12 volt acid lead battery, which amp fuse?

Discussion in 'The Projects Forum' started by Tony Elliott, Jul 11, 2015.

  1. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    Hi everyone,

    I have a 12v battery (the Yuasa attached) and I need to attach a fuse between the battery positive and an oscillator. Does anyone know of the value fuse I should use?
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,809
    Depends on the oscillator. The fuse is supposed to keep the wires from melting if the oscillator goes bad, not so much to protect the battery.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Do you have any idea what the current draw of the oscillator is? That is kind of the key.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Does this battery supply power to anything other than the oscillator?
     
  5. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    The battery only powers one oscillator, it is a 555 timer square wave based, when I measured the voltage where the frequency pot is I measured 6 volts. below is a schematic.
    n41fl_datasheet.jpg
     
  6. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    It is good that you posted the schematic. The oscillator itself draws less that .1 amp, but Q2, the output mosfet can drive many amps. Based on the spec of the IRF530 with no heat sink, I would fuse it at no greater than 5 amps. A 2 amp fuse would be very safe.

    Edit: I looked this kit up. If you are going to just drive a small speaker, 2 amps will be more than enough.
     
    Last edited: Jul 12, 2015
  7. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    Thank you for your advice thats great! The oscillator will be plugged into a amplifier to power a speaker system.

     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,540
    1,251
    In that case there might be some interface issues. The output of this circuit can sink current but not source any, a requirement for an audio source. A simple fix is to connect a 1 K resistor across X2-1 and X2-2 terminals to act as a pull up resistor. Second, the output is a 12 V square wave, huge in terms of a normal audio amplifier input range. Might not be a problem for your particular amp, but don't be surprised if your amp is running full blast with the volume control set very low.
    One way to solve both issues is this:

    1. Do not install Q2.
    2. Install a 47 ohm to 100 ohm resistor from the Q2 Gate location to the Q2 Source location.
    3. Install a jumper from the Q2 Gate location to the Q2 Drain location.
    4. Reduce the size of the external fuse to something between 0.1 A and 1.0 A

    An output that is much better suited to an audio amp input will be on X2-1. Important - do not connect X2-2 to anything. The return wire (shield) for the audio cable connects to the oscillator circuit Ground. This can be X1-2 or anywhere else in the circuit labeled GND.

    ak
     
    Tony Elliott likes this.
  9. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    Thank you I'll try cutting out Q2 and seeing what happens.

     
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