Fuse voltage drops

Discussion in 'General Electronics Chat' started by Luv2Learn, Apr 5, 2015.

  1. Luv2Learn

    Thread Starter New Member

    Dec 6, 2014
    I am new to fuses and am trying to decipher what I have read online.

    1. Is it true that a fuse in a noncomputerized circuit will have no voltage drop?

    2. Can a 2 volt drop be normal in a 25 volt circuit (assuming it's computerized).

    3. If 2 volts is too high of a voltage drop, is that always an indication of a parasitic draw? in other words, is a parasitic draw the ONLY way voltage drops on fuses become too high?

    4. How does a parasitic draw occur? Does that mean an electronic component is allowing current to leak to ground? or to another circuit?

    Thank you in advance for your help.
  2. crutschow


    Mar 14, 2008
    A fuse is just a low value resistor that will open when the current going through it causes enough I^2×R power dissipation in the fuse element to melt the element.
    That has nothing to do with whether the circuit is computerized or not.
    The voltage drop across a fuse is generally low and depends upon the fuse rating. A lower current rated fuse has a higher resistance. This resistance is usually stated in the data sheet for the particular fuse you are using.

    Where is the 2 volt drop occurring? That's too high for a fuse carrying its rated current.

    I don't understand what a parasitic draw as related to your question. Where did you read about that?
  3. dl324

    Distinguished Member

    Mar 30, 2015
    Hi Luv2Learn,

    Not sure what you mean by computerized vs noncomputerized circuits. Voltage drop is voltage drop regardless of whether it's a linear or analog circuit.

    Fuses are usually in series with the line voltage, so drops of a volt or two out of 120 or 240VAC are insignificant. I've seen low current fuses, like 0.125A, that had resistors in them. If a fuse like that was put on the output of a low voltage supply, the drop could be a problem; but not a typical application. Fuses protect things from bursting into flames; they're not really to protect circuits from damage.
    Last edited: Apr 5, 2015
  4. wayneh


    Sep 9, 2010
    crutschow nailed it, but in case it helps:
    No, there will be a finite ∆V across any conductor - including a fuse - in a circuit carrying a current. It will be small for a typical fuse, in the millivolt range. Computerized or not, doesn't matter.

    Nope, that's 8% of the circuit's power being used to make the fuse hot. You might be able to find such a situation, but it would be only a matter of time until the fuse blows.

    Unfortunately I don't understand these questions either. What's the context of "parasitic" power draw?
  5. MaxHeadRoom


    Jul 18, 2013
    There are delay fuses, fast blo, slo blo & rectifier fuses, to name a few.
    When you consider the old method of rewirable fuses, the rating for a 15amp circuit was a short length of single strand 25g copper wire.
  6. dl324

    Distinguished Member

    Mar 30, 2015
    Tektronix used carbon comp resistors as module level fuses in some of their equipment.
  7. Luv2Learn

    Thread Starter New Member

    Dec 6, 2014

    I was reading a document at the web page above. Reading this document was the first time I had heard the term "parasitic current." Searching for "parasitic current" introduced me to the phrase "parasitic draw." Also, at the web page above there is a caption for the only picture on the page that says......

    1. Computer/Module
    circuits with excessive
    voltage drop:
    2. All non-computer/
    module circuits with
    any voltage drop.

    To me item number 2 implies that there should be no voltage drops (in the mV range which the article's author tells you to set your meter at) for "All non-computer/module circuits.."

    Most of the web pages that I was reading all dealt with automotive fuses, and really, I am more concerned with fuses in electronic equipment. I don't even know if the principles are fundamentally the same in both automotive and electronic situations. ( I understand that fuses burn out when current gets too high and that that is what they are designed to do, but this whole concept of a fuse functioning at an abnormally high voltage drop and that being indicative of a particular problem in the circuit is new to me.)

    So, to rephrase an earlier question, what is happening in a circuit to make a fuse drop more voltage than it is supposed to? is it an intrinsic problem with the fuse or an indication of something going on in the circuit it happens to be in, or both?

    Thanks everyone.
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
    If your link was intended for teaching servicing it is not suprising it deals with real world situations, not idealised ones.

    It is interesting the my answer concerning connection resistance, in post#8 in another recent thread, has relevence here.


    Crimp on connectors can carry quite high currents safely, if properly made,
    But poorly made connectors can introduce high connection resistances.

    And, of course, any connection resistance intoduces unwanted voltages.
  9. Luv2Learn

    Thread Starter New Member

    Dec 6, 2014
    I read about parasitic currents in an automotive servicing webpage. The 2 volt drop is occurring across the fuse - this is a fictitious fuse; I do not have a circuit but am trying to solve a hypothetical question.
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014

    The document you reference and "parasitic draw" is specific to automobiles. The modern automobile has some systes that draw power all the time. The clocks in your radio. The alarm system. The Body Control Module. It is true that some cars have systems that may only go to sleep after a few hours.

    An example. When you turn certain GM cars off, the radio stays on until the door opens. The trunk light may go off after 20 minutes. This isn;t parasitic, but it's systems you have to wait until they "go to sleep".

    Usually a customer complains about the battery running down. It's the technician's job to verify and isolate. The main battery cable(s) is one place to start. You can verify that an excessive quiesient current exists.

    Automotive blade fuses have two test points on them that are accessible without removing them. The idea is to use the fuse as a "tiny value resistor" and measure the voltage across it, turning it into an ammeter. Not a very accurate one, but one, nonetheless.

    So instead of removing fuses, one ATTEMPTS to guess which circuit it might be by using the voltage drop.


    An aside. I;ve had this happen twice to me. A 120 VAC piece of equipment would not power up. I removed the fuse and checked it's resistance. The measurement suggested that the fuse was good. I then turned the equipment one and measured the voltage across the fuse. If the circuit was a plug, fuse and transformer and switch, why would I see 120 VAC across the fuse? Because the fuse was bad only under load. It was capable of intermittent contact internally. It never blew.
  11. Luv2Learn

    Thread Starter New Member

    Dec 6, 2014
    That's good to know! Thanks.
  12. RichardO

    Well-Known Member

    May 4, 2013
    I had a friend that had a job repairing automotive equipment that used standard fuses (not the automotive kind). He commented that a large number of the equipment failures were caused by bad fuses. His guess was that the fuse failures was caused by vibration.
  13. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    FWIW.. Just had some equipment running in the lab..
    48VDC power source.. 15A fuses running 10.5A current.
    Voltage drop across the fuse itself was .15 volts.
    Lundwall_Paul and wayneh like this.
  14. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    Older vehicles used to use 3AG fuses ot 1-1/4 x 1/8". No real issues. The blade style fuses seem to use less material.

    A co-worker had a French automobile with those European colored fuses and all he had as electrical problems. I have a lighter plug that uses those fuses. Much better than the Chinese stuff which fall out when your not looking. Case in point - an automotive USB cell charger.