Zener regulation

[Elesquare]

Hi,

I've got a cheap led light that didn't work. Supply voltage is made by zener regulation and there was a shorted diode (can't be another zener, can it? No markings, however) in parallel with zeners. After removing this, the light is working again.

What's the purpose of that diode?

[T3sl4co1l]

It's literally on a resistor and cap, and nothing else??

What markings on it?

[Elesquare]

Good morning,

after a good night's sleep it looks that I shouldn't have forgotten the second half of the circuit: there's a light sensor, too.
The mysterious diode must be making proper voltage for darlington! So with diode removed the switching point may be affected. But if I can live with it, there's nothing dangerous about that fix. Am I right?

[Seekonk]

It may not work for long if current can get high enough.  D form a reference voltage for the constant current amplifier.  Figure what current the LED should be at.  Multiply that by the 75 ohm resistor for the voltage and add 1.2V for the darlington. That will be the rough voltage for the zener.

[Elesquare]

I have no idea of led (panel) current or even number of leds. Absolute maximum current with 240V mains and 100 ohm resistor after bridge is about 1.2A. Transistors are propably BC817, so max collector current is 500mA. Thus max voltage for D is 36V?

[Fank1]

It's a classic zener voltage regulator circuit, designed to keep the voltage on the 75 ohm resistor constant and as a result the current thru the led's is regulated.

[Seekonk]

Lets throw out some numbers. Suppose that 75 ohm is a 1W.

100ma X 75 ohms = 7.5V

100ma X 7.5V = .75W

7.5V + 1.2V = 8.7V .......8.2V zener

Using ohms law you can solve this for any size resistor.  Your actual current is probably 15-20ma.

[Elesquare]

Just installed 5V1 zener and lamp seems not too bright at all in full darkness. Going to give it a chance now!

Thank you for help, all of you!