basic dc current limiting question

Discussion in 'General Electronics Chat' started by aseb, Jan 10, 2012.

  1. aseb

    Thread Starter New Member

    Jan 10, 2012
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    0
    A very basic question- I know about current limit resistor when using leds but I'm unsure about a DC charger. A friend has a small rechargeable kitchen appliance but the charger has been misplaced. The appliance is marked as 3V 150mA but not battery type. I've looked for suitable mains adapters and found them with 3V 300mA, 500mA, 800mA etc. Can a higher rated charger just be used or should a current limiting resistor be used? How do I calculate it's value since both the supply and appliance voltage ratings are the same?
    Thanks!
     
  2. Robert.Adams

    Active Member

    Feb 16, 2010
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    5
    Was the charger that was lost a simple wall wart? I'd be careful about trying to hack together a replacement since the previous charger may have used specific charging (constant current, constant voltage, etc) profiles to prolong the battery life of the appliance.

    Also for a battery charger you can't simply use a current limiting resistor. A battery has different requirements for current draw and voltage at different states of charge. An LED works with a current limiting resistor because you want to keep it at a set forward current and voltage drop. For battery charging, these parameters change as the battery charges.
     
  3. colinb

    Active Member

    Jun 15, 2011
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    You don't current-limit a 3 V supply to reduce its current output. 300 mA is the maximum output current but the actual output current is determined by the load device.
     
  4. Stuntman

    Active Member

    Mar 28, 2011
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    This is an interesting topic that doesn't get spelled out well very often.

    First and foremost, know that there are two common low-cost DC converters that work in very different ways.

    First, the infamous wall-wart, known for cluttering up power strips and wall outlets for years. These are the large, usually heavy, blocks that plug into the wall. These generally use a simple transformer (hence the big heavy block) with rectification and varying degrees of smoothing. The trick to these, is they often give a (roughly) constant power output. Ie. If the wall-wart is rated at 5VDC @500mA, then hooking this supply to a load that that draws 250mA, the voltage will be roughly 10V. In practice, I have found these to be far from truly linear, but the principle still applies.

    These supplies are arguably wasteful (you probably remember unplugging one of these and finding it quite warm), and are largely being phased out by switching power supply technology (next paragraph) and due to the one watt initiative that is lowering the wasted standby power of household electronics.

    Second, the switching power supply is really a win-win. Not only are these "plug packs" much lighter, smaller, and waste less energy to heat, but they also are designed to provide a stable output voltage regardless of current (with current/power limitations of course). These supplies are widely used and readily available. One good source is discarded cell phone chargers.

    So, to answer the question. If you find a wall-wart that meets your voltage requirements, you probably won't need to limit current, but if anything, actually bleed enough current to ground so that the supply works for your device (provides the correct voltage). If you find a switching power supply (which I would recommend), then makes sure the voltage is correct and that the supply current exceeds the demand current.
     
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  5. aseb

    Thread Starter New Member

    Jan 10, 2012
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    Thanks guys. I've found some that say eco, using less power, so I assume that these will be the switching type as well. I believe that the original supply was small and very light so it also seems to fit.

    Your help is greatly appreciated!
     
    Last edited: Jan 10, 2012
  6. colinb

    Active Member

    Jun 15, 2011
    351
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    Well, the main point to remember is that the current rating of a power supply is just a maximum! They are nominally constant-voltage supplies, so as long as the current rating is at least what the circuit demands, any supply should be OK.

    However, be aware that many are unregulated, meaning that the output voltage is not exact and will likely vary a lot depending on the load current. See Jon Chandler's Wall Wart Analysis for details.
     
  7. aseb

    Thread Starter New Member

    Jan 10, 2012
    3
    0
    Yes, I'm aware that the voltage could double when using an unregulated supply with a much lower than rated current draw, that's why I wanted to develop my knowledge wrt the rated currents of regulated supplies- not worth risking a fire!
    Thanks
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    We need to distinguish between ordinary power supplies and dedicated chargers.

    • Typically, power supplies have more or less a fixed output voltage, though some are more stable than others.
    • Chargers may have tailored current / voltage characteristics.
    I would advise a little caution when replacing a power supply used for charging. If the original device is a constant-voltage type, no harm will be done by replacing it with one of a higher current rating. If, on the other hand what you have is a current-limited charger, replacing it with a higher current rated model could cause damage.
     
  9. colinb

    Active Member

    Jun 15, 2011
    351
    35
    Once you get the power supply (a switching supply is probably best and should provide reasonable regulation if it's a quality product and not a cheap knockoff; see Crappy PSU Analysis) you can check its open-circuit output voltage and also measure its output voltage under various loads using some power resistors of varying values, to satisfy yourself that it won't put out over 3 V and cause a problem.
     
  10. colinb

    Active Member

    Jun 15, 2011
    351
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    Good point, Adjuster, thanks for bringing that up. Perhaps the device being charged could be opened up and the battery type identified, and perhaps see if there is a charge controller in the product or just a direct connection to the battery from the dc input jack.
     
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