Comments on: Measuring current through USB

By: dcsledhead

dcsledhead — Mon, 05 Jan 2009 20:31:20 +0000

I think a DVD burner requires a wall wart power supply for power. The 120ma may just power logic in the interface.

By: TheBakachan

TheBakachan — Sat, 03 Jan 2009 21:52:20 +0000

I’ve never bothered to check the accuracy, but if you go into the windows device manager and check your USB root hubs, it tells you what devices are connected, and what the draw…or maybe just what they’re SUPPOSED to draw… Since it says ‘Power Required’

By: carlton

carlton — Sat, 27 Dec 2008 18:40:11 +0000

120 ma is the average current draw for the burner. laser applications (I believe) are typically pulsed current, which could be on the order of amps, but, with a low duty cycle, the average current measured on the input to the laser power supply (i.e. from the usb) is much less. for example, 1.2 a with 10% duty cycle would be only 120 ma on the input side. a cap provides this pulsed current which the input supply (usb) doesn’t see.

By: nubie

nubie — Fri, 19 Dec 2008 04:03:39 +0000

Thanks, it still amazes me that Harbor freight can move these Digital Multi Meters for $3, every time I visit.

I totally get that Instructables is on the simple end of the spectrum sometimes and I think it is great.

By: Vincent (gmgfarrand)

Vincent (gmgfarrand) — Thu, 18 Dec 2008 16:05:07 +0000

nubie,
Nice thinking….. might have to try that..

By: nubie

nubie — Thu, 18 Dec 2008 16:03:38 +0000

If you want to make this prettier you can build it into a Digital Multi-Meter yourself.

Cost: $3 DMM from Harbor Freight, $1 USB extension from Dollar Tree.

Matter of fact you can use two DMM and get the voltage of the connection while you are building a meter.

By: Vincent (gmgfarrand)

Vincent (gmgfarrand) — Thu, 18 Dec 2008 14:25:31 +0000

“It’s okay and nice if someone hacks this together at home. But it’s not sophisticated, anyone who has ever done current or voltage measurement can do it by himself, without instructables.”

I agree as well, but sometimes, in order to come up with something more sophisticated, you need to start simple.

With the Instructables I have seen/done, when the complication level goes up, the people on the lower end of the skill spectrum bombard me with sometimes “moronic” questions, or just don’t “get it”

For the sake of those, and some of the “slower” techs I work with, I keep me better/more complicated projects to myself or to the places/people that have more skill.

By: Vincent (gmgfarrand)

Vincent (gmgfarrand) — Thu, 18 Dec 2008 14:10:18 +0000

hex,
No, I mean 120ma.. I thought it was weird as well, which lead me to test it in the first place.

The burner actually has 2 USB ports on the back and a DC jack.

One port is just for USB power, and the other is for DATA.

Really annoying….

By: JohnZero

JohnZero — Thu, 18 Dec 2008 08:47:04 +0000

“On another note: I’d love to be proven wrong, but it seems all electronic projects on Instructables seem to… errm… lack the quality that most electronics sites have.”

I have to totally agree.

It’s okay and nice if someone hacks this together at home. But it’s not sophisticated, anyone who has ever done current or voltage measurement can do it by himself, without instructables.

By: hex4def6

hex4def6 — Thu, 18 Dec 2008 08:15:50 +0000

Yeah — I think you must mean 1.2a, not 120ma :)… it would take 120ma just to power up the laser on the DVD drive. I’ve seen DVD drives sometimes use a split USB cable that allows you to use two USB connectors to get within the power spec of the port.

By: Steve

Steve — Thu, 18 Dec 2008 05:21:17 +0000

@ngth If everything you intend to plug into your USB ports came from the store then you’re on the wrong site.

By: Peter

Peter — Thu, 18 Dec 2008 03:12:32 +0000

Not to nitpick, but mAh != mA

mA = milliampere, a measurement of current (coulombs/second)

mAh = milliampere hour, a measurement of charge (coulombs/second * hour = coulombs)

I’m actually surprised that your dvd burner only pulls 120 mA, a cheap optical mouse I just got pulls 100 mA (manufacturer) and it doesn’t use a laser diode.

By: Vincent (gmgfarrand)

Vincent (gmgfarrand) — Thu, 18 Dec 2008 01:56:33 +0000

Also….
All devices are not created equal when it comes to current draw.

Sure the USB standard is 5V for VOLTAGE..
For example, my Lenovo USB Slimline DVD burner pulls about 120mah, my GPS when it charges about 340mah, my ZEN MP3 player, 450mah.

When modding USB devices or ports, I need to know what kind of numbers I am working with to avoid a “bad day” ;)

By: Vincent (gmgfarrand)

Vincent (gmgfarrand) — Thu, 18 Dec 2008 01:53:32 +0000

I realize that its not pretty or very functional in the grand scheme of things, but it’s just something I slapped together for basically monitoring current draw for a previous project.

I’m working on a prettier/smaller solution.

By: ngth

ngth — Thu, 18 Dec 2008 01:48:34 +0000

Let me ask you this: why would you want to know how much current is being drawn out of your USB device?

The devices that you buy from the store will give a current rating @ 5V. You can safely assume it will never go above the max current rating. For all estimation purposes, you can assume “typical” power consumption at about 70% of the total power.