Repairing AC powered white LED table lamp

[olcal]

Hi

I need to repair an AC powered white LED table lamp. The AC mains voltage is 230Vrms. The LED's are a single string of 40 white LED's in series. When I switch it on all the LED's light extremely dimly and show a rapid steady flicker. Maybe it is 25Hz flicker, I really do not know

I have done the following...
1. Replaced the 1n4007 diodes
2. Replaced the 220nF 400V capacitor
3. Replaced the 3.3uF 250V electrolytic capacitor

Even after I did the above changes there is no change. All the 40 LED's light extremely dimly and it shows a rapid steady flicker. I measured the DC voltage across the 40 LED chain. It shows 310 VDC.

Can any one suggest what the problem is ?
Should I replace the resistors also ?
Attached is the circuit diagram that I got by tracing it out from the PCB.

Regards
olcal

 

[ljcox]

Are you sure that R1 = 1M & C1 = 220nF?

Because the reactance of 220 nF @ 50 Hz is about 15k Ohm.

That seems a bit high to me.

Have you tried to make one LED glow across a battery with a resistor in series?

See how much current is required to obtain a reasonable light intensity.

 

[olcal]

I am sure that R1= 1M and C1=220nF because I have the circuit board in front of me. What do you suggest the correct values should be ? The LED lamp appears to be one of the cheap unbranded types. The LED's are fitted in a plastic circle, imagine the letter O with the LED's fitted around the periphery. If I attempt to desolder any one LED I run the risk of melting the plastic. The LED chain terminates in two wires on the PCB.

 

[Diver300]

It sounds like one of the LEDs has broken and has gone open circuit. Maybe there is a small gap that conducts at high voltage.

There has to be a problem like that because 40 LEDs would only be around 140 V.

You may have damaged C2 with the large voltage.

 

[olcal]

When the problem first appeared I had replaced both capacitors in case the problem lay there. However there was no change.

 

[alamgir80]

I think some LED damage, please check led.

 

[ljcox]

Yes, one of the LEDs may be faulty.

Can you access the LED terminals with your meter probes?

What voltage do you measure across the LED string?

What voltage do you measure across R3?

 

[olcal]

Ok, I will check

 

[ljcox]

This thread may help you.

https://www.electro-tech-online.com/threads/ac-powered-leds.116674/

I have not read it myself yet. Will do so later

 

[ben7]

Hi

I need to repair an AC powered white LED table lamp. The AC mains voltage is 230Vrms. The LED's are a single string of 40 white LED's in series. When I switch it on all the LED's light extremely dimly and show a rapid steady flicker. Maybe it is 25Hz flicker, I really do not know

I have done the following...
1. Replaced the 1n4007 diodes
2. Replaced the 220nF 400V capacitor
3. Replaced the 3.3uF 250V electrolytic capacitor

Even after I did the above changes there is no change. All the 40 LED's light extremely dimly and it shows a rapid steady flicker. I measured the DC voltage across the 40 LED chain. It shows 310 VDC.

Can any one suggest what the problem is ?
Should I replace the resistors also ?
Attached is the circuit diagram that I got by tracing it out from the PCB.

Regards
olcal

One of the leds is damaged.

-Ben

 

[unclejed613]

40x3.6V= 144V you have an intermittently open (or in the process of burning open internally) LED. the flashing is possible caused by the cap charging to the point where the bad LED begins to arc over, discharging the cap to 144V, the arc extinguishes, and the cap begins charging again. the open circuit in the LED is likely a bond wire that has separated from the LED chip, or broken inside the clear epoxy. it's very small and isn't easily visible. with the voltage applied, divide and conquer. measure the voltage across the first 20 LEDs, then the second 20 LEDs. whichever side has the higher voltage across it is the side you're looking for. then divide that set half and measure across 10 LEDs at a time. one half of that will have a higher voltage, so divide it into two strings of 5. once you narrow it down to 5, measure across each individual LED until you find the bad one (the one with the highest voltage wins).