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Thread: OT - Need sparky for tecnical question - florescent lamp ballast

  1. #1
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    Default OT - Need sparky for tecnical question - florescent lamp ballast

    Greetings all:

    I have a couple of portable florescent work lights that use a F382D/835 lamp (like this one: https://www.1000bulbs.com/product/53768/FC38-38011.html ). One of the lights quit working a year or so ago - worked fine one day and did not light up the next day. Today while hibernating in my basement I finally took the light apart to find the problem. The "ballast" has what appears to be a tiny fuse (wire thru a glass sleeve, labled F1 on the PCB) with a small dark spot (and no continuity) - rest of the board visually looks OK. So I temporarily jumpered this fuse and the light functioned correctly.

    So how do I size the replacement fuse? The lamp is 38W so would that imply a 0.35A fuse? Are there typically losses in the circuit that should be factored in and the fuse upsized by some factor? Or can I temporarily jumper the fuse and use my Amprobe to measure the current on the lead as it enters the PCB to determine the fuse size? All of these questions presume that the fuse was marginal in capacity and or quality and that there is no other problem on the board.

    Yes, I know this is a cheap "disposable" light but I dislike throwing stuff away that can be fixed (cheaply, if Radio Shack were still around; less so if I need to mail order a fuse).

  2. #2
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    1) Check that fuse over for markings. I'd be surprised if there were no markings stamped into the ends.

    2) Measure the current it draws. You know it will have to be rated higher than that. Depending on just what the ballast is, the fuse may be rated at some multiple of the actual running current.

    If the fuse blew, it either was incorrectly sized, OR it is fine, and blew due to a surge just as it should, OR it was just a defective fuse. I would tend to assume it was poorly sized, or blew as it should.

    Fuse sizing is best done by checking the turn-on surge and duration. But, since that takes some equipment and judgement to measure, one can do some estimates based on the current draw, which may be decently close. Fuse rating is not a precise calibration.

    Generally, a fuse within a decent range of the right size will be fine, it will pass normal current, and it will blow in case of a genuine fault. Nobody counts on a general purpose fuse to blow at exactly a certain current, fuses have tolerances, and there are physical limits on what is possible. The fuse works by melting, but you cannot let it get too close to melting in normal use, or during a turn-on surge. So there is always a range of fuses that will function.

    That is not an excuse for putting in a fuse rate at 10x the optimum rating, however.
    1601

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    Hashim Khan

  3. #3
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    I doubt that the value is all that critical. You calculated about 1/3 Amp for the operating current but there will/may be some inrush current when it is turned on. I would try a 1 Amp fuse and see how it does. If it blows, try a 2 Amp. If there is a serious short, the circuit breaker would blow so you should be safe. This fuse is probably to prevent that from happening for a problem in the light so with multiple lights on one circuit only the bad one would go out.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  4. #4
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    I would not exceed 1 amp, but it might be OK to use a time-delay slow-blow fuse if surge currents are high. Most modern ballasts are essentially switching power supplies that have a rectifier and good size capacitor, probably 400V and maybe 50-100 uF. Ballasts also often have surge suppressors across the line after the fuse, and repetitive surges will cause them to draw current and eventually short.

    I would not rely on the distribution panel breaker to provide much protection. A 15 amp branch breaker may take an hour to trip at 20 amps and several seconds at 60 amps. Instantaneous is likely over 100 amps, and the line cord will probably melt before it trips.

  5. #5
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    If you have another working fixture that is the same look at the fuse. You can get a pretty good idea by the size of the wire in it.
    If it looks like a piece of hair it's probably 1 amp or less. If it's a flat strip than it's probably well over 1 amp.
    Like previously mentioned I wouldn't go over 1 amp. I have seen some solder in place fuses that have no markings on the ends.
    I've also seen some that have markings that don't correspond to amp ratings. Internal mfg. codes probably.

    JL...............

  6. #6
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    Sounds like an F fuse.

    Fuses do get tired, and perhaps that one blew for no other reason. I would just put in a 1 or 2 amp fuse and then keep an eye on the light for a while. If there is some intermittent short you will still have protection.

    A great many devices have a thermal fuse in them, usually buried in motor or transformer windings. If your ballast has an intermittent short, and the replacement fuse doesn't blow, the thermal one will still work to prevent the temperature rise from becoming a fire hazard. That's assuming that it has one of course-

    Just to mention- fuses are connected in different ways. Some clip into a holder, some have press-on end caps that are pre-wired for soldering to a pc board or to terminals- others are meant for you to solder wires directly to the ends. If you have to solder directly to the fuse, keep it all clean and do the soldering quickly so you don't blow the fuse just by installing it.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  7. #7
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    Thank you all for your responses.

    Yes, this fuse is tiny - the glass is (eye ball measurement) about 0.070 to 0.080 in diameter with a very fine small gage wire coming out of the ends. The leads of the fuse are soldered directly to the PCB. I was concerned about exceeding 0.5A, but based on several of your responses, I'll look for a fuse with a value approaching 1A.


    New info - I just went to the basement to check the value of the big cap to respond back to PStechPaul, afterwards I flipped the pcb over and looked at all the tiny caps (each about the size of a grain of rice). When I got to C12 it was missing - I could see where the remains of the cap were soldered to the PCB. I'm guessing that is why the fuse blew. So even thou the lamp lights up when I jumper the blown fuse, I would imagine a new fuse will fail again if I were to try to solder in a comparable fuse with the PCB missing the (damaged) cap.

    Once again - thank you all for responding (and educating me a bit).

  8. #8
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    Good that you have another one like it to see what that cap is supposed to be.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  9. #9
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    Quote Originally Posted by darryl View Post
    Good that you have another one like it to see what that cap is supposed to be.
    The blown off cap (rice grain sized) is surface mounted. I have seen others solder similar surface mounted components but this is beyond my skill level and cost/value is suddenly tilting away from trying to repair it - I'll keep the lamp as a spare for the other light (with my luck the same cap will fail in the future on the other light).

  10. #10
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    PostScript

    Since I already had a 0.5A fuse and a 5 pack of 1.25A slow blow fuses, I decided to see how the light would function with one (small) cap missing. After 2.5 hrs of illumination, I tried to turn the light off using the inline cord switch - except I turned the switch the wrong direction. There was a very brief sizzling sound and then darkness. I had fried the fuse again. Installed a 1.25 slow blow fuse, light works OK again. Now I intentionally turn the cord switch the wrong direction and the lamp dims, flickers and makes a sizzling sound. So now I no longer have an inline cord switch, but I do have a functioning light again.

    Just loaded tapatalk after reading about it in a recent forum discussion. This post is in large part my trial of the app and how to load images with the spp. First image original fuse just above the black power lead on the PCB. Second image is the backside of the PCB blown cap C12 to the far right. Third image shows the ungainly fuse holder I replaced the OEM fuse with. Forth image, functioning well, no noticable flicker and no noise on the nearby FM radio with the one missing cap.

    Sent from my 5049W using Tapatalk

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