12 volt supply to 5 volt system

Discussion in 'General Electronics Chat' started by JoGe, Feb 5, 2018.

  1. JoGe

    Thread Starter New Member

    Feb 5, 2018
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    Someone checked a 5v max DC resistor with a 12v DC supply. The circuit was said to be ok, but the next day tested again by someone else and it was dead.

    My recollection is that you can't do that, only with amps, but the person who tested it swears up and down that as long as it is grounded the volts will dissipate.

    Can anyone confirm/refresh my memory and point me in the right direction to resolve this through an authoritative reference?

    Thanks
     
    Last edited: Feb 5, 2018
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Welcome to AAC!

    Resistors typically have much higher working voltages. When checking a resistor with a 12V supply, you only need to worry about power dissipation.

    Since you don't provide resistor value or wattage, can't give any useful information.
     
  3. JoGe

    Thread Starter New Member

    Feb 5, 2018
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    At the moment all I have is the manufacturer's 5v max specification.

    I will try to get more.
     
  4. WBahn

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    How about a link to the manufacturer's specifications?

    What's the manufacturer and part number?

    What does it mean for this resistor to be dead? What is it doing/not doing that makes you think it's "dead"?

    How much current can this 12 V supply deliver?

    I have no idea what it means for the "volts to dissipate" and how that might relate to whether something is grounded or not.
     
  5. JoGe

    Thread Starter New Member

    Feb 5, 2018
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    12,672 watts (12v @ 1056 amps) were put through the 5v max circuit.

    Does that help; the manufacturer will take some time to get info from.
     
  6. panic mode

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    that is a lot of amps... i don't recall seeing more than 100A on a 5V rail... ever...
    can you post photo?
     
  7. WBahn

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    That means that the 12 V supply is more than capable to delivering a lot of power to that resistor.

    How much current is the 5 V resistor rated for? Assuming it's less than about 440 A, you dissipated nearly six times the power in it that it was rated for.
     
  8. Reloadron

    Distinguished Member

    Jan 15, 2015
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    12 V / 1056 Amps = 0.01136 Ohms

    I tend to think something is very, very wrong with all of this. When we apply 12 Volts across a resistor the current through the resistor will be equal to the Voltage divided by the Resistance. You would need one very, very powerful 12 Volt supply to deliver 1,056 Amps.

    While resistors do have a working voltage it is seldom as much concern as the power rating of the resistor. If for example I apply 12 volts across a 2 ohm resistor the current will be 12 volts / 2 Ohms = 6 Amps. The power is equal to the Voltage * Current so 12 Volts * 6 Amps = 72 Watts so to comfortably dissipate the power the resistor should be rated well above 72 Watts. Using the same 2 Ohms with % volts applied I get 5 Volts / 2 Ohms = 2.5 Amps and 5 Volts * 2.5 Amps = 12.5 Watts.

    You need to brush up on or understand Ohms Law as well as what Power expressed in watts is all about as well as the formulas used to derive it. Now if I apply 12 volts to a system designed for 5 volts bad things are likely to happen. The circuit, if unprotected will draw excessive current likely and some things may smoke or simply fail depending on the circuit.

    Ron
     
  9. WBahn

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    Actually not that uncommon for many industries. Industrial plating supplies routinely deliver thousands or even tens of thousands of amps at well under 5 V.

    The system we used for testing superconductors had a 4000 A, 4 V plating supply in parallel with a 1000 A battery supply (powered by 2 V submarine batteries) in parallel with a 10 A battery supply (powered by a car battery). The 1000 A supply was used to supply the first 800 A and, after that, to cancel out the noise in the 4000 A supply. The 10 A supply was used to cancel out the noise in the combined supply. The result was that we could get 5000 A with noise that was down in the tens of milliamps range.

    The busbars mounted on the wall were 6" x 1/4" OFHC copper and there were four paralleled in each direction. When operating near full current they would be warm to the touch (and, yes, you could put your hand across the supply without risk of injury -- but we didn't because it injected very noticeable noise into the measurements).

    Typically all of this current would be eventually going through a superconducting wire that was between 0.5 mm and 1.0 mm in diameter and even then we couldn't drive it normal until we had a background field that was at least several tesla.
     
  10. JoGe

    Thread Starter New Member

    Feb 5, 2018
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    Just found some info online:
    Rated for 4.4v & 25 amps

    Edit: the supply was verified to: 12.82 volts & 1,056 amps (it is a fairly powerful supply)
     
    Last edited: Feb 5, 2018
  11. Reloadron

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    Do you have a link? 4.4 Volts & 25 Amps is exactly what?

    Ron
     
  12. WBahn

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    And they certainly exist.

    When using just our 1000 A supply (2 V max output) we used a current sensing resistor that was 0.01 mΩ. It was a big, honkin' serpentine resistor that dissipating 10 W of power.

    We used that same resistor for the 5000 A combined supply and had to go to rather herculean efforts to keep in cool enough so as to not mess up the current measurement (and it was specifically designed for current measurements at those kind of currents, but not with the accuracy we needed).

    Fortunately, our duty cycles were low so we could let it cool between ramps, but each ramp might take 30 seconds or so, it still got toasty by the end of the measurement.
     
  13. WBahn

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    Just what kind of resistor is this? What is it used for?

    Online where? Please provide a link!
     
  14. JoGe

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    Feb 5, 2018
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  15. Reloadron

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    Yes and I have used them in plating applications as you mentioned. They are however not very commonplace. They are also used for large welding power supplies. My point was the initial post made little sense to me. I have also used plenty of high power resistors as loads for aircraft generator test but again they are not something we see everyday. This is where I got lost:
    "12,672 watts (12v @ 1056 amps) were put through the 5v max circuit". That would be an interesting resistor. :) I am missing something in all of this? :(

    Ron
     
  16. be80be

    AAC Fanatic!

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    GE105: Glow Plug
     
  17. WBahn

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    I'm hoping that we can get some clarification about what this "5 V max circuit" is. Is it just a resistor, as indicated in the OP? Or is it really a more complex circuit of some kind.

    The rating of 4.4 V and 25 A would amount to 110 W if it rated voltage at the same time as rated current is possible. But it would also mean that, if it's a resistor, we are talking about something in the vicinity of 175 mΩ. Apply 12 V across that, and you only get something under 70 A.

    The only thing that makes any sense to me, at this point, is that we are talking about perhaps a 10 mΩ current sensing resistor that is part of a larger circuit and they applied 12 V across the naked sensing resistor. The heating effect might have raised the resistance so that it only allowed 1056 A instead of the nominal 1200 A, or the 12 V supply could be limiting out. Or I could be so far off that none of that makes any sense.

    But what does it mean that the resistor was "dead" the next day?

    I wish it weren't SO hard to get the relevant information about these things.
     
  18. sghioto

    Well-Known Member

    Dec 31, 2017
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    Someone connected 12 volts across a 4.4 volt Glow Plug and burned it open. If you are going to apply 12 volts then you need a .304 ohm resistor in series rated a minimum of 200 watts.
    SG
     
  19. Reloadron

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    Well I figure if it is dead then some arrangements are in order. This likely should be done by immediate family and close friends. If it was a resistor of sorts maybe cremation is in order? I guess if the original poster provides more information we may actually get a handle on what this is all about. Until then I shall remain Clueless in Cleveland.

    Ron
     
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  20. BobaMosfet

    AAC Fanatic!

    Jul 1, 2009
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    If you put a glow-plug that is rated for 5V between the poles of a 12V power-supply, it will eventually kill it. The volts do not 'dissipate', there is no such thing.

    Considering volts as a measure of how badly electrons want to flow from point A (PSU negative) to point B (PSU positive) is the key. If your glowplug is rated for 5V, it means the materials it is made up of are able to withstand up to 5V potential and work properly. Once you exceed 5V, you are putting stresses on the glow plug that are beyond what it's materials and design allow it to handle, so it will in time fail.
     
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