Power adapter voltage?

Discussion in 'General Electronics Chat' started by frank55, Jun 19, 2015.

  1. frank55

    Thread Starter Member

    Dec 6, 2013
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    Hi guys; can you tell me what is the correct load Voltage on a Power Adapter,
    for example i have here a 3v DC 200 mA power adapter when i test it on my Fluke it reads about 7v,without load, is this too much ?
    i rarely find a Power adapter that reeds the exact voltage that is in the adapter specs always have 2 to 3 volts or more ,what is too much voltage?.Thanks
     
  2. #12

    Expert

    Nov 30, 2010
    16,248
    6,744
    Depends on the load. Will the load break when it gets too much voltage?
    If you have a switch between the power supply and an appropriate load, the voltage will drop into range instantly when you flip the switch on.
    Demanding a power supply act correctly with no load?
    Only if you have something better than the cheapest you can buy.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
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    If this is the older Wall-Wart etc type supply these typically were linear supplies and relied on the rated load to bring the voltage down to the stated V.
    Recent ones for some time now are practically all SMPS and should measure the label voltage.
    Max.
     
  4. frank55

    Thread Starter Member

    Dec 6, 2013
    253
    10
    Thanks; i found this very small adapter out in the street on top of a garbage can, this could be a phone charger or something else i don't know,
    it's a Class 2 power supply input 120VAC 60hz 7w output 3vdc 250ma
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    Unregulated vs regulated power supplies..
    Most new ones are regulated (aka they will output their marked voltage regardless of load)
    Most older ones and still some today are unregulated.. (aka they will output their marked voltage only when loaded properly)

    If its a 3V and you measure 7V without a load its unregulated (or your meter is broken :) )
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Most of the ones that utilize a real transformer are unregulated, some are AC out, some have just one diode (half-wave), some have just a bridge rectifier, some have the rectifier(s) and an electrolytic filter capacitor, which is usually relatively small, so when load, there is a lot of ripple...
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    7W in and only 0.75W out implies an old-type non-SMPS unregulated supply.
     
  8. frank55

    Thread Starter Member

    Dec 6, 2013
    253
    10
    So this is a Phone charger? can't use it in anything else?
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Most phone chargers I have seen are between 5 and 7V. More likely a rechargeable flashlight charger..
     
  10. #12

    Expert

    Nov 30, 2010
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    I remember, I have an old charger rated for 3 volts. It went with some computer speakers that also ran on a pair of C batteries. Presto, a reason for that charger to live. :p
     
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