Is my wall adapter putting out too much current for my pic16f690 circuit?

Discussion in 'Embedded Systems and Microcontrollers' started by johnnyinwa, Aug 16, 2015.

  1. johnnyinwa

    Thread Starter Member

    Jun 24, 2013
    Heh guys,
    Please look at the attached schematic to this post for my question. I have a wall adapter putting out 5 volts at 300 mAmps (regulated). I am worried that my wall adapter will put out too much current for my project and fry my circuit. Up till now I have used batteries to power my projects and have no clue as to how the current output on a wall wart works. I could use some help here. ;)
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    hi Johnny,
    The PIC will only draw as much current as it needs from the power supply.
    The 300mA is only the specification for the maximum current that the PSU can supply.

    If your 5V power supply was not regulated this could cause a problem when the psu is lightly loaded, as the output voltage could be much higher than 5V
    johnnyinwa likes this.
  3. johnnyinwa

    Thread Starter Member

    Jun 24, 2013
    Thanks for your help ericgibbs!!
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    A voltage supply simply puts out a voltage. The curcuit determins how much current is drawn, up to the limit of the supply.

    This is a case where MORE is BETTER, your supply needs to be able to put out as much current as the circuit draws. Less is bad, somewhat more is good, lots more may be bad as things may tend to smoke lots worse if there is some problem.
  5. tjohnson

    Active Member

    Dec 23, 2014
    Your adapter should work fine as others have said, but I wanted to add that you should be careful. Although your circuit will only draw as much current as it needs (unlike voltage - it will always draw 5V from the adapter) if wired correctly, it can draw 300mA if it has a short in it.

    I learned this from experience. When I built a circuit powered by a 9V battery, it wasn't very serious if it got shorted for just a short time. But when I powered it from a USB port instead (5V, 120mA) and the power wires touched each other, the wires instantly started smoking and made a small flame.:eek: Thankfully the carpet my circuit board was laying on didn't catch fire and my computer's USB port still works!
  6. OBW0549

    Well-Known Member

    Mar 2, 2015
    That's not quite correct: what you have is a wall adapter that puts out 5 volts at up to 300 milliamps. 300 mA is its maximum rating-- the maximum amount of current you can draw from it before unpleasant things (such as overheating, automatic shutdown, etc.) start to occur.

    That's a non-worry. Your adapter will take care of regulating its output to 5 volts, but it does not determine how much current is drawn; your load determines that.

    Power supplies generally do not force current through whatever they're connected to; they only allow current to be drawn by whatever they're connected to.
  7. ScottWang


    Aug 23, 2012
    If you see the thing in different angle, maybe you will more easy to understand how the relationship between the supply current and the draw current of load, the supply current as the money in your pocket and the draw current of load as the money you have to pay for what you buy.
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    Everything mentioned above is correct of course, but to address the matter described by @tjohnson , some people will put a small resistor, say 100 Ω. in the positive supply line before the circuit is first turned on. That can help protect against excessive currents due to wiring errors, shorts, or just plain mistakes in design. Then, when you know the circuit doesn't draw excessive current, you remove it.

    Of course, adjust the resistor to suit what you expect the maximum current will be. The circuit doesn't actually have to work or work well with the resistor. It's just a safety device for start up, kind of like a fuse.