LED Bulb Repaired
I found few dead LED Bulbs in my Junk Box I have not tested because of busy schedule. Bulb’s domes Opened for visual inspection. All of them are of 7W.
Visually tested everything seems fine and there is no damaged /burnt sign found on any LED.
As usual first tested Voltages on driver Unit. It shows 92V DC without Load it means driver voltage is Ok and problem is with LED Circuit (Driver Voltages vary on different wattage of bulb).this voltage reduces to actual voltage when circuit is in working condition. How to find voltage of LED used on this bulb……………………
So for testing of led I took 9V battery and connected one 1.5K resistor on positive side of the battery and checked LED one by one. I found Bad LED which I marked with red dot.
Mostly in LED bulb led’s used are of 3v/6v/9V or 12V as per circuit design and as per wattage marked on the Bulb. In 2nd photograph you can see the numbers of Led used on each circuit are different 3 out of 4 bulbs are having identical LEDs and one is having 18 LEDs on it but all bulbs are of 7W.
As per standard circuit all Led’s on PCB are connected in series hence if one LED goes bad it will affect working of whole circuit. Before removing the LED I have to make one Bulb as a donor to use working Led’s. So I put a side one bulb which is having 18 Led’s on it because none of other bulb LED type is matches.
From rest of three I have to use one bulb as a donor because all of them having identical Led’s. Now here is the trick to find out voltage of led.
Connected the bulb on power now using one tweezer I “shorted that bad LED” when power is on while doing this whole bulb was light up perfectly. Now it is confirmed the problem is that bad led and it need to be replace. Same time I tested voltage on load. It gives 54V dc.
So divide 54VDc with 6 led it comes 9V of each LED. (This PCB is having 6 LED). If 7 led’s used divide 54VDc with 7 Led’s it comes 7.71V (Approx 8V) you have to consider nearby value on higher side i.e. 9V this confirms in this circuit all Led’s are of 9V. If you count for 18 led’s divide 54VDc with 18 Led’s it comes 3V each led voltage.
For desoldering SMD LED easiest way is to use gas lighter. (Don’t try to desolder LED using soldering iron you will damage the LED). I took one helping hand made out of old CFL Bulb casing. I grabbed the PCB and put PCB holder on edge of my table and with help of Lighter I heated up PCB from beneath the LED which I wanted to replace. After few seconds of heating I just try to move the SMD if solder is melted properly and when I pushed the SMD lightly it moves & I pulled up the SMD using tweezer. Same way for resoldering SMD back just heat up & when you see solder is melting just place the SMD on it & done. Be care full about polarity of LED.
PCB Holder Helping Hand……………………
Made out of old bad CFL Bulb casing & one crocodile clip which I fixed on top using Araldite epoxy. For make it little heavier I filed the hollow casing using white cement paste.
After replacing LED on PCB I plugged the bulb on power and it light up perfectly.
This way I fixed two bulbs back to the life.
This article was prepared for you by Yogesh Panchal who works as a Computer Hardware Engineer in Mumbai India.
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Note: You can check his previous repair articles on Multimeter That can’t be Repaired