Measured amps and fuse don't match.

Discussion in 'General Electronics Chat' started by Bjornk, Apr 15, 2013.

  1. Bjornk

    Thread Starter New Member

    Jan 28, 2013
    12
    0
    Hi.

    I have a problem I just can't figure out. I trying to measure the amps of a small motor with a pump (rs 360sh). I measure the motor to have around 1.4 amp went the motor is pumping water. When I block the hose so the water can't flow through I measure the amps to be 1.5. At 1.5 amps I want a fuse to blow. But when I insert a 1.5 amp fuse it won't blow. So I tryed with a 0.8 amp fuse I didn't blow eather, but when I insert an 0.5 fuse it blows streit away. This seems very strange. Also I tryed to stall the motor, then I measure around 8 amps.

    Thanks for any suggestions or tips.

    Bjørn
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Fuses have a time curve stated on the datasheet...
     
  3. Mike33

    AAC Fanatic!

    Feb 4, 2005
    349
    25
    Right -there are fast blow, slow blow, etc. Maybe a breaker would be a better choice, altho fuses ARE cheaper.

    Try a fast blow of about 2A - but be advised that when you start the motor, you'll have inrush current that will surge to rather higher levels than you might expect, so you may find that you just blow fuses right off! :eek: It might be a useful experiment, though. To let you know you're on the right track.
     
  4. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    On a motor circuit I use slow blow fuses, a fast blow fuse of the same current rating will mot likely blow this is caused by the inrush current which is much higher that the 1.5 running amps.
     
  5. #12

    Expert

    Nov 30, 2010
    16,338
    6,823
    Fuses just aren't that dead accurate that you can use them to differentiate between 1.4 amps and 1.5 amps with any reliability. If you really want to blow the fuse at 1.5 amps, you will have to build a current measuring circuit and use that to blow the fuse, possibly with a crowbar circuit. Then, of course, you will have to make sure it has a delay time to allow for the start-up current surge.
     
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