Stepping down amps but retaining voltage

Discussion in 'General Electronics Chat' started by wario, Sep 17, 2010.

  1. wario

    Thread Starter New Member

    Sep 17, 2010
    I have a small battery powered device (several blinking LEDs in a circuit) that require two 3V 30mAh "button-cell" batteries. I would like to power these with a transformer. I have a "wall-wart" style transformer that outputs 6V at 600mA. What type of resistor (if that is all) do I need to place in the circuit to reduce the current from 6V 600mA to 6V 30mA. I used an online calculator and the online book on this website, and I am guessing a 10 Ohm resistor might work, but am unsure of the wattage required on the resistor as well. Any help is appreciated. I did not have any electronics classes back in school :)
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    If the circuit needs 6 volts then you must give it six volts. It will draw only the power that it requires. The 600ma rating you quoted is the MAX. that the wallwart could put out. NOT the amount it has to put out.

    6 volts is the pressure pushing it. and the amps are the amount of electricity that flows. its like a water faucet in your house.

    If you only turn the valve on just a little bit, a very small stream of water comes out. (Low amps). If you open the valve all the way, the maximum amount of water the pipes can deliver will come out.

    The pressure(voltage) is what ever the city water main pressure is. It remains the same no matter how much or little you open the valve.
    wario likes this.
  3. Audioguru

    New Member

    Dec 20, 2007
    But a cheap wall-wart is not regulated. Its output is supposed to be 6VDC at 600mA but might be 12V at only 30mA. Then the 6V circuit might blow up.
  4. BillB3857

    Senior Member

    Feb 28, 2009
    The first thing you need to do is find the current draw from your circuit while using the button cells. The internal resistance of that type cell may well represent a fairly large portion of the overall current limit function for the LEDs. One way to tell if they are indeed limiting the current is to measure the voltage without your circuit being ON and then measure again with it turned off. If there is much of a difference between the two voltage readings, the batteries are helping to limit the current.
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    So begins a journey into understanding electronics. Next time around young grasshopper "knows" he needs a wallwart that is Regulated at 6 volts. They are more expensive than "cheap" ones. Knowledge is almost never free. :)
  6. Chrisoborski


    Oct 12, 2009

    This Is WHAT you Need..

    Read This. This is what i used to build an LED Circuit. For me at least this was the most clear cut. I found it to make the most sense. I used these formulas, I found it to be the most digestible. I'm not sure what other formula's you were using or others advice ,but this reminded me of this web page i have saved on my computer. That's how much i like it.

    Scroll down abit to get to the formula, Check it out! [​IMG] [​IMG] [​IMG]
  7. Wendy


    Mar 24, 2008
    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers

    One of the problems you may face is while not obvious the button cell batteries have an internal resistance. So if you get a 6V power supply you will still need a resistor. LEDs always require a resistor, they get around it with the small batteries by using the internal resistance build into the battery.

    The resistor, not the voltage, is the thing. If you meet the minimum voltage you need to power the LEDs you can use any voltage you want, and adjust the resistor accordingly.

    So if you want to use the 12VDC inside a computer it is no problem. I've done this.
  8. wario

    Thread Starter New Member

    Sep 17, 2010
    Thanks everyone for your help! I think I have a much better understanding of what to look for and how to make my circuit work. I like the two LED websites--they will be great for reference of formulas and examples.

    Thanks again!
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    Ohm's law.

    Its not only a good idea, its the law. ;)