Simple Voltage Drop Question....

Discussion in 'The Projects Forum' started by davekeck, May 19, 2006.

  1. davekeck

    Thread Starter New Member

    May 19, 2006

    I'm working on a small RF project that requires 2.4 volts. For testing I'm trying to rig the device up to a power adapter that I cannibalized from some old computer equipment. The power adapter outputs 5 volts DC, so based on some voltage divider circuits I found online I was able to drop the voltage to 2.4 volts using two resistors.

    The problem is, the device simply doesn't turn on when connected to the power adapter, but does when connected to batteries. I've verified the power adapter is putting out 2.4 volts on the dot, so I'm at a loss to see what the matter could be; I'm assuming it must be the current.

    Assuming it is the current, would the problem be the current is too high, or too low? I'm guessing it's too low since the circuit hasn't been fried when attempting to use the power adapter with it... so if my assumption is correct and the current is too low, I guess my question is:

    How can you reduce the voltage of a circuit, but meanwhile retain the original current?

    Hope this question isn't too obvious, heh :)


  2. Battousai

    Senior Member

    Nov 14, 2003
    A simple voltage divider won't regulate the voltage to 2.4V: That is, when you draw 0 current from the divider tap it will be 2.4V, but as soon as you begin to draw current, the voltage will drop because you pull the current from the battery through R1, dropping additional voltage.

    You could try making the values of R1 and R2 very small so that the voltage drop suffered across R1 is negligible, but then you would be drawing A TON of current from the 5V battery, you would probably need flameproof resistors for any significant load.

    Here are some really crappy ways to make a 2.4V reference:

    -Take your 5V battery and simple tie two-three diodes (0.7V) in series with a resistor to ground (5V to anode1, cathode1 to anode2, cathode2 to anode3, cathode3 to a resistor to ground) . Connect your load to anode3.

    -Set your voltage divider to 3.1V, and then use an NPN 2N3904 transistor and connect the collector to the 5V supply, the base to 5V supply and tie the emitter to a resistor to ground. Your 2.4V will appear at the emitter of the NPN.
  3. richbrune

    Senior Member

    Oct 28, 2005
    can you send a drawing of the the reference with the 2N3904?