Connect with us

Help me diagnose circuit problem

Discussion in 'General Electronics Discussion' started by fscii, Nov 18, 2012.

Scroll to continue with content
  1. fscii


    Nov 13, 2012
    I have before you, the ballast circuit board from a fluorescent light fixture (27w FML bulb).

    Specs on outside of ballast say: 120v .38A 60hz 27w
    (How is that accurate when P=IV 45.6W = 120v * .38A) ?

    I have tested wire continuity from plug to switch to ballast input on both hot and neutral wires. I have checked continuity of all four
    output wires from ballast to bulb socket. It all checks out. I believe the starter exists in the base of the bulb so none exists in the fixture.

    The only reason remaining is that my ballast is bad. Now normally I'd buy a new ballast but not only is this one impossible to find but more importantly its an exercise in learning for me that I don't want to pass up.

    At some point I'd like to take this circuit and master it. In other words its AC and I haven't gotten to that point yet. I'm not sure if the output is supposed to be ac or its being converted to DC - how can I tell? Someday I want to be able to trace the circuit path and draw it out in software and understand exactly whats going on electrically. For right now I just want to identify which part (or parts) are faulty, desolder them, and solder in replacements. Can you guys help me on this?

    I will have many questions as I go along on this project and am genuinely grateful for your help.

    1. Behind each of the 13003's there is a diode that you can't see in the photograph! They test VERY low using diode test mode of meter, .007 volts (James Bond diodes?)

    2. I have 7-8v when I test the potential of one yellow wire to one red wire on the output side and ditto for the 2nd red/yellow pair. Seems way too low.

    3. I don't have any obviously cooked parts (burst capacitor, melted resistor etc) so its going to be all up to me and the meter though I do see some yellow discoloration on part of the trace beneath one of the 13003's.


    1. Do I have to desolder ANY device in order to test it (even 1)? It has resistors, capacitors, diodes, dual 13003 (mosfet?), transformer and a few parts I can't identify yet because I'm noob lol.

    2. What is the green thing next to the transformer? Looks like a green donut with 3 wires wrapped around it coming into 6 contact points to the board?

    3. What path/process of elimination would you take in trying to find the faulty component? I suspect the 13003's due to yellowing of the traces beneath one (very yellow) and slightly yellow on the other. Except I have no idea how to test them. I looked up a data sheet and found this:

    Notice it doesn't say "Hey this is a MOSFET or qfet or baked potato" nor does it say NPN or PNP but I assume (ugh) its NPN. This gives me a side question. Is NPN tied to Emitter Base Collector/ Source Gate Drain? In other words if we rearrange the pins, the name NPN remains even tho the physical layout of the pins can be NNP or PNN etc?

    Sorry for the long post but I really do want to be able to identify and repair this thing even tho its presently beyond my abilities so for now I'll be more than satisfied to test each and every part (since so few of them on the board - only 32 components in all) to identify the bad ones to replace.

    [note in photo the trace photo is upside down as I had to flip it over for photo]

    Attached Files:

    Last edited: Nov 19, 2012
  2. shrtrnd


    Jan 15, 2010
    Somebody here might help you, but I recommend against it.
    Replace the ballast with the correct part.
    You're messing with a fire hazard.
    I know you want to learn, play with it all you want.
    When you reinstall the lighting fixture, don't leave your experiement with it in the ceiling.
    Good Luck.
  3. KJ6EAD


    Aug 13, 2011
    Check the fuse.
  4. fscii


    Nov 13, 2012
    I get that but isn't every electronic thing a fire hazard when done incorrectly? But I see your point in that its 120vac not a 9v battery :p I may still replace the ballast but since it is just a small simple circuit I really would like to repair it anyway as a learning experience.

    I have a bunch of questions in the original post that I don't know.
  5. fscii


    Nov 13, 2012
    I did. Strangely enough its soldered to the board so it can't be replaced easily but its in tact via continuity test across its terminals.
  6. duke37


    Jan 9, 2011
    Power = V * I * cos(theta) where theta is the phase angle. A resistive load will give a phase angle of 0, reactive loads will give lower power even with the same current.

    A diode with 0.007V drop would be worth a fortune. I suggest you buy 1000 ballasts, take the diodes out and sell them for $1000 each! A silicon diode will drop 0.7V or so.

    I am not sure how the ballast works but some early fluorescent fittings had the filament electrodes heated with a transformer. This seems to be the same with 9V across each filament. This type of circuit is used with compact fluorescent lamps, there will be no starter.

    My guess is that the ballast is working and the bulb is faulty. Check with another bulb and also check for filament continuity at each end.
  7. fscii


    Nov 13, 2012
    hmmm I never heard of the * cos part of the formula!

    That .007 is what I read across the diodes that cross the 13003's when the unit is off. Maybe they are fried? Ok thx for info that there is no starter. This is a brand new bulb I am using.

    There is some browning of the traces on bottom of the pcb at the 13003 points im wondering if thats a normal sign of wear or showing that they are cooked?

    I have continuity from plug to switch to ballast, and from ballast to all 4 pins of the socket.

    I just checked the new bulb (doesn't hurt to check as it was shipped in the mail) and i do have continuity across the pins of the bulb itself.
    Last edited: Nov 19, 2012
  8. fscii


    Nov 13, 2012
    I talked to a guy that sells ballasts online and ordered a replacement but I still want to fix this one - to learn how. I found out that most of the compact units, instead of a starter just use capacitors to dump the voltage and start the bulb up.

    Is it possible to test components while they are still on the circuit board or must I remove each one to test them?
  9. jimbo


    Nov 15, 2012
    Testing components on a circuit board

    Please note that the voltages and currents found on this board when it is live are lethal and so any component testing should be done with the power disconnected. I also agree that modifying the circuit/repairing it could well result in a fire hazard. Use the board as a learning experience and buy a new light.:eek:

    On a more positive note:-
    Check the diodes by unsoldering one end and testing them with the diode tester on a multimetrer and you should get a 0.7v diode drop. There is a good tutorial on rectifier diodes which gives you all the information you need.

    Check the resistors by unsoldering one end and testing them with the multimeter and again there are good tutorials and even a colour code value calculator to help you.

    Check the transformer at the right of the photo by unsoldering the end of each winding and testing the winding resistance, its just a piece of coper wire and should have a low resitance but if faulty will be an open circuit.

    The donought is a choke with copper wire wrapped around a ferromagnetic core and so the wires should just be like a short circuit or piece of copper wire
    The capacitors can be tested by unsoldering them and checking them on a multimeter.

    Unfortunately the brown marks on the board at the 13003's suggested heating and as these appear to be silicon power switching transistors they are quite likley to be faulty. Unsolder them from the board and check for a diode from base to emitter and base to collector.

    There is a lot of great testing experience to be gained with this board between reading the htutorials and trying out the tests and fitting it all together in your head!

    After that keep the bits for future referenece and snip all the wires of the board for safety.
  10. Rleo6965


    Jan 22, 2012
    If fuse was still okay or not open. Replaced first the 2 electrolytic capacitor with correct value and voltage. Be sure to test terminal of capacitor before desoldering.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
Electronics Point Logo
Continue to site
Quote of the day