While the battery voltage will decrease during discharge, the battery current will increase, to keep the output power constant. The USB voltage will always be the red line that starts at around 5 volt, the battery voltage will start just above 4 or 8 volt, depending on number of batteries. I will specify the usb current in the text, it can also be calculated from ohms law: voltage/resistance->current and with usb voltage of about 5 volt the current in the above chart will be 5/12 -> 0.417A This power supply could not sustain a 1A load for more than 23 minutes and 10 minutes later it starts doing some silly stuff on the output.īut reducing the load to 0.9A makes it possible to sustain it for one hour and probably also longer.įor battery powered equipment I run the test until the battery is empty and the chart will show data for both the battery voltage/current and for the usb voltage. To check sustained current I also do a load test.įor battery powered equipment the maximum voltage and current might depend on how much charge there is in the battery.įor mains powered equipment I run this test for one hour and only shows the output voltage and current: The red line show the voltage as the current increases, because the test is done in a few minutes there is no gurantee that the voltage and current can be sustained. The yellow lines shows the USB voltage limit, chargers does only need to stay above the lower limit up to 0.5A, but equipment might not charge when the voltage is below that limit. This test is used to get an idea about the performance of the usb power supply, the test only takes a few minutes, but requires the electronic load. What I am interested in is not the resistance, but if the insulation breaks down. It will apply a high voltage and measure the resistance. With the above box I can connect to all outputs at once (This only works if they are running on the same regulator inside the power supply) and get a fairly low resistance, making high current tests possible.Īt higher currents I nearly always uses the electronic load, except for noise measurement that is done with a resistor load.įor mains powered equipment I can also use the mega ohm meter or insulation tester. When testing a usb power supply with many outputs a single usb cable is not enough. I do have boxes for all 3 sizes of USB connectors, the mini and micro is mostly used when I need to supply current to a charger.
The cable can be used for load tests, but the breakout box is needed when I want to check what standard the usb power supply is following. None of the above equipment can be connected directly to a usb connector, for this I have made a special cable and a breakout box. For my usage a usb version is very useful, this means I get the curves directly on the computer.
To see how clean the output is, I am using a oscilliscope. The electronic load is usual busy doing battery tests, making the resistors the most used load. To do any test, I need to put a load on the usb output, for this I can use either a electronic load or resistors. This feature is not very common on DMM's. With AC power it is not enough to measure voltage and current and then mulitply them, it is also necessary to measure a "power factor". The Gossen has one special feature that is important when checking mains powered equipment, it can measure power/watt. I must be able to measure and log the result to a computer, for this I uses a couple of Fluke DMM and a Gossen DMM. I uses a couple of different methodes to test them, this article will show what type of test I do. I test different equipment with usb outputs:
How do I test a usb power supply/charger How do I test a usb power supply/charger/power bank
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