In this article I will show you how to build a very simply 220 V or 110 V operated 1 watt LED driver circuit,that would cost you not more 1/2 a dollar, excluding the LED of course.
I have already discussed capacitive type of power supply in a couple posts, like in LED tube light circuit and in a transformerless power supply circuit, the present circuit also utilizes the same concept for driving the proposed 1 watt LED.
In the circuit diagram we see a very simple capacitive power supply circuit for driving a 1 watt LED, which may be understood with the following points.
The 1uF/400V capacitor at the input forms the heart of the circuit and functions as the main current limiter component of the circuit. The current limiting function makes sure that the voltage applied to the LED never exceeds the required safe level.
However high voltage capacitors have one serious issue, these do not restrict or are not able to inhibit the initial switch ON mains power in rush, which can be fatal for any electronic circuit LEDs are no exceptions.
Adding a 56 Ohm resistor at the input helps to introduce some damage control measures, but still it alone cannot do the complete safeguarding of the involved electronics.
An MOV would certainly do, also what about a thermistor? Yep, a thermistor would also be a welcome proposition.
But these are relatively at the costlier side and we are discussing a cheap version for the proposed design, so we would want to exclude anything that would cross a dollar mark as far as the total cost goes.
So I thought of an innovative way of replacing an MOV with an ordinary, cheap alternative.
What is the function of an MOV? It's to sink the initial burst of high voltage/current to ground such that it is ground before reaching the LED in this case.
Wouldn't a high voltage capacitor do the same function if connected across the LED itself. Yes it would surely operate the same way as an MOV.
The figure shows the insertion of another high voltage capacitor directly across the LED, which sucks the instantaneous influx of voltage surge during power switch ON, it does this while charging up and thus sinks almost the entire initial voltage in rush making all the doubts associated with a capacitive type of power supply distinctly clear.
The end result as shown in the figure is a clean, safe, simple and a low cost 1 watt LED driver circuit, which can be built right at home by any eletronic hobbyist and used for personal pleasures and utility.
CAUTION: THE CIRCUIT SHOWN BELOW IS NOT ISOLATED FROM THE AC MAINS, THEREFORE IS EXTREMELY DANGEROUS TO TOUCH IN POWERED POSITION.
In the above shown simple 1 watt led driver circuit, the two 4.7uF/250 capacitors along with the 10 ohm resistors form a kind of "speed breaker" in the circuit, this approach helps to arrest the initial switch ON surge inrush which in turn helps to safeguard the LED from getting damaged.
This feature can be replaced with an NTC which are popular for their surge suppressing features.
This enhanced way of tackling the initial surge inrush problem could be by connecting an NTC thermistor in series with the circuit or the load.
Please check out the following link for knowing how to incorporate an NTC thermister in the proposed 1 watt LED driver circuit
Another interesting way of controlling the involved switch ON surge could be by using a SCR crowbar circuit stage as shown below:
Here it is fixed at 4.7V, which implies that the voltage across the SCR anode/cathode terminals can never exceed the 4.7V value.
This further ensures that the connected 1 watt LED would be always getting a safe voltage of around 3.5 to 4V which is quite manageable for the LED, provided it's mounted over a suitable heatsink.