Safe to pulse power supply over rated current?

Discussion in 'The Projects Forum' started by jellytot, May 11, 2015.

  1. jellytot

    Thread Starter Member

    May 20, 2014
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    I have a 2 Amp power supply. My project normally draws less than 0.5 Amps, but about 5 times a minute a Solenoid activates for a fraction of a second and pushes the total power draw to around 2.3 Amps (for a fraction of a second). Is this dangerous, or is it fine? My project has an MCU, could there possibly be reset issues when the Solenoid hits (because it's going above rated current)? Should I buy a 3A power supply, or is that unnecessary?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What kind of regulator in your supply?
    How big a filter capacitor can you put near the solenoid to supply the transient load?
     
  3. jellytot

    Thread Starter Member

    May 20, 2014
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    RE: What kind of regulator in your supply?
    It's a wall-wart power supply. I don't know what regulator is in the supply. If it helps, the supply is a "ag2412-b". It feeds the voltage regulators in my project to power the different device components, and the voltage regulators have bulk and decoupling capacitors on them.

    RE: How big a filter capacitor can you put near the solenoid to supply the transient load?
    I'm still quite new to electronics, so I'm not sure what you mean. I have a diode on the solenoid to absorb kickback voltage. The largest capacitor I have on hand is 470uF.
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    Yes you can.

    There is often a large margin to accomodate hot weather, no ventilation and so on.
    More even, the engineer didnt calculate it much.
    So the spec. is just a tested value, or the rating of some parts.

    20% is normally safe for a longer time, pulses could be double current.

    The average current counts for linear supply, while for switching, the saturation is the limit.
    HV switchers also have a tendency just to explode (the small ICs), it happens on a surge, for instance when you turn it on and it gets max, voltage instantly, and also there is abnormal condition at the same time.

    So these ICs are rated 700v or 800v.

    Overloading such ICs does increase the chance for failure, but pulses once the IC is ON, are OK.

    You can short circuit smaller transformers for a minute, larger ones heat up fast,
    while a really big Toroid will start smoking after 2 seconds.

    You could tap it very briefly and hear some weird noise, dont do it, the wires will weld together....

    2

    You can buffer the MCU with a diode and a capacitor so its unaffected by the surge.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
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  6. jellytot

    Thread Starter Member

    May 20, 2014
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    Thanks for your post.
    I already had that in place, but I'm not sure I have them in the right configuration. Can you please tell me if this looks/sounds correct:
    I have a large capacitor across VCC and GND on the MCU. Capacitor is positive on VCC and negative on GND. Diode placed between bulk capacitor (positive end) and positive power feed. Small decoupling cap 0.1uF placed between bulk capacitor. (Ignore the periods, I have to put them there because spaces are deleted)

    .................................../--[+ bulkDecouplingCapacitor -]--\
    ................................./.../--[+ smallDecouplingCap -]--\.....\
    (+ power) --[-diode+]-------------[vcc MCU gnd]----------------(- power)
     
  7. takao21203

    Distinguished Member

    Apr 28, 2012
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    sure that works. Note any IO can drain off current if it connects to lower voltage.

    its not a full short but can draw a lot of current. A controller works even with quite mad supply waveform, like almost a sine with some DC bias. But its pheripherals like A/D of course not, as well writing to FLASH will fail.

    If you do that, check actual voltage and use a capacitor as above.

    A/D needs its own reference if you use switcher supply they can have 50mV ripple and more,
    and capacitors dont filter it well.

    Worse even are IR receivers.
     
  8. jellytot

    Thread Starter Member

    May 20, 2014
    72
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    Thank you! But what is A/D? And what does "need its own reference" mean?
     
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