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How complex would it be to make a usb power source imitating small battery?

  1. Apr 20, 2012 #1
    there are some computer related gadgets out there which, courtesy of the OEM decisions, have to be used with a small battery. E.g. some of the graphics tablet related pens are that way. Perhaps in some situations (like "battery prices just went up by 1000%" situation) it might be nice to be able to mod such gadgets into not using batteries but rather using a power source rated as functionally equivalent to the battery in question.

    Let's say we would connect this power source gadget via professional "tuning" on the end user gadget, inserting a battery emulating "plug" into the OEM battery holder and supplying power to that plug via a wire going through a hole drilled in the case.

    What would be the complexities inherent in creating such a power source gadget? Is this something that can be straightforwardly prototyped in a lab and then easily made in an electronics plant? Or are these things not as easy as my naive view of it?
  2. jcsd
  3. Apr 20, 2012 #2
    I'm not sure what you mean by tuning. Do you mean modify the gadget?

    Its not that difficult at all, and you can hack it easily although it might not look professional. You can splice the USB cable at one end and solder the +5V and gnd to the power pins of the gadget. If you use a USB cable, there are a couple uknowns, but its not that hard to overcome:

    1. USB ports power pins are regulated at 5V.
    The issue here is that if the gadget is using small batteries, its voltage requirement might be too low for 5V, and so you risk damaging the device. Make sure the device can use 5V or else you will need some kind of simple regulator to get the correct voltage.

    2. USB ports are rated for 500mA current draw.
    Motherboards usually have an overcurrent detection switch that will isolate the device from the USB power if it draws too much current, so make sure your device won't use over 500mA at 5V.
  4. Apr 20, 2012 #3
    Sorry I don't quite understand your writing after reading it a few times. But it is quite common for small gadget using power from a USB port of the computer. My latest cell phone using a USB connector to charge the battery!!! I have seen device charge on USB port. There as so many little stuff that just plug into the USB port and not require battery. I have a 250M zip drive that use the power from the USB port and not require to plug into wall or battery.

    I remember I read there are specification on the power of the USB port, you know the voltage specification and current specification of the port. If you can conform to that, you can use the power.
  5. Apr 20, 2012 #4
    DragonPeter, thank you for your response. My use of the word "tuning" is inspired by "car tuning" and maybe there is a more appropriate term to this, like "modding".

    I do realize, as you have explained, that the output of USB port is likely not what the gadget expects. So, I am proposing/discussing a power source that would in fact output the current and the voltage expected from the battery. It could be power source just for that battery brand, or maybe it could be a generalized power source that could imitate several different batteries, as long as it is connected with appropriately shaped plug to the gadget.

    So, I guess, you are saying that, yeah, this is straightforward enough to do, right? USB powered power source isn't hard, the DC wire that will go to the gadget and plug into the battery holder isn't hard, and basically the gadget will never know what hit it?
  6. Apr 20, 2012 #5


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    No it is not hard. But the problem is that you can never get more than 2.5W (5x0.500)out of a USB port, and if you change the voltage using a DC-DC converter (or simply a regulator) there are additional losses (and you can still not pull more than 500 mA).

    Hence, there is a good reason for why some devices use batteries.
  7. Apr 20, 2012 #6
    I thought they sell these kind of stuff. It is quite common to have a power source with multiple connectors for different devices. I am looking at my AC power source for all my guitar effect pedals that use 9V battery!!! Am I missing something?
  8. Apr 20, 2012 #7
    f95toli, if power and current limitations you mention prevent us from using the cheap linear regulator, would a circuit that transforms USB DC to AC, steps it down in transformer and then rectifies DC for gadget input be "excessively" expensive? Or are such circuits fairly routine and doable for ten dollars from commoditized components?
  9. Apr 20, 2012 #8
    That circuit you describe is not very routine and is rather elaborate and non straight forward for what you're wanting to do, and it won't help your situation that f95toli pointed out. Your situation is that you cannot exceed the maximum power rating (voltage x current) of the USB power source. It doesn't matter what you do with this voltage on the gadget end, as long as it doesn't load the USB power source by more than its max power rating.

    As far as changing DC voltage levels, you should read about LDOs, buck and boost converters, charge pumps, and maybe a simple emitter follower zener regulator. Those are all much more common ways to switch DC voltage from one level to another than the elaborate way you described.
    Last edited: Apr 20, 2012
  10. Apr 20, 2012 #9
    hmm, then perhaps a better approach to this might be an AC adapter working with the wall outlet instead of USB power.

    E.g. in the worst case (assuming minimal circuit fabrication ability in the mod lab) using an off-the-shelf mass produced adapter, like for cell phone, and running its output through a regulator should probably add up to a very inexpensive device.

    In the better case if we could make our own custom adapter with transformer geared closer to the expected voltage without raising price too much, then of course it would be even nicer, without unnecessary heat dissipation in the regulator.
  11. Apr 20, 2012 #10
    Ya just buy something like one of these, cut off the connector and solder the + and - to the battery terminals of your device and you're good to go. http://www.powerstream.com/power2-3-6.html

    If you wanted to get fancy, you could not cut off the plug, and instead buy a mating plug and screw a hole in your battery cover and put the mating plug inside the battery cover facing out through the hole, and solder the mating plug's terminals to the battery terminals.
  12. Apr 20, 2012 #11
    Transformers aren't so important for a design like this. I would stop thinking in that mindset when you are talking about converting voltages, since these are logic level DC voltages. All you need with an AC wall powered version is a rectified AC voltage, with possible transformer step down to a certain voltage level (dont' really need to custom spec this), and then a DC-DC voltage regulator. If you use USB or some other DC voltage, rarely would you ever consider converting it to an AC and sending through a step up/down transformer and rectifying to get another DC voltage.. the transformer method you're thinking of is logical, but not very practical and also less efficient.
  13. Apr 20, 2012 #12


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    They don't use a transformer, but they do use an inductor, which can be physically small since the circuits operate at much higher frequences than the 60 Hz mains.

    Chips are available for a few dollars, e.g. http://www.maxim-ic.com/datasheet/index.mvp/id/1901 will handle inputs and outputs from 1.8V to 28V and currents up to 6A.
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