Help with converting from battery to wall wart!

Discussion in 'The Projects Forum' started by miahg17, Jan 17, 2011.

  1. miahg17

    Thread Starter New Member

    Jan 17, 2011
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    Hello everyone. I am new to these forums and fairly new to circuit building. I am currently trying to convert an LED children's toy that operates on 3 AAA batteries to power it from a 5v wall adapter. I know that 3 AAA's in series is 4.5 volts. Would a single resistor placed in front of the wall adapter's cable to bring the voltage down .5 suffice or is there something more efficient/ stable that I need to do? Any input is greatly appreciated.
     
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    A simple 1N4001 diode will drop the voltage by ~0.6V
     
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  3. miahg17

    Thread Starter New Member

    Jan 17, 2011
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    Thank you for the timely response. From what I understand about diodes (very little but learning daily) this will not only drop the voltage but should regulate the voltage to protect from any power spikes that may occur from the wall transformer. Is this correct?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Wall-warts are generally not regulated, although some are.

    They'll have specifications such as "6v @ 300mA". Such a wall-wart might measure 9v with no load on it, and within ~10% under load. They might have small capacitors in them, but you'd really need an external capacitor to provide more smooth output.

    Do you know how much current this toy requires?
     
  5. miahg17

    Thread Starter New Member

    Jan 17, 2011
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    Without cutting into the materials of the toy I cannot find specifics, it just says 3*AAA @1.5v. The wall wart is an old cell phone charger 5vdc @ 550mA. I was planning on soldering the wall wart cables to one of the battery connections and just soldering the other two battery connections with a small gauge wire + to - . Like I said I'm new and seeking info and definitely don't want to short out the toy. It's a Cloud B Twilight Turtle if it helps. I've googled for specs but can't find anything. I will probably just have to cut into it. If a generic regulator can be build without knowing amps then a link or specific parts would be very appreciated. And from from one Sgt O' Marines to another, Semper Fi!
     
  6. SgtWookie

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    Jul 17, 2007
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    Have you measured the output voltage of the cell phone charger with no load on it?
    From the Google search I did, looks like those Cloud B turtles project various star constellation patterns on the ceiling - and they aren't cheap, either.

    If you don't have a multimeter, I suggest that you pick one up. Harbor Freight Tools has some very cheap DMM's (digital multimeters) that work just fine for hobby stuff - they're like $4 on sale.

    How long does a set of alkaline batteries last? That'll give a good indication of the turtles' current usage.

    S/F, Brother :)
     
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  7. miahg17

    Thread Starter New Member

    Jan 17, 2011
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    Okay so I tested the output voltage of the wall wart multiple times with both of my multimeters and it peaked at 5.14 vdc each time. I read up a bit and a lot of cell phone chargers are now regulated (switched transformer was another term I found). With this finding I think it is safe to put a diode or resistor in the circuit that will knock off .5 or more volts like the diode Bill mentioned above and it should be fine. That said, I also found information on using a 7805 Dc/Dc converter/regulator for 12vdc to 5vdc. Since I'm finding out this is a pretty standard conversion in electronics I think I'll buy the parts to build this with a 12vdc wall wart since I'll need this knowledge for future use anyway. I'll just throw a resistor on the end to drop .5 volts. Of course, if I had mentioned 12vdc in the original post this is probably the answer I wouldve gotten up top. Thanks for the help.
     
  8. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    You tested the wallwart without a load? I doubt that .14 volts without load is going to mean much once you connect to your toy. But maybe a more senior member can weigh in to confirm.


    You should be far more concerned about polarity. You do not want to hook up the wallwart backward to the toy. It may not have polarity protection.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    OK, you can get away with a 1N400x series diode (1N4001-1N4007) in the connections. It must be a regulated supply after all.

    [eta]
    An LM317 is a more versatile regulator than the 7805 or 7812.

    With an LM317, you use a 120 Ohm resistor (call it R1) from OUT to ADJ, and then another (call it R2) from ADJ to GND.
    Your output voltage will be roughly 1.25v + (R2*0.0104)
    So, if you want 4.5v out, then R2 = (4.5-1.25)/0.0104 = 3.25/0.0104 = 312.5 Ohms.
    That's not a standard value of resistance. Here's a page that helps you calculate resistors in series and parallel;
    http://www.qsl.net/in3otd/parallr.html
     
    Last edited: Jan 17, 2011
  10. miahg17

    Thread Starter New Member

    Jan 17, 2011
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    Yes the 5.14vdc was without load, and thank you for emphasizing polarity. I've made a few mistakes in the past in regards to polarity and have since learned to follow directions for all components precisely, and if none are supplied I ask experts like those here.

    Thanks. I'll look into the LM317. As far as R2 is concerned I guess I'll use a value < to be safe since it's not a standard value.
     
  11. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Sorry I was ASSUME ing you needed 5 V and you need 4.5 V so a diode or voltage regulator is a good idea.

    Don't ask me how I know about the importance of polarity. :) And a well regulated supply for that matter. :)
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Most cellphone chargers are USB 5v regulated, so very nice for this application.

    Me, I'd probably just hook it up directly and not worry about the over-voltage. Polarity yes, voltage no. But if the toy is valuable and you really want to be careful, the diode is a good way to go. A resistor could work, but unlike the diode, ∆V across a resistor depends on current. We don't know what that is and it may vary during use.
     
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