Three 1.5 AAA vs. 4.5 wall wart

Discussion in 'General Electronics Chat' started by TexAvery, Mar 12, 2010.

  1. TexAvery

    Thread Starter Member

    Oct 7, 2009
    58
    0
    I am powering two LED fans with a 4.5 VDC 600 mA. wall wart instead of three AAA batteries.
    The fans spin a bit faster compared to using three AAA batteries that the fans were originally designed for. After 3 hours the LED's burn out.
    I think the current is too high compared to three AAA's.
    If this is correct how can I reduce the current?

    BTW... I added a zener diode but now the voltage is 3.7 VDC.. too low to make all the LED's light.
     
    Last edited: Mar 12, 2010
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    There is likely high ripple on the wallwart, you are only seeing the DC component. Add a large capacitance (for testing purposes) and measure the DC.

    Remember, it is the load that dictates how much current is pulled. If there are pulses hidden in the pulses then they will be current spikes.

    What is the value of the zener?

    The zener is the best evidence that you aren't using filtered DC.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The wall wart outputs roughly 4.5v when there is a 300mA load applied. The problem is likely that your LED fans require far less than 300mA current, so the output voltage of the wall wart is too high, resulting in your LEDs getting fried.

    What does the output of the wall wart measure with no load on it?

    How much current does one of these fans use when running from three AAA batteries?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I like Wookie's answer better overall. Have you measure the voltages while running this?
     
  5. TexAvery

    Thread Starter Member

    Oct 7, 2009
    58
    0
    4.5 Vdc 1600 ma. wall wart (I am worng about the 600 mA. original post)
    No load = 4.66 v
    Load = 4.48 v
    Diode = 1N4148
    When diode is in series voltage=3.7
    Each fan current must be over 200mA. because my toy multimeter only goes to 200mA. and it reads OVERLOAD
    with a rapid beep-beep
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
  7. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    You can get a suitable multimeter for less than 10 bucks from Harbor Freight (I've seen them on sale there for $4).
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It is best to measure current by measuring the voltage drop across a 1 Ohm resistor.
    If you measure current directly, you stand a very good chance of blowing your meter's fuse - or the meter itself.

    If you place a 1 Ohm resistor in the current path, and measure the voltage drop across the resistor, you get a direct correlation from Volts to Amperes.
    This is because Ohm's Law states:
    I=E/R, or Current in Amperes = Voltage / Resistance in Ohms
    If R=1, and I=E/1 ...
     
    PackratKing likes this.
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, the Harbor Freight meters DO have a 10A setting, but you have to plug the positive lead into the 10A jack instead of the V/Ohms/mA jack. That's pretty typical for any meter though.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The cheap fans were made in ***** which is a country where junk is made. You are lucky that they lasted 3 hours.
     
  11. hwy101

    Active Member

    May 23, 2009
    91
    28
    Ha Ha Ha...so true, i miss the days when electronic devices were actually made in north america, they were built to last, then that cheap junk from overseas stated to flood the market and now people are just dumping that crap into our landfills because it quits working in less than a year.
     
Loading...