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protection for short circuit

Discussion in 'General Electronics Discussion' started by plumber1969, Feb 15, 2013.

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  1. plumber1969


    May 7, 2011
    Hello im a newbie to electrionic`s but keen to learn
    After spending much time reading on forums and google i have put my circuit together but now im stuck

    Im simply now trying to prevent a short circuit going through a negative wire

    In the past when connecting up the battery source v4 to the mosfet i have t wired it up to the negative of the alarm by mistake and blown them up .

    Ive tried using a blocking diode on the negative wire of the alarm but that dosnt allow the circuit to complete ,i can turn the diode round to allow the circuit to comlete but that dosnt prevent postive from the v4 source

    Is there a diode that allows 0 voltage through to comple the circuit but will prevent any voltage going through ?would a scokkty diode work?

    Is there any sort of fuse that pops as soon as voltage is detected

    I have attached a picture of my circuit

    Thanks for any help
  2. BobK


    Jan 5, 2010
    I am having trouble understanding what you are asking. It sounds like you are trying to prevent damage when a battery is hooked up backwards, but then you keep talking a short circuit. These are two different things. A diode would be all you need to prevent the first problem. If it is really a short circuit you are trying to prevent, what is it that is causing the short circuit?

  3. duke37


    Jan 9, 2011
    You do not have any current limit through the leds.
    The input voltage is not specified, if the signal produces high voltage pulses, you might have popped off the fet.
    Please show the entire circuit.
  4. plumber1969


    May 7, 2011
    protection short circuit

    Thanks for the replies
    I can see i have not explained myself properly
    Im using car parking units to power up some leds
    The leds are less than a watt they do all light up

    By removing the buzzer piezo the wires have a variable voltage 0 -11
    ie bleep
    bleep bleep

    Im simply trying to connect all the postives and all the negatives to power a mosfet to power up a some leds

    if the postives are link together as one parking unit detects it send that voltage up the other postives and blows the up the unit ,from experiance,hence the blocking diodes then connecting together .

    If the postive touches the negative direct it blows up the unit

    The voltage for the units is 12 volts

    Where im stuck I have wired the mosfet up incorrectly in the past and made contact with the 12 volt from led side direct to negative side of the buzzer wire whilst connecting to the mosfet and blown them up

    Problem one
    I just want to know if i could put something on the negative buzzer wire to protect it from myself mainly just incase i touch it with a 12 volt live again

    Problem two

    I also have which is not in the attached diagram a switchable control panel with indvidual switches for all five parking units.The negatives to the parking units are all linked together then connect to the battery and the postive to there own switch then linked then to a battery

    I have wired up just two units without the mosfet test .

    BOTH positive and negative buzzer wires are now connected

    If i turn on both parking units no problems sensor detect and buzzer sounds

    But if i turn only one on unit by the switch The negative buzzer wire form the unit that has been switched off gets postive voltage across its negative buzzer wire and making the buzzer make a slight sound

    Hope this explains it better thankyou again for any help

    Attached Files:

  5. john monks

    john monks

    Mar 9, 2012
    I dont't see any switch in your schematic. I'm having difficulty getting a handle on what's going on. Is the switch in the negative lead of your Car parking unit?
  6. duke37


    Jan 9, 2011
    You have 16 leds all in series. If each one needs 2 or 3V then a 12V battery will not drive them.

    A piezo is driven with a high voltage, this may be more than the fet can stand. Put a 12V zener gate to source to limit the voltage. A capacitor gate to source may be advantageous to lengthen the ON pulse.

    As a test, disconnect the piezos from the fet and connect the gate to ground, the fets should be off.

    You would be wise to keep the ground connected and put any switches in the positive line.
    Connect the gate to 12V, the leds should be on.
  7. plumber1969


    May 7, 2011
    alarm problems

    Hi thanks for all the replies its difficult to explain the word come out quicker than i can wirte lol

    Leds are all wired up at home and all come on flashing

    Piezo is on side of battery not mosfet

    Have put resistor on gate and source to ground used a 1k resistor works fine stopping lights coming on

    Switch to each parking unit are on the live side of battery not shown in diagram

    Ok my problem is self created by connecting all the negatives from all the buzzer wires together as i get reverse voltage to negative via the other units somehow

    The problem im now stuck on after doing some more testing is how can i link the postive and negative buzzer wire together individually and then to one wire to give me that voltage to open a gate on a mosfet

    Can i use a mosfet to connect a postive and negative only and then use the other pin to power or open another gate on another mosfet

    Or is there an electronic component and that can handle 0 to 11 volts variable that can then simple send of that voltage in a single wire to open a gate on a mosfet

    Thanks Tony
  8. plumber1969


    May 7, 2011
    alarm problem

    Hi Again

    Just detected why the negative wire become live which i switch one unit off

    the power drain from the unit is going through the negative on the buzzer wire

    can i short this which a resistor

    Thats Tony
  9. duke37


    Jan 9, 2011
    The battery negative lead should go to the fet and all the buzzers and sensors. Do not short anythig!

    The negative lead goes round in a loop, this is not a problem if things work but if they do not, you do not know which way round the current is coming.
  10. plumber1969


    May 7, 2011
    alarm problems

    Thanks Duke

    I thought the power drain would of just gone through the negative of the unit to the battery ?

    As it drains through the negative of the buzzer wire as well this causes a problem for me as i have link them altogether any voltage going into any of negative wires blows them up

    When i said short circuit i meant with a resistor to to ground the drain quicker

    Last edited: Feb 17, 2013
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