Spark with connecting power supply to board

  1. When I connect a DC power supply to a board, why do I get a spark.
    I connect the ground terminal first and then the Vcc terminal(24V). When I connect Vcc with an alligator clip, I get a spark.
    Is it because of charging of the bulk storage capacitor?
  2. jcsd
  3. davenn

    davenn 4,368
    Science Advisor
    Gold Member
    2014 Award

    quite possibly

    what is the board ? got a pic or a circuit diagram of it ?

    with the lack of info supplied, its kinda difficult to give a better answer :smile:

  4. Baluncore

    Baluncore 3,305
    Science Advisor

    Yes, the capacitors require the high current, but the high voltage spark occurs due to the inductance of the circuit when disconnected from the board once a current has begun to flow.

    As you make the connection, there are short periods where the connection is intermittent. Once a current is flowing through the inductance of the circuit, (power supply, both jumper leads and the PCB), any break will cause a voltage spike to maintain the current. V = L * di/dt.
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  5. As a follow up, how do you reduce the inductance.
    Putting a Cap at the input of the circuit should take care of it. right?
  6. Baluncore

    Baluncore 3,305
    Science Advisor

    Because the inductance is distributed about the circuit it is hard to cancel it with one capacitor.
    More capacitance just makes more current to spike a higher voltage during intermittent connection.

    I would first approach the problem by trying a simple snubber across the power supply output.
    To make a snubber, experiment with a resistor of from 47 to 150 ohm in series with a 0.1uF capacitor.

    If that does not fix the spark problem, place a snubber across the PCB also.
    Beyond that, as davenn pointed out in #2, we need more information to resolve the situation.
  7. jim hardy

    jim hardy 5,460
    Science Advisor
    Gold Member
    2014 Award

    Another approach is to connect through a current limiting resistor that charges the bulk capacitor more slowly, then short out that resistor, perhaps using a spdt switch..

    Some folks use a thermistor with negative temperature coefficient. It starts out high resistance and decreases as it's warmed by the current. That gizmo is called "inrush current limiter" and works well provided your load current is enough to keep it warm.
    Last edited: Jun 23, 2014
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