ampere usage

Discussion in 'General Electronics Chat' started by gamba, Aug 27, 2012.

  1. gamba

    Thread Starter New Member

    Aug 27, 2012
    I have a problem grasping Ampere usage... is ampere used "as needed" or is there some fixed value being pushed through?

    suppose I have a 192 led array connected in parallel, each using 20ma.
    the amount of "on" leds is unkonwn. it could be that only 1 is on so that the whole array is using 20ma or all the leds are on thus needing 3.84A.

    I want to control the array, turning everything off, or allowing power through using an arduino. since an arduino pin can only source about 40ma - I would need a transistor capable of sourcing 4A.

    my question is - what happens when only 1 led is turned on while the transistor is capable of sourcing 4A? will the ampere be used "as needed", just 20ma, or will things go boom?

    another question - if the above setup is ok, is an amplifying transistor what I need? one with a gain of ~200 (considering I put 20-40ma in the base)? or maybe a different type?

  2. takao21203

    Distinguished Member

    Apr 28, 2012
    Learning how to control a large number of LEDs is a process which can take years in some cases.

    Giving well-meaning answers can even be more confusing than the original questions.

    Simply you can not use 384 LEDs directly through Arduino. The chip does not have so many digital I/O lines.

    You need extra circuitry.

    I only want to give the information that often digital MOSFETs are used for such circuits, among other components.
  3. gamba

    Thread Starter New Member

    Aug 27, 2012
    well, the arduino is only used to control the transistor - either giving power to the base, and the whole array or not.
  4. elec_mech

    Senior Member

    Nov 12, 2008
    Welcome to AAC Gamba.

    The amount of current (amperes) used by a load is determined by the load, not the source (power supply). If something requires 1A and you use a 15A power supply, only 1A will be used. You only run into trouble if your supply cannot provide enough current to the load, e.g., your load requires 2A but your supply can only supply 500mA. Then the supply will get hot, pop a fuse (if there is one), or maybe burn up.

    Voltage is another subject altogether, but we can address that later if you still have questions.

    So, if your supply can provide 5A and your transistor can safely handle 4A, but you're only powering one LED, no problem. Be sure you use a current-limiting resistor on each and every LED in a parallel configuration. I do not know your background, so if you're not familar with this, please let us know and we'll explain in greater detail. Your LEDs will burn up without the resistor regardless of your power supply.

    I would verify your supply can safely supply at least 5-6A and your transistor or MOSFET (more below) can handle at least double your expected maximum load, in this case 4A x 2 = 8A. Why? If you use a transistor at its maximum current rating, it's going to get very, very hot and require a heat sink and possibly a fan to prevent it from burning up. Using a transistor with a higher current rating will help address this. Alternately, you could use multiple transistors or MOSFETs in parallel so you don't need one large, potentially hot, transistor to handle 4A.

    Transistors are great, but they do require some forethought and calculations to get the base resistor correct and to verify they will work with different voltages (if that is the case here).

    I'd recommend a logic level MOSFET. These work with logic level voltages, usually 5V (I'm not use if the Arduino uses 5V or 3.3V), and allow you to work with higher voltages, if needed. The great thing about these is that you don't need to worry about calculating or adding a base resistor. Unlike transistors which are current-controlled devices, MOSFETs are voltage-controlled devices. This also means they consume less current in your circuit.

    We may be able to help further if you can post a schematic of what you have in mind. I'm not sure how you'd use a transistor to control all of the LEDs but only have one LED on unless the LEDs are also controlled with something else. You can go as simple as making a rough drawing on paper and taking a picture of it. A schematic goes a long way here.

    Hope this helps.
    Austin Clark likes this.
  5. takao21203

    Distinguished Member

    Apr 28, 2012
    OK let say, Amperage flows as caused by circuit.

    You may want to clear up basics of Ohms law.

    Electronics is not difficult but understanding of Ohms law is required.
  6. #12


    Nov 30, 2010
    Look at the top listings on the "Chat" page where your question is posted. Find, "Ohm's Law for Noobies". Read it.