Why no fuses In VOM/DMM?

Discussion in 'General Electronics Chat' started by Chris Wright, Oct 21, 2006.

  1. Chris Wright

    Thread Starter Well-Known Member

    Jul 26, 2006
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    I need to test both electric fuel pumps in my car. If the pumps are bad there is the potential that the current draw could be over 10 amps and both of my meters are limited to 10 amps. I have both a digital DMM and an old Simpson analog meter and they are both limited to 10 amps and although the DMM is fused at the lower current ranges, neither one is fused on the 10 amp range!

    Why no fuses? If I made a test lead with a 10 amp fuse, would this protect the meter or will that not work and there is there something I'm missing, which is why the manufacturer didn't use a fuse in the first place?
     
  2. EEMajor

    Well-Known Member

    Aug 9, 2006
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    Your fused test lead will work just fine.
    I see no reason not to put in a fuse, unless they are just trying to save a buck on manufacturing cost by omitting the fuse. (Or make you buy more meters by blowing them all! :) )
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You could take a foot of wire ... use the resistance of the wire and measure the voltage drop across the wire and calculate the amperes that is more than your DMM or Simpson 260 can measure.
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
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    My DMM has fuses but it's limited to 10 amps. I never needed to measure more but if I did there are attachments for that which will do much higher amps. The other method is what joe said which is exactly what your meter does.
     
  5. EEMajor

    Well-Known Member

    Aug 9, 2006
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    I don't know exactly which model of Simpson you have, but some of them do have internal resettable overload protection, perhaps yours has something like this? I learned that just by doing a search for the Simpson meters in google.
    Just a thought!
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    If you have a regular need to measure greater than 10 amps, I would purchase the clamp-on attachment. If you don't have a regular need, I would use the voltage drop method. It's your money that your spending.
     
  7. richbrune

    Senior Member

    Oct 28, 2005
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    I've been looking for a clamp-on meter that works on DC, and I'd like a model number Rich
     
  8. Chris Wright

    Thread Starter Well-Known Member

    Jul 26, 2006
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    Thanks for all of the replies,

    EEMajor, I said old Simpson didn't I?. I should have said antique Simpson, it's a 260 series 2, circa 1940's/'50's. I have a PDF of the original manual plus I've been poking around inside it trying to solve a problem with the Ohm meter not reading right, so I know that there is no internal protection.

    Mrmeval, My DMM also has fuses on the lower ranges, but NOT on the 10 amp range. I'd be curious if you could look in your manual to confirm that there is a fuse on this range, especially If it has a separate plug for the 10 amp range like mine.

    JoeJester, I don't need to measure more than 10 amps, only about 6/7 amps, it's just that I can see the possibility of the current spiking over 10 amps on start or a bad pump drawing more because it is worn, which is why I want to protect the meter.

    As to using a length of wire as a shunt, it made me think of an old automotive meter (Dwell, tach, etc.) that I have kicking around somewhere that I think has a cable with a flat copper bar set up as a shunt like you say. Even if I find it, I'll still try the wire, it sounds like a good trick to have.

    I had already trotted out an Amprobe clamp-on ammeter and was disappointed to find that is was apparently broken until I started kicking myself as I remembered it ONLY WORKS ON A/C! :rolleyes: They do make Hall Effect clamp-on DMM's that work on DC. It seems like all of the inexpensive DMM clamp type add-on accessories are for A/C. Does anyone know of an inexpensive hall effect clamp-on DMM accessory?

    The Simpson doesn't have any fuses for anything so at least it is consistent. What I can't understand is why the DMM would only protect some ranges and not others unless there was a reason, like the fuse can't blow fast enough to protect the circuit at 10 amps or something?
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I believe the old SEARS DWELL/TACH meter had the flat copper bar you can use as a meter shunt for testing out the charging system. At least I remember it having one when I used one this summer working on a boat motor.


    Your back to protecting your meter with the meter lead inline fuse ... a fast blow 10A model.
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I think they don´t use fuse on 10A range, because the fuse is the nasty thing that gets blown quite often when measuring currents, and I hate changing them all the time.
    The 10A circuit should be capable of taking serious current for a short time, so you can measure higher than 10A spikes (the meter may show overflow) without damaging the meter or changing the fuse. And if you do overheat the shunt wire and melt your meter, be sure it is the 4.99$ type.
     
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