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Misc Electronic Questions

Discussion in 'General Electronics Chat' started by Billy Mayo, Apr 4, 2013.

  1. Billy Mayo

    Billy Mayo Member

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    Where do you put milliohm resistors in a circuit at mostly?

    I sometimes see Milliohm resistor in circuit , but where do you put them in a circuit?
     
  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Values of that size are typically used for current sensing and they MAY be 4-terminal resistors.

    This http://www.newark.com/simpson/06700... Products&MER=PPSO_S_C_Resistors_PrivateLabel is one that's a little on the wierd side, but it is a 4-terminal resistor. Note the scres and the Bolts on the resistor. Between the screws is a precision resistor. The bolts are use to attach the wire carrying the current.

    They can also be used when transistors are placed in parallel and they are used in series with the emitters of some transistors.
     
  3. Billy Mayo

    Billy Mayo Member

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    yes i do seem them at work used on the emitters or a FETS source

    why do they use milliohms resistors on the emitters or FETS source pins to ground?

    Current sensing what? what is current sense used for?
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Emitter resistors are described here: http://en.wikipedia.org/wiki/Common_emitter

    Current sense: Dunno why it might be used. If this were a motor, measuring current would conceivability be necessary.
     
  6. Billy Mayo

    Billy Mayo Member

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    When have u seen milliohm resistors used?
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Nearly all decent class AB audio amplifiers.
     
  8. Billy Mayo

    Billy Mayo Member

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    In a class AB amplifier , where do they put the milliohm resistors mostly? they either use the milliohm resistor to do current sensing or to act like a fuse

    Do you know any other circuits that uses milliohm resistors like a fuse or current sensing?
     
  9. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  10. Billy Mayo

    Billy Mayo Member

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    Where have you seen these fuseable resistors at in circuits? are they in series or in parallel?
     
  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Billy Mayo,

    I believe it would be more helpful to everyone, if the you created a new Thread with a specific question about a Topic, rather than this never ending 'Misc' Thread.

    The reason being, is that if any member does a Forums Search on a topic, this Thread will keep appearing in the listing and he/she will have to trawl thru hundreds of Posts in order to find the required information being sought.

    Eric
    Moderation.
     
  12. Billy Mayo

    Billy Mayo Member

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    Today at work I have noticed that when One Decoupling cap is backwards it will pull down the Rail voltage, why does it pull down the DC buss rail voltage

    They have multiple Decoupling Caps from Vcc to Ground, if one of the Electrolytic Capacitor is put in Backwards it will pull down the VCC to a lower voltage

    12vdc VCC, if one of the decoupling caps is put in backwards it will pull it down to 10 volts or lower , but why?

    When an Electrolytic capacitor is in backwards it acts like a short or open?
     
  13. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    That decoupling cap is likely a Tantalum capacitor. They are polarized. They don't like surges either. Trash the cap and install a new one in it's place.
     
  14. Billy Mayo

    Billy Mayo Member

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    No, They are electrolytic capacitors, they get put in backwards by accident

    When One of the electrolytic capacitors is backwards, it pulls down the VCC voltage, why is that?

    The VCC's are either 12 volts or 5 volts

    If there is a electrolytic capacitor put in backwards by accident it will pull the VCC down lower

    I just don't know why it pulls down the voltage when a polarized capacitor is put in backwards

    When a Polarized capacitor is reversed in a circuit it acts like a short or open? or how does it operate when its put in reverse or backwards?

    What is this called? is this "reverse biasing" the electrolytic or polarized capacitor?
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  16. Billy Mayo

    Billy Mayo Member

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    My Manager said that when you put a Polarized Capacitor in Backwards it acts like a Resistor, that's why the VCC rail voltage sinks and gets pulled down

    Why does it act like a Resistor when a polarized capacitor is in reverse polarity?
     
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    From the link above "This is because a reverse-bias voltage above 1 to 1.5 V[4][5][6] will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Following the loss of the dielectric material, the capacitor will short circuit, and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst, often in a spectacularly dramatic fashion."

    Here "short circuit" is relative. For a capacitor autopsy see: http://www.pa4tim.nl/?p=1385
     

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