2 Best Current Limiter Circuits Explained

In this post I have explained 2 simple universal current controller circuits which can be used for safely operating any desired high watt LED.

The universal high watt LED current limiter circuit explained here can be integrated with any crude DC supply source for getting an outstanding over current protection for the connected high watt LEDs.

Why Current Limiting is Crucial for LEDs

We know that LEDs are highly efficient devices which are able to produce dazzling illuminations at relatively lower consumption, however these devices are highly vulnerable especially to heat and current which are complementary parameters and affect an LED performance.

Especially with high watt LEds which tend to generate considerable heat, the above parameters become crucial issues.

If an LED is driven with higher current it will tend to get hot beyond tolerance and get destroyed, while conversely if the heat dissipation is not controlled the LED will start drawing more current until it gets destroyed.

In this blog we have studied a few versatile work horse ICs such as LM317, LM338, LM196 etc which are attributed with many outstanding power regulating capabilities.

LM317 is designed for handling currents up to 1.5 amps, LM338 will allow a maximum of 5 amps while LM196 is assigned for generating as high as 10 amps.

Here we utilize these devices for current limiting application for LEds in the most simplest possible ways:

The first circuit given below is simplicity in itself, using just one calculated resistor the IC can be configured as an accurate current controller or limiter.

Calculating the Current Limiter Resistor

The figure shows a variable resistor for setting the current control, however R1 can be replaced with a fixed resistor by calculating it using the following formula:

R1 (Limiting Resistor) = Vref/current

or R1 = 1.25/current.

R1 wattage = R x I²

Current may be different for different LEDs and can be calculated by dividing the optimal forward voltage with its wattage, for example for a 1watt LED, the current would be 1/3.3 = 0.3amps or 300 ma, current for other LEDs may be calculated in similar fashion.

The above figure would support a maximum of 1.5 amps, for larger current ranges, the IC may be simply replaced with an LM338 or LM196 as per the LED specs.

Application Circuits

Making a current controlled LED tubelight.

The above circuit can be very efficiently used for making precision current controlled LED tube light circuits.

A classic example is illustrated below, which can be easily modified as per the requirements and LED specs.

30 watt Constant Current LED Driver Circuit

Assume the LEDs to be 3.3 V, 10 watt, and Supply input to be 12 V

Current of LED becomes = 10 / 3.3 = 3 amps

The LM338 current limiter can be calculated using the formula

R1 = 1.25 / 3 = 0.41 Ohms

Wattage = R x I² = 0.41 x 3 x 3 = 3.69 watts or 4 watts

The series resistor connected with the three LEDs is calculated by using the following formula:

R = (supply voltage – Total LED forward voltage) / LED current

R(watts) = (supply voltage – Total LED forward voltage) x LED current

R = [12 - (3.3+3.3+3.3)]/3amps

R= (12 - 9.9)/3

R = 0.7 ohms

R watts = V x A = (12 - 9.9) x 3 = 2.1 x 3 = 6.3 watts

Restricting LED Current using Transistors

In case you do not have an access to the IC LM338 or if the device unavailable in your area, then you could simply configure a few transistors or BJTs and form an effective current limiter circuit for your LED.

The schematic for the current control circuit using transistors can be seen below. The design is an example for a 100 watt LED current limiter, with 35V as the input supply and the 2.5 amp as the maximum current limit.

PNP Version of the Above Circuit

How to Calculate the resistors

In order to determine R1 you may use the following formula:

R1 = (Us - 0.7)Hfe/Load Current,

where Us = supply voltage, Hfe = T1 forward current gain, Load current = LED current = 100W/35V = 2.5 amps

R1 = (35 - 0.7)30/2.5= 410 Ohms,

Wattage for the above resistor would be P = V² / R = 35 x 35 / 410  = 2.98 or 3 watts

R2 may be calculated as shown below:

R2 = 0.7/LED current
R2 = 0.7/2.5 = 0.3 ohms,
wattage may be calculated as = 0.7 x 2.5 = 2 watts

Using MOSFET for Higher Current Applications

MOSFETs are more efficient than BJTs in terms of handling higher current and wattage. therefore, for applications that require high current limiting, for high wattage loads, a MOSFET can be used in place of T1.

The current handling capacity of the MOSFET will depend on its V_DS and I_DS ratings, with respect to the case temperature. Meaning, the MOSFET will be able to tolerate the amount of current defined by the product of its V_DS x I_DS, provided the case temperature does not exceed 40 degrees Celsius.

This may appear practically impossible, therefore the actual limit will be defined by the amount of V_DS and I_DS that allows the device to work below the 40 degrees Celsius mark.

The above BJT based current limit circuits can be upgraded by replacing T1 with a MOSFET as shown below:

The resistor value calculations will remain the same as discussed above for the BJT version

Variable Current Limiter Circuit

We can easily convert the above fixed current limiter into a versatile variable current limiter circuit.

Using a Darlington Transistor

This current controller circuit features a Darlington pair T2/T3 coupled with T1 to implement a negative feedback loop.

The working can be understood as follows. Let's say the input supply the source current I starts rising due to high consumption by the load for some reason. This will result in an increase in the potential across R3, causing the T1 base/emitter potential to rise and a conduction across its collector emitter. This would in turn cause the base bias of the Darlington pair to start getting more grounded. Due to this the current increase would get countered and restricted through the load.

The inclusion of R2 pull up resistor makes sure that T1 always conducts with a constant current value (I) as set by the following formula. Thus the supply voltage fluctuations have no effect on the current limiting action of the circuit

R3 = 0.6 / I

Here, I is the current limit in amps as required by the application.

Another Simple Current Limiter Circuit

This concept uses a simple BJT common collector circuit. which gets its base bias from a 5 k variable resistor.

This pot helps the user to adjust or set the maximum cut off current for the output load.

With the values shown, the output cut off current or current limit can be set from 5 mA to 500 mA.

Although, from the graph we can realize that the current cut-off process is not very sharp, yet its is actually quite enough to ensure proper safety for the output load from an over current situation.

That said, the limiting range and accuracy can be affected depending on the temperature of the transistor.

Adjustable Current Controller Circuit using IC 741

The following IC 741-based arrangement can be used if you want a preset current limit which is adjustable across a wide range.

When R9 is adjusted to zero ohms, the lowest current limit for the components depicted is around 47 mA. Add a suitable fixed-amount resistor in series with R9 if you want to set a specific high current limit.

As you can clearly see, implementing current crowbars offers the excellent method of safeguarding electronics from short circuit destruction.