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15 August, 2014

New gadget measures negative resistance

If you are like me, you appreciate electronic gadgets with dials and displays. So when I discovered this "USB detector",  I thought to myself that I really always wanted to know the voltage as well as the current consumption of my USB devices. And since it is more or less impossible to connect a multimeter, this is exactly what I need.

The device fully satisfied my curiosity. Actually one surprising result was that the charger for my Samsung Galaxy Note 8 has a negative output resistance.

With a load it outputs 5.27 Volts as shown in the top image. Usually one expects the voltage to increase when the load is removed. But for this charger the voltage dropped instead to 5.13 Volts (second image). That should mean that there is the equivalent of a series resistance of (5.13-5.27)/0.98 which is about -0.14 Ohm.

I measured other chargers also, without finding a negative output resistance, so it seems as if it applies to this particular charger only. Out of curiosity, I also measured the current consumption of my Arduino Mega to 0.08 A without any shields connects.

The unit has two outputs which are different from each other. Output 1 is a fully functional USB port, while output 2 only connects DC power. What is that good for? Well, the epanoroma blog opened my eyes to the utility of this. If you charge your phone at some public place, then this feature isolates the data port of your phone. That may protect you from being hacked.

So there you see. The €5.13 were well spent and I even learnt something new by giving in to the temptation to click "Buy It Now" on Ebay.

But why does the Samsung charger have what amounts to a negative output resistance, is it by design or by accident?
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4 comments:

  1. LA2RL15 August, 2014 22:25

    I guess this is due to the no load situation. The charger is surely using a switch-mode supply and many of these designs have problems under no load conditions. Many of them loose control and can even output much higher voltages than the design voltage. It is possible that the Samsung Galaxy charger guards against this situation by forcing the output to a fixed lower value when the feedback loop is open.
    73 Frode, LA2RL

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  2. Sverre Holm16 August, 2014 10:49

    There a good comment on Amateurradio.com by WA8WHP:

    "The difference may be there is some ripple on the output which is filtered more by the batteries and not showing up on the DC voltmeter without the filtering."

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    Replies
    1. LA2RL16 August, 2014 20:12

      Well this theory can easily be tested by replacing the Samsung Galaxy with a resistive load that draws 980 mA and see what happens then when there are no extra filtering by the batteries. Another test would be to test the voltmeter and ampere meter in the USB Detector with a good and stable multimeter and see what differences there are if any.

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  3. G1KQH16 August, 2014 16:00

    I found them cheaper here:

    http://www.ebay.co.uk/itm/Red-and-Blue-LED-USB-Charger-Doctor-Voltage-Current-Meter-Tester-Power-Detector-/281353987047?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item41820123e7

    73 Steve

    G1KQH

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