The post presents an effective method for protecting capacitive LED driver circuits through an SCR shunt regulator circuit and explains how it can prevent filter capacitors from blowing of and LEDs from getting destroyed.
The remedy was requested by Mr. Max Payne.
Protecting High Watt LEDs
Please suggest regarding 3W, 5W LED bulb circuit design fault. the LED bulb running very cool. The electrolyte capacitor (47uf to 100uf , 50v rated) keeps explode. without any reason. and wasting all LED's but smd resistor (474 & 560), MB10S, 105j400v has no problem, This is the problem pcb of the cheapest 3W, 5W LED set.
How can i solve the problem permanently or Pls suggest me few cost effective designs ... I noted that these explosion happening when i operate induction cook-top.
The Design
A filter capacitor in a capacitive power supply will mostly blow-of due to an insurge of voltage higher than its rated value. For example if the filter capacitor breakdown voltage is 50V, and if the surge voltage exceeds this limit could instantly cause the capacitor to explode or gas.
A rather simple solution to protect the vulnerable parts in a capacitive transformerless power supply such as the capacitors and the LEDs is to introduce a shunt regulator circuit as indicated in the following diagram:
Circuit Diagram
Here the SCR forms the main element for curbing the initial high voltage, high current surge and to ensure a regulated constant supply regardless of the input fluctuations.
If we recall, this concept has been already discussed in one of my earlier posts titled how to make a high current transformerless power supply circuit
Circuit Operation
The idea here is to switch ON the SCR or a triac as soon as the input supply tries to surpass the stipulated safe voltage limit of the circuit, which in turn depends on the indicated zener diode voltage.
Referring to the above SCR shunt circuit for protecting capacitive LED drivers, whenever the input from the mains capacitor tends to go above 12V, the zener diode conducts fully, triggering the SCR, and causing its anode cathode leads to short circuit the supply instantaneously. This response becomes enough to stop the voltage to rise any further and ensure that it's limited within the assigned 12V range.
The stage comprising the SCR, the zener and the 1K resistors act like a high current zener diode and is employed here since the input current is much higher and beyond the capacity of a normal 1 watt zener diode, however for low current transformerless power supply circuits the SCR and the 2nos 1K resistors may be eliminated and only the 12V zener could be used for the intended regulation.
Using High Voltage Transistor
Another effective shunt regulator could be built using a high voltage transistor and a variable zener diode, as shown below:
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