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How determine fuse amperage?

Discussion in 'Troubleshooting and Repair' started by FredVH, Nov 16, 2018.

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  1. FredVH


    Nov 16, 2018
    Hi. I have a Sharp VC-H972U VCR whose label says "AC 120V 60Hz 16W"
    Its main 5mm x 20mm power fuse is missing and I want to find the right one.
    Would it be correct to divide the 16W by the 120V, times 1.25 and come up with a desired amperage of 0.16 amp fuse?
    Pardon my ignorance; but I'll greatly appreciate any advice.
    Thanks, Fred
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    16 W at 120 V is 133 mA. A 250 mA fuse should work.
    However, there are considerations like inrush current, temperature etc. to be considered. Normally the manufacturer puts information about the correct fuse somewhere near the fuse on the PCB, on the rating label or in the manual. Always use the rated fuse for safety reasons.
  3. kellys_eye


    Jun 25, 2010
    If your mains plug doesn't have a fuse and the fuse you're referring to is on the MAINS side of the supply then the fuse should be rated to 'save the mains cable' and not the equipment. If the cable is 1.0mm2 then any fuse up to 3A or 5A will work. It's not as if the fuse can possibly 'save' the equipment in the event of a fault!

    If, however, the fuse is on the regulated side of the supply then the appropriate value needs to be fitted as per Haralds post above.
    Arouse1973 likes this.
  4. 73's de Edd

    73's de Edd

    Aug 21, 2015
    SIR FredVH . . . . . as in VHS ?. . .

    Compliments . . . . of my inheritance of brother in laws (RIP) tech references archives, now stored inside the mule barns NORTH side wall, further buffering the cold wind for Francis.

    No more guessing . . . .1.6A . . . . and it is associated with feeding directly into a switch mode power supply.


    So, if you are not being electronically inclined nor edjumicated, I will now sub in a real world comparison.

    You are in your penthouse 18 stories up and it has an upright piano on its balcony and in some heavy partying last night it got flipped on its back and slid a bit. it just needs uprighted now and then a check on its tuning.

    That is where you are now with your blown / missing fuse (why else would someone pull a fuse unless blown . . . and then take it and expect it to be an easy-peasy snap to locate a replacement . . . ha ha).
    Now if you just find the PROPER ? fuse and pop it in and immediately turn the unit on, you very likely will be in the first piano situation, but you have caused the upended piano to slide and it is now 18 stories down and ALL O. . . .V . . .E . . R . . . the street.

    Should you want to take a nondestructive testing approach and be a smart feller . . . instead of a fart smeller . . just do this.

    Commandeer one of Mama Cass's stand / table / desk lamps (or your Droplight out in the garage) that uses a common incandescent filament lamp, with a 60-75 watt globe in it.

    Plug in " El Lampo " and " Let there be light ! " test and leave the switch turned on, then only unplug and move to where the Vee Cee R'uh has its top cover already lifted.
    Zero in on the vacant fuse clips and tilt the plug prongs such that they can pair up and safely have one blade touching one fuse spring clip.
    PRESTO . . . . Your lamp is now filling in for the electrical connections of the vacated fuse.
    To concentrate on holding your solid connection, non slipping, request that Little Duddley Do-Rite, or Polly Purebred, press the now plugged in VCR's power on button.

    Now if it has 4 diodes and 1 power transistor solid state parts inside that have so damaged as to becoming into even MORE SOLID STATE than they should be, that lamp will light up vewy bwightly.

    A normal power supply unit will have an initial glow at turn on and slightly taper down to a reduced, or no glow as the units main filter cap initially takes on its full charge.
    The display may or may not illuminate, as there is not enough power being let thru by the lamp, for full system operation.

    If you pass that last dimmed light test, you are then free to try that fuse, but, if trying in a BRIGHT lamp condition, you have SOME thousands of piano pieces to clean up.

    73's de Edd

    Shot my first turkey yesterday ! . . . . . scared the sh%# outta everyone in the frozen food section.
    Richard9025 and Harald Kapp like this.
  5. FredVH


    Nov 16, 2018
    Hey, 73's De Edd! You sound like a real kick in the pants! You talk kinda like I probably would if I knew anything.

    Yes! You correctly id'd my fuse as a 1.6A. On your VCR diagram, I noticed it said not only the number; but also1.6A and 120V.

    I went back to my circuit board and saw all 3 on it, too.
    I missed them the first time around because that info was slightly away from the fuse; but in a box with a giant arrow pointing to the fuse housing. I had thought that maybe that info was for some other position on the board.

    I lied - there was a (blown) fuse in the holder when I decided to use the VCR.
    I bought that fuse, in Slow, Medium and Fast.
    They all blew.
    That's when I decided to buy another VCR, right after figuring out whether I had the right fuse in the first place.
    Now that I know, my course seems clear.

    Just for fun, I followed your lamp instructions to blow up the whole world, and got a steady light.
    While it was not THAT bright, it was certainly steady; and so I interpreted that as meaning that something deeper was wrong with the VCR. If that's true, I think that might be beyond my pay grade and so - on to the next device.

    If you ever pass by Wrightwood, CA and have an hour or so, a free sandwich is waiting for you. You sound like maybe a fun guy to talk to. [email removed]

    I also appreciate the replies from Harold Kapp and kellys_eye. Sorry I was too ignorant to "get" your replies.
    Thanks, Fred
    Last edited by a moderator: Nov 17, 2018
  6. davenn

    davenn Moderator

    Sep 5, 2009

    That points to one or both of either the bridge rectifier or the switching transistor/IC as the most likely problem(s)

    Sometimes the switching transistor/IC will blow and take out the rectifier and fuse
    other times it's just the rectifier that dies and takes out the fuse

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