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  • OT - Electrical Question

    My neighbor and I have a six-pack riding on the answer to this question.

    We talking about the electrical power consumption of devices in the home. The item in question were low-voltage converters for recharging electronic devices like cell phones. The ones which use 120 VAC input and step it down to DC power that charges the battery of low-voltage electronics. He contends that when the device is disconnected from the charger, there is no electricity flowing in the converter because the device has been removed.

    I contend there is electricity flowing in the converter device as long it is plugged into the wall, even though the device being charged has been removed. The only way to stop the current flow in the converter is to unplug the converter from the wall. This would then reduce the power consumed by devices in the house.

    The analogy I tried to explain is a simple transformer with a single primary and single secondary. If power is applied to the primary, a magnetic field will be produced as long as power is applied to the primary, independent of what is happening to the secondary electrical current produced. I contend the low-power chargers are exactly the same. He says not.

    Would appreciate any help in this discussion.
    Last edited by BigBoy1; 09-12-2010, 10:56 AM. Reason: Spelling
    Bill

    Being ROAD KILL on the Information Super Highway and Electronically Challenged really SUCKS!!

    Every problem can be solved through the proper application of explosives, duct tape, teflon, WD-40, or any combo of the aforementioned items.

  • #2
    You are correct. A charger will consumer power when plugged in regardless if it is charging a device or not. Although the current drawn is very very small, there is current being drawn when not actively charging.

    Comment


    • #3
      Originally posted by Richard-TX
      You are correct. A charger will consumer power when plugged in regardless if it is charging a device or not. Although the current drawn is very very small, there is current being drawn when not actively charging.
      That's what I've also heard. To completely stop power consumption it must be unplugged. Ever notice how the lights stay on cordless power tool chargers.

      Comment


      • #4
        Yes, you are correct.

        I have designed a few devices of that general type.... in most cases they are still active with the powered device out. In a few cases they are in a "sleep" mode which draws a VERY little current , just enough to power something that determines if there is a load plugged in.

        SOMETHING has to determine whether there is a load, and that "something" needs power.
        1601

        Keep eye on ball.
        Hashim Khan

        Comment


        • #5
          Originally posted by J Tiers
          Yes, you are correct.

          I have designed a few devices of that general type.... in most cases they are still active with the powered device out. In a few cases they are in a "sleep" mode which draws a VERY little current , just enough to power something that determines if there is a load plugged in.

          SOMETHING has to determine whether there is a load, and that "something" needs power.
          Soaking the step down transformer at best. The remainder of the circuitry depend on the powered device to complete the circuit and draw current.

          But I agree - it will draw current - how ever minuscule it may be

          Comment


          • #6
            Originally posted by JoeFin
            Soaking the step down transformer at best. The remainder of the circuitry depend on the powered device to complete the circuit and draw current.

            But I agree - it will draw current - how ever minuscule it may be
            Most cellphone chargers are SMPS.

            The SMPS will usually go into a "burst" mode, where it turns on to produce real power every so often, but is drawing only the small "startup" power most of the time. Some pass "energy star" requirements, which are under a half watt.

            A device with an actual 60 Hz transformer will draw a percentage of max current as "magnetizing current"..... with small cheap transformers, that is often significant.... could be a couple watts of power. Way more than "energy star rating" allows.

            It's easy to find out.... see if it gets warm...... if warm with no load, it draws significant power.
            1601

            Keep eye on ball.
            Hashim Khan

            Comment


            • #7
              Different Animal entirely

              Originally posted by Dr Stan
              That's what I've also heard. To completely stop power consumption it must be unplugged. Ever notice how the lights stay on cordless power tool chargers.

              That charger with the lights (LEDs) and such is a different animal than just a transformer. There is other circuitry that is being powered even when a battery is not being charged. A transformer should, idealy, draw no current when nothing is attached to the secondary. BUT, in the real world, the small ones (wall warts...) are designed to be cheap and easy to make so I doubt much care goes into making them as efficient as possible.

              Large transformers can have quite an inrush current when first powered, but the primary current with no load is minimal. The problem comes with time and things loosening up and any vibration gets turned into noise and heat. The more noise and heat, the moe losses.

              Comment


              • #8
                One way to find out would be a short test harness with a milliameter plugged inbetween the wall socket and the unit....I think the amount of current would be quite small but there would be some given that we are talking about a transformer which can probably use some current, even in an idle state.

                In fact, I've noticed that some of the "wall warts" will stay warm even if left plugged in without a load. Of course, I'm excluding any current drain from an indicator LED....those can draw 20-30 ma.

                Comment


                • #9
                  The old style, linear type regulator wall warts were pretty thirsty, and soaked up a fair amount of juice even with the load off.

                  Through a series of regulations driven by the DOE, small wall power supplies are getting quite efficient. Without load, many do not get warm to the touch. As someone mentioned, they are pretty much switched mode power supplies at this point.

                  Nevertheless, plugged in, they are drawing some current.

                  Comment


                  • #10
                    Any circuit draws current, even the best electrical switchs still given a rated 'off' resistance, Infact even mechanical switchs have a insulation resistance.

                    On the one hand, a charger can sense/detect a battery, or even have a mechanical switch that disconnects the charger from mains without a battery inserted (havent seen one that does this yet but..)

                    On the other hand, even a off switch still allows a few picoamps of current through. And most supplys will draw a lot more if they are just electricaly sensing the battery.

                    Basicly: yes everything draws power pluged in, a mechanical off switch is the best at preventing signifigant power usage, while electrical sleep modes have varying success depending on thier design, but will allways have some parasitic power draw.

                    PS: Did you know for most nicad/nimh chargers, its very bad to leave a fully charged battery in the charger for long periods? Most chargers trickle charge the battery once fully charged, and many will end up overcharging it given enough time.
                    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                    Comment


                    • #11
                      I have to agree with everybody. They will all draw some small amount whenever they are plugged in. That amount will vary, primarily depending on the design and quality of the transformer. The primary winding of the transformer is essentially a wire connected across the two power leads. Only the self inductance of this coil opposes the current. In a good transformer, it will produce a high percentage of the line Voltage as a back EMF (Voltage) and the current will be quite small, but low quality ones there will be less back EMF and the current drawn will be more.

                      I like to use a switched power strip to hold these devices. Turn it off when they are not in use. Of course, it will probably take 100 years to make back the cost of these strips.
                      Paul A.
                      SE Texas

                      Make it fit.
                      You can't win and there IS a penalty for trying!

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                      • #12
                        Originally posted by kf2qd
                        Large transformers can have quite an inrush current when first powered, but the primary current with no load is minimal.
                        Not true, (unless you consider 2 to 5 % minimal.) From http://elektro.fs.cvut.cz/en/SSem/21...ansformer.pdf:
                        "The value of idling current is between 2 and 5 % of rated current in big transformers and up to 15 % in smaller transformers."
                        Location: Saskatoon, Saskatchewan, Canada

                        Comment


                        • #13
                          Originally posted by Arcane
                          Not true, (unless you consider 2 to 5 % minimal.) From http://elektro.fs.cvut.cz/en/SSem/21...ansformer.pdf:
                          "The value of idling current is between 2 and 5 % of rated current in big transformers and up to 15 % in smaller transformers."

                          And it is at a horrible power factor........ which causes the power company trouble.
                          1601

                          Keep eye on ball.
                          Hashim Khan

                          Comment


                          • #14
                            You win a six pack.

                            The control circuitry on the hot side takes some small amount of power to run. It may not produce pulses on the transformer as long as the output capacitor is fully charged. However, the low voltage side circuitry (which also needs a small amount of power) signals to the high voltage side that it doesn't need power through an optoisolator. Driving the LED on the optoisolator will take a few milliamps (at the DC voltage, not AC) and will slowly discharge the capacitor until a pulse is required to top it off. I just measured the current on my cell phone charger (small switching supply) with no load: 1.5mA. Then I measured a USB charger, also switching. 1.44mA. So, about 1/6 of a watt or about 1.5KWhr/year.

                            Comment


                            • #15
                              Originally posted by J Tiers
                              And it is at a horrible power factor........ which causes the power company trouble.
                              Ah.. but that trouble doesn't mean much on the residential watt meter. Crappy power factor is is great

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