SCR Shunt for Protecting Capacitive LED Driver Circuits

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 destryed.  The remedy was requested by Mr. Max Payne.

The Request

hi swagatam,
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.

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:

SCR Shunt for Protecting Capacitive LED Driver Circuits

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

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.

17 thoughts on “SCR Shunt for Protecting Capacitive LED Driver Circuits

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Thx swagatam. nice work. limited space in smd board to implement that, instead i added C1 on another side parallel to existing one & R1 series to LED line, but I'll test that circuit, and Happy Ramzan!

    Iwanna post some interesting Electronic articles here, pls gv me ur gmail id.

  3. I am trying to use MB10s with an 8mm 2Chip LED so that I can use the LED without worrying for the polarity but the brightness is very less. What can I do to increase the brightness. Input voltage is around 5V AC

    • rectifier bridge will drop 0.7 + 0.7 = 1.4V, so 5 – 1.4 = 3.6V, that looks fine if the LEd is the normal 3.5V rated

      by the way the MB10 is rated only at 0.8A, and will be extremely hot at this full current level.
      what is your LED amp rating??

      you may have modify the bridge current accordinly

  4. Hello swagatam,

    here in this circuit X rated capacitor used to step down the voltage ….but for the 5uf 400V capacitor how much voltage will get drop down ….this has any calculations ?? how much voltage will get before bridge and after bridge….

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