Wall transformer (wall wart) voltage reduction.

Discussion in 'The Projects Forum' started by cpqfe29, Feb 17, 2010.

  1. cpqfe29

    Thread Starter New Member

    Jan 30, 2010
    I have a solar powered address sign that is more in the shade than it is in the light. It uses two rechargeable AA batteries. I would like to run power directly to it through the brick and up into my attic. It uses six white LEDs. Of course I would cut the leads to the solar panel and remove the batteries.

    I have a spare 8v 10A powered transformer that was used for a second door bell. (Don't ask me why the previous owners had separate door bells and two transformers - I replaced that system with one 16v transformer, etc)

    I was going to put a 3volt 300mA voltage regulator and components inside a box and use some 18ga thermostat wire to hook it up. I was going to research this further but...

    I have another option.
    I have unused outlets in the attic right above that area. I found an old cell phone charger (wall wart) with an output of 4VDC / 200mA.

    I believe the second option is easier, quicker, and more cost effective but I do not have the knowledge to pull this off -- yet.

    I have no specs on the LEDs. There are six LEDs but only three wires coming from that section (glued - no access) There's a circuit board near the batteries where the three LED wires go to. All three wires appear to be voltage/current regulated. So I think the 200mA output should be fine as I hope the board will regulate it. Even so most white LEDs appear to use 25mA or greater it seems. So 25mA x 6 = 150mA on the low end.



    How do I easily limit the current to 3 volts from the 4 volt sealed transformer? The wire will be cut and soldered/spliced to 18ga wire about 15-25 feet long.

    I found this calculator on line [credit given]:


    I have 10 Ohm 1/2 watt resistors. Is this enough? According to another calculator 200mA @ 4v gives me 20 Ohms resistance with 0.8 watts. I take it that if resistors will work for my situation then I will need at least one watt sized resistors.

    The only other thing that I need to wrap my head around is the +/_ in that picture.
    Is the top line (Vin) the positive side from the transformer and the bottom (Vin) line the return/negative line with a resistor going in between the two?? I like to occasionally release the white smoke but I usually do not 'cross the streams' ha-ha. But this is DC and low voltage so I am not sure.

    This is what I picture -

    _+__10ohm/1watt__|______________+ to LED PCB
    / |
    Transformer< |30ohm/1watt
    4V/.2A \_ -_______________|______________- to LED PCB

    Thank you for any help. The more that I learn the more that I second guess the simple stuff. If I am totally off base with this then please steer me in the right direction. I do try to research before posting. Sorry for the length but it should be complete.

    My 'art' did not come out right. See below please.

    Last edited: Feb 23, 2010
  2. Audioguru


    Dec 20, 2007
    Rechargeable AA battery cells are about 1.25V each, not 1.5V like a new throw away alkaline cell.

    Resistors are used to reduce a voltage when the load does not consume any current. But your load consumes at least 150mA and cannot be fed from the voltage divider.

    The 4V/200mA adapter probably has an output of 6V or more when it has a load of only 150mA because it is cheap and is unregulated so the LED circuit will probably blow up.

    Add a few diodes in series with the adapter's output to reduce its voltage.
    cpqfe29 likes this.
  3. cpqfe29

    Thread Starter New Member

    Jan 30, 2010
    Thanks for that information. I looked up unregulated adapters and I now see the light. That is good to consider. I scrapped the idea of using a transformer because I forgot that it is still AC and that I would need a rectifier.

    What I did (if anyone cares to know the outcome) :

    I used the 110v socket in the attic and used that to send power to the unit (address sign w/ backlight) using a 12v 1.2A UNREGULATED spare adapter. I then took a voltage regulator/heat sink off of an old motherboard (Micrel 29502 5 lead TTO-220) and used it instead of buying a LM375 or LM1117 or the. I did it the old fashioned way and just soldered the wires to the pins and also used some caps from the same motherboard (1500uF, 16v -- overkill but free). Used a 1 watt 240 and 220 resistor wired the way a LM375 would be (1.24 ref voltage instead of 1.25). Since the pins were 'free floating' and not on a board I used some fast set epoxy to encase the regulators pins and all exposed wiring so nothing would short.

    I was sitting at 2.6 volts which is a good number. I was going for 2.4v so this is good with what I had. The mA draw was around 45mA or so on the batteries and it was about the same with this setup. The parts fit easily into the cavity where the battery holder sat. The wires from the solar panels were cut. The regulator was running for about twenty eight hours and only gets slightly warm. I left in the built in photo resistor so that it will not turn on the light until dusk. (Masked it for testing).

    Thanks for the information and the great site.
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    Good going. However, a fuse wouldn't be a bad idea.