Transformerless Constant Current LED Driver Circuit

In this post we learn how just a single IC MBI6001 can be used as a transformerless constant current LED driver circuit for illuminating a chain of many LED in series.

The MBI6001 series of ICs are designed to work with mains AC inputs and convert it into a lower voltage DC output which may be suitably used for driving a group of series connected LEDs.

The IC features a pulsed current PWM output which enables setting of current to the precise levesl as per the rating of the LEDs.

The IC marked N1x are specified to operate with 110V AC inputs while the N2x series with 220V inputs.

Using the IC MBI6001

Warning: Circuits explained below are not isolated from mains AC, and therefore are extremely dangerous to touch in the powered and open condition. You should be extremely careful while building and testing these circuits, and make sure to take the necessary safety precautions. The author cannot be held responsible for any mishap due to any negligence by the user.

Referring to the standard transformerless constant current LED driver circuit using the IC MBI6001, we can see hardly any external components being used except a few resistors.

Here the resistors R1, R2, and R3 help to determine the correct PWM setting for achieving the intended constant current output from the IC.

The values of the resistors are recommended by the manufacturer and may be used as per the given instructions. We’ll talk about this in the later part of the article.

How many LEDs can be used at the Output.

The number of LEDs that can be safely used at the output of this IC is actually not critical. One may use any number of LEDs across the shown output pins of the IC, the voltage across the series is automatically adjusted by the ICs internal circuitry.

However the maximum combined forward voltage of the connected LED series cannot exceed the the input AC voltage value, otherwise the light from the LEDs may get reduced and dull.

Selecting Constant Current Limit for the LEDs

As explained earlier the IC uses PWM for controlling the current to the LED, and this may be set as per the requirement or the maximum safe limit of the LED string.

The above is determined by the various resistors included externally with the IC and is implemented by either increasing the PWM duty cycle or by decreasing the duty cycle of the PWM.

However 90mA is the highest amount of current that may be achieved from this IC, that implies high watt LEDs cannot be used with this transformerless constant current LED driver IC circuit.

Also, above 23mA the IC might start heating up, reducing the overall efficiency of the circuit, therefore above this limit the IC must be stuck with a piece of aluminum heatsink in order to maintain optimum response.

LED Specification Chart

The following table shows the values of R2 which may be appropriately selected by the user as per the preferred LED specs.

The resistor R1 may be replaced with a 1K resistor and is not much critical, although its purpose is intended for fine tuning the intensity of the connected LED string, therefore may be tweaked a bit for getting the desired intensity from the LEDs.

R3 is optional and may be simply omitted, its use is restricted for some advanced requirement and may be ignored for general application as described above.

Using a MOSFET

If you find the above mentioned IC obsolete, you can try the following universal MOSFET based constant voltage, constant current transformerless LED driver circuit.

The series bulb can be eliminated if the load current is within the MOSFET's handling capacity.

R2 can be calculated using the following formula:

R2 = (Supply Voltage after bridge - LED total forward voltage) / LED Current