.

If one end of the resistor is disconnected, and another component D

is connected to the disconnected power supply terminal and resistor so that R

and D are in series across the power supply, the circuit still

behaves like a voltage to current converter if the voltage drop across the

component D is very small or relatively constant. This

component could be a diode, LED, or zener diode, or even a low-value resistor.

The diagram below shows these possible combinations.

The

resistor R can also be thought of as a

current limiting resistor for the added component D. The current that flows through D

is determined by the simple formula: I

= (V – VD) / R, where VD is the voltage drop across

the added component.

resistor R can also be thought of as a

current limiting resistor for the added component D. The current that flows through D

is determined by the simple formula: I

= (V – VD) / R, where VD is the voltage drop across

the added component.

For

constant values of VD

and R, the current only depends on V.

For forward biased diodes, VD is about 0.3 - 0.35 volts for germanium, and 0.6

- 0.7 volts for silicon diodes, and is relatively constant over a wide range of

currents. LEDs are similar to diodes,

except that they are constructed using special materials that emit light. They

have a forward bias voltage that is a little higher than regular diodes, and

could be anywhere from about 1.4 volts

to over 3 volts, depending on the color. LEDs operate efficiently at about 10

mA to 40 mA, and a current limiting resistor is almost always connected to one

of the LED terminals to prevent any damage due to high current. There are

slight changes in the voltage drops of diodes and LEDs for different current

levels, but these can usually be neglected in the calculation.

constant values of VD

and R, the current only depends on V.

For forward biased diodes, VD is about 0.3 - 0.35 volts for germanium, and 0.6

- 0.7 volts for silicon diodes, and is relatively constant over a wide range of

currents. LEDs are similar to diodes,

except that they are constructed using special materials that emit light. They

have a forward bias voltage that is a little higher than regular diodes, and

could be anywhere from about 1.4 volts

to over 3 volts, depending on the color. LEDs operate efficiently at about 10

mA to 40 mA, and a current limiting resistor is almost always connected to one

of the LED terminals to prevent any damage due to high current. There are

slight changes in the voltage drops of diodes and LEDs for different current

levels, but these can usually be neglected in the calculation.

Zener diodes are

different in that they are connected with reverse bias. This sets a fixed

voltage drop VD across the

zener diode that could be anywhere from 2V to around 300V, depending on type.

In order for any of these devices to work, the supply voltage has to be higher

than the voltage drop VD. Any value

of resistor would work, as long as its value is low enough to allow sufficient current

to flow, while at the same time being high enough to keep excess current from

flowing.

different in that they are connected with reverse bias. This sets a fixed

voltage drop VD across the

zener diode that could be anywhere from 2V to around 300V, depending on type.

In order for any of these devices to work, the supply voltage has to be higher

than the voltage drop VD. Any value

of resistor would work, as long as its value is low enough to allow sufficient current

to flow, while at the same time being high enough to keep excess current from

flowing.

Usually there is a

switching component inserted somewhere in this series circuit, which turns an

LED on or off, etc. This could be a transistor, FET, or the output stage of an

op amp. An LED flashlight basically consists of a battery, switch, LED, and

current limiting resistor all connected in series.

switching component inserted somewhere in this series circuit, which turns an

LED on or off, etc. This could be a transistor, FET, or the output stage of an

op amp. An LED flashlight basically consists of a battery, switch, LED, and

current limiting resistor all connected in series.

Sometimes, the

current limiting circuit consists of two resistors in series across a power

supply, instead of a resistor-and-diode type device. The second resistor RD

has a much smaller value than the current limiting resistor, R, and is

often called a "shunt" or “sense” resistor. The circuit can still be thought of as a voltage to current converter,

as the above formula can now be reduced to

I = V / R,

since VD

is negligible compared to V. The current will now only depend on the

voltage, since R is constant.

current limiting circuit consists of two resistors in series across a power

supply, instead of a resistor-and-diode type device. The second resistor RD

has a much smaller value than the current limiting resistor, R, and is

often called a "shunt" or “sense” resistor. The circuit can still be thought of as a voltage to current converter,

as the above formula can now be reduced to

I = V / R,

since VD

is negligible compared to V. The current will now only depend on the

voltage, since R is constant.

This kind of circuit

can often be found in various sensor circuits, such as temperature and pressure

sensors, where a defined amount of current is to flow in a device with a small

resistance. The voltage across this device is usually amplified to measure any

change as the sensor resistance changes under varying conditions. This voltage

can even be read by a

can often be found in various sensor circuits, such as temperature and pressure

sensors, where a defined amount of current is to flow in a device with a small

resistance. The voltage across this device is usually amplified to measure any

change as the sensor resistance changes under varying conditions. This voltage

can even be read by a

__multimeter__if it has sufficient sensitivity.If the

formula I = V / R is flipped around to become a voltage function V = I R, the simple two-resistor series

circuit can be thought of as a current to voltage converter as well. The

current limiting resistor still has a value much higher than the sense

resistor, and this sense resistor small enough that it doesn’t affect the operation of the circuit in any meaningful

way. A current is converted to a voltage by the fact that that the small

voltage VD across the sense resistor can

be detected by a multimeter, or it can be amplified and applied as a signal

into an A/D converter. This measured voltage indicates the current flow with

the Ohm’s law formula V = I R. For example, if 0.001 A flows through 1 ohm, the voltage

reading is 0.001 V. The conversion is simple for a 1 ohm resistor, but if this

value is too high, another value - like 0.01 ohms - can be used, and the

voltage could easily be found using V = I R.

formula I = V / R is flipped around to become a voltage function V = I R, the simple two-resistor series

circuit can be thought of as a current to voltage converter as well. The

current limiting resistor still has a value much higher than the sense

resistor, and this sense resistor small enough that it doesn’t affect the operation of the circuit in any meaningful

way. A current is converted to a voltage by the fact that that the small

voltage VD across the sense resistor can

be detected by a multimeter, or it can be amplified and applied as a signal

into an A/D converter. This measured voltage indicates the current flow with

the Ohm’s law formula V = I R. For example, if 0.001 A flows through 1 ohm, the voltage

reading is 0.001 V. The conversion is simple for a 1 ohm resistor, but if this

value is too high, another value - like 0.01 ohms - can be used, and the

voltage could easily be found using V = I R.

The actual value of

the sense resistor is not important in this discussion. It can be anywhere from

0.1 ohms to 10 ohms, as long as the current limiting resistor is much higher.

In high-current applications, the value of the sense resistor should be very low

in order to prevent excess power dissipation. Even with a value around 0.001

ohms, a reasonable voltage can be sensed across it because of the high current

flow. In cases like this the sense resistor is normally called a “shunt” resistor.

This kind of circuit is often used to measure the current though a DC motor,

for example.

the sense resistor is not important in this discussion. It can be anywhere from

0.1 ohms to 10 ohms, as long as the current limiting resistor is much higher.

In high-current applications, the value of the sense resistor should be very low

in order to prevent excess power dissipation. Even with a value around 0.001

ohms, a reasonable voltage can be sensed across it because of the high current

flow. In cases like this the sense resistor is normally called a “shunt” resistor.

This kind of circuit is often used to measure the current though a DC motor,

for example.

It is a simple

matter to use a multimeter to measure AC or DC voltage at any point in an

electronic circuit, such as on a PC motherboard. An appropriate voltage scale

is set on the multimeter, the black probe connected to a ground point, and the

red probe connected to the check point. The voltage is then read directly.

Hopefully the impedance of the probe input circuitry is high enough that it

doesn’t

affect the circuit’s operation in any way. The probe input impedance should

have a very high series resistance along with a very low shunt capacitance.

matter to use a multimeter to measure AC or DC voltage at any point in an

electronic circuit, such as on a PC motherboard. An appropriate voltage scale

is set on the multimeter, the black probe connected to a ground point, and the

red probe connected to the check point. The voltage is then read directly.

Hopefully the impedance of the probe input circuitry is high enough that it

doesn’t

affect the circuit’s operation in any way. The probe input impedance should

have a very high series resistance along with a very low shunt capacitance.

Measuring AC or DC

current at any point in a circuit instead of voltage becomes a little more

tricky, and the circuit might have to be modified a little to accommodate this.

It might be possible to cut the wiring of a circuit at the point where

measurement of the current flow is desired, and then insert a sense resistor

with a low value at the two contact points. Again, this resistor's value should

be low enough that it doesn’t affect the operation of the circuit. The multimeter

probes can then be connected across this sense resistor using the appropriate

voltage scale, and the resistor voltage would be displayed. This can be

converted to the current flowing through the test point by dividing by the

sense resistor value, as in the formula I

= V / R. In some

cases, the sense resistor can be kept in the circuit permanently if the current

at a particular test point needs to be measured frequently.

current at any point in a circuit instead of voltage becomes a little more

tricky, and the circuit might have to be modified a little to accommodate this.

It might be possible to cut the wiring of a circuit at the point where

measurement of the current flow is desired, and then insert a sense resistor

with a low value at the two contact points. Again, this resistor's value should

be low enough that it doesn’t affect the operation of the circuit. The multimeter

probes can then be connected across this sense resistor using the appropriate

voltage scale, and the resistor voltage would be displayed. This can be

converted to the current flowing through the test point by dividing by the

sense resistor value, as in the formula I

= V / R. In some

cases, the sense resistor can be kept in the circuit permanently if the current

at a particular test point needs to be measured frequently.

It would

probably be much easier to measure current flow with the multimeter directly,

instead of having to use a sense resistor. So after cutting the wire at the

point to be measured, the sense resistor can be left out and the multimeter's

leads hooked up directly to the two contact points. A current flow indication

would be displayed on the multimeter if the appropriate AC or DC current scale

is set. It is always important to set the correct voltage or current scale on a

multimeter before hooking up any probes, or risk posting a reading of zero.

probably be much easier to measure current flow with the multimeter directly,

instead of having to use a sense resistor. So after cutting the wire at the

point to be measured, the sense resistor can be left out and the multimeter's

leads hooked up directly to the two contact points. A current flow indication

would be displayed on the multimeter if the appropriate AC or DC current scale

is set. It is always important to set the correct voltage or current scale on a

multimeter before hooking up any probes, or risk posting a reading of zero.

When a

current scale is set on a multimeter, the input impedance of the input probes

becomes very small, similar to a sense resistor. The probe input of a

multimeter can be thought of as the sense or “shunt” resistor, so the multimeter itself can be included in

place of the RD resistor in the above

diagram. Hopefully, the input impedance of the multimeter is low enough that it

doesn't affect the circuit operation in any way.

current scale is set on a multimeter, the input impedance of the input probes

becomes very small, similar to a sense resistor. The probe input of a

multimeter can be thought of as the sense or “shunt” resistor, so the multimeter itself can be included in

place of the RD resistor in the above

diagram. Hopefully, the input impedance of the multimeter is low enough that it

doesn't affect the circuit operation in any way.

The

simple current-to-voltage and voltage-to-current conversion techniques

discussed in this article are not as precise as those that are based on a

transistor or amp, but for many applications they will work just fine. It is

also possible to do other types of simple conversions using the series circuit

shown above. For example, a square wave input can be converted to a saw-tooth

waveform (integrator) by replacing the

D component with

a

the time constant RC should be large relative to

the period of the square wave signal.

simple current-to-voltage and voltage-to-current conversion techniques

discussed in this article are not as precise as those that are based on a

transistor or amp, but for many applications they will work just fine. It is

also possible to do other types of simple conversions using the series circuit

shown above. For example, a square wave input can be converted to a saw-tooth

waveform (integrator) by replacing the

D component with

a

__capacitor__. The only restriction is thatthe time constant RC should be large relative to

the period of the square wave signal.

Anonymous says

LED on AC Line 230Volt AC

RED LED =1.5 volt

Current= 20milli amp

Voltage drop = 228.50

I=V/R

R= 228.5/20= 11.5K

Power= Isquare*R

P=400*11.5 =4600 ml watt

Wattage of Resistor 5 watt

Can you pls confirm if the calculations are correct. else explain the concept pls

Swagatam says

That's perfect, but the resistor will get pretty hot….

Anonymous says

reason pls, can we increase the resistor and if so what is the basis. kindly let me know

Swagatam says

Theoretical calculation results are based on ideal component conditions, but practically things are never ideal.

To reduce heat you may have to increase the wattage many many folds.

You can incorporate a 0.22uF/400V capacitor with a bridge and a zener network for illuminating the led without any heat.

Anonymous says

Thank you, yes i can use a X rated capacitor, just want to know the practical reason behind such a situation, can we increase the resistor to 47K with 1 watt to connect the LED directly to AC volt.

Swagatam says

Increasing the resistor value will decrease current to the LED, at 47K even an ordinary 20mA LED will not illuminate properly.

Anonymous says

I have noticed LED are used as indicator in Mosquito Repellant, directly in 230volt ac using a resistor

Swagatam says

You can try a 47K resistor and check the illumination yourself.