Hi, I’m Bill Griffith, an Application Engineer with Keysight Technologies and today we’re
going to look at powering devices over USB.
I have several USB devices and a couple of our DMMs, one to measure current and one to
measure voltage.
So let’s go ahead and get started.
The first thing I’ve connected is a USB mouse and this is really where USB started.
This is really designed to power low power devices such as a keyboard and mouse and so
in this, the mouse is currently pulling about 30 milliamps in an idle state and we move
the mouse.
You can see it jump up to 60 milliamps and so this is well below the current threshold
of 100 milliamps which is considered a lower power device.
So I want to show a device now that’s a bit higher than the 100 milliamp range and
we consider a USB high power device.
Let me go ahead and make that change.
Okay, so I’ve gone ahead and connected a web camera instead of the USB mouse and as
you can see, it pulls 135 milliamps so it’s just above the threshold for being a low power
device so this is considered a higher power device.
Let’s go ahead and change it to even a device that pulls even more power.
I have a power cell so I’m going to go ahead and now connect it
and you can see the power cell’s determining how much power it can draw over this connection.
It’s connected through a USB hub and to a power port on the PC.
Since it’s a power port, it can pull up to around 1000 milliamps and so it’s currently
pulling about 700 milliamps, and you can see the voltage drop a bit and this is really
due to the resistance in the USB cable.
The more power you pull through the USB cable, you’ll start to see voltage drop which we’re
seeing now.
This power cell is really smart and it actually has the adaptive power feature of USB.
I’m going to go ahead and hook it up to its charger to show you what that looks like.
Let me go ahead and make that change.
Okay, so now we’ve connected up the charger for our power cell and so before we were only
charging at five volts, but now when we hook up the power cell to its own charger, it will
negotiate and it will determine that it can actually charge at nine volts.
So this is the adaptive charging and it’s pulling about 820 milliamps at nine volts.
By using the higher voltage, it can actually charge much faster than just with the five
volts that’s available through a USB power port.
The next thing I wanted to show you is using our BenchVue software.
It makes doing some of these things a little bit easier and it can data log over time.
Okay, so now we’re using our Keysight BenchVue software and one of the first things that’s
really nice about it is that if you’re using multiple instruments, you can have them both
displayed on your bench top at the same time.
For our application of looking at voltage and current simultaneously, it makes for a
really easy way to look at our two DMMs.
What’s a little bit more interesting is we can move onto the graphing mode and we
do have both R charts as well as just a trend chart.
We’re going to use the trend chart to go ahead and look at the voltage and current
over time.
We’ll go ahead and get that started and you can see the voltage on the top as well
as the current on bottom over time.
That’s a handy feature to be able to track the USB power over time and again, there’s
a lot of other features we can use for analysis on these two signals.
Okay, so today we’ve looked at several USB devices and we were able to measure the amount
of power that they take.
We looked at a lower power device, devices that use less than 100 milliamps.
We looked at a high power device, ones that use less than 500 milliamps and then finally,
we looked at an adaptive power device.
I hope you found that useful.
Thanks for watching.