Measuring current with a multimeter - weird results

Hardware
#1

Hi,
I’m getting some weird results from my multimeter. Basically, I’ve cut the red wire on a USB cable and attached a multimeter in series of the red wire (+5v)

When I plug it in, I see the Spark go up to about 180mA (about right because I have a music shield and RFID shield attached). It starts running my setup() code. I can tell because I have it play a test beep through the music shield. Then, around when I call Spark.connect(), the solid cyan turns into a breathing yellow. It doesn’t seem to get to the flashing green stage.

So I tried changing from my powered USB port to my computer USB so I could connect a serial connection to see what’s happening. Connected to my computer USB, it beeps from setup(), then the solid cyan turns into a dim solid red. Almost pink because it’s flashing so fast.

Of course, when I just connect the two leads without the multimeter, everything is perfect.

The multimeter is an ex330 on the mA setting.

Help?

Thanks!
Ryan

#2

Does sound like a power supply issue. You might want to test out with a simple tinker firmware and see that everything runs well with the multimeter setup first :wink:

#3

Read up on burden voltage of DMMs…the impact will increase as your device draws more current.

Some good videos on Youtube.

If your USB voltage is already at a lower threshold and you introduce the DMM,reducing the voltage level further,then you may find one of the key components(WiFi?) is operating out of spec.

Also consider the quality of your USB cable, there can be significant additional voltage drop over thin or long cables. Try with a few different & preferably good quality short USB cables.

I got hit with a cheap USB extension cable recently.

#4

Thanks for the replies. I understand the problem now.
But how do I fix it? How does one measure the current consumed by a setup like this? Would a better USB cable that I hack apart solve the problem? I’m already running the rig off of a 2A USB power source.

Thanks!
Ryan

#5

did you try a shorter & better Usb cable?

It might be worth trying a different DMM as well, if available.

You could also try a 1R or 0.1R resistor in series(instead of your DMM) and measure the voltage drop across it with your dmm (I=V/R). Sounds like you would need a .25W(1R) or .125W(0.1R) resistor!

There are also dedicated USB current/voltage/power meters available on eBay, but could suffer the same.

FYI: the burden voltage of your DMM seems to be 2.5mV per mA - so at 200mA you already have a 0.5V drop, which isn’t great.

3 Likes
#6

Awesome. Thanks, all. I’ll try the suggestion of shorter, better USB cables, and the voltage drop idea. I hadn’t thought of that :smile:

I’ve also come up with two other suggestions from Google that I’ll try:

  1. powering the core from a 5v supply to Vin rather than from USB.
  2. This thing looks pretty cool: http://eevblog.myshopify.com/products/ucurrent

Thanks for the help, gentlemen.
Ryan

#7

Yes, the uCurrent Gold is what I use & should sort out the burden voltage. Very handy to see what is happening, particularly when used properly with scope (scope GND passes thru to your circuit) and you can see the current draw as your device goes thru its various stages/functions.

The 5v supply, won’t fix the burden voltage of your dmm, but ‘may’ at least give you a starting level of a clean 5V vs as low as 4.735V for USB 2.0

you should check the voltage where you cut the red wire and at the device, to give you an idea of the voltage levels you are dealing with.

#8

A clamp-based multi-meter would allow you to unobtrusively measure the current. You will however need to ensure that the device supports measuring DC as I believe many are limited to AC currents.

#9

I’ve been using this to watch current usage.