Javascript Disabled Detected

You currently have javascript disabled. Several functions may not work. Please re-enable javascript to access full functionality.

The Netduino forums have been replaced by new forums at community.wildernesslabs.co. This site has been preserved for archival purposes only and the ability to make new accounts or posts has been turned off.

Measuring current draw

Started by roguemat, Feb 13 2013 04:25 PM

Best Answer CW2, 13 February 2013 - 06:09 PM

Then, how do I actually work out how much current something is using? Can I use a voltmeter to test?

An ampere-meter is much easier to use, as it can measure the current directly. If by 'voltmeter' you mean only voltmeter (and not multimeter), then you'd need to convert the current to voltage, which it can measure (for example a resistor can do that, although there will be rather low voltage for current in mA range; technically ampere-meter works exactly like that, it measures voltage over special low-resistance resistor).

And how do I know where the current is coming from? So lets use the ping))) as an example. It's data sheet says it uses 20mA. So, does that 20 count towards the Digital pin thats sending it signals or towards the 5v/3.3v power pin?

It depends on the connection, but at the end the current will be drawn from the power supply, the sensor current consumption counts towards its 5V pin.

Let's say I wanted to work out how many ping)))'s I could attach - how would I do that?

You can connect as many sensors as their total current consumption does not exceed power supply limit, if you decide to power them all from Netduino 5V pin then there is about 800 mA "available" current, minus what is consumed by Netduino. You can increase number of sensors, if you don't use them simultaneously, but switch them on demand (multiplexing).

Go to the full post

Please log in to reply

4 replies to this topic

#1 roguemat

Advanced Member

Members
54 posts

LocationSouth Africa

Posted 13 February 2013 - 04:25 PM

My background is purely software, so please excuse me if my questions are silly.

I have a Netduino 1 and want to know how to tell that I'm not going to kill the poor thing. So, how many mA can I draw from each pin? I read 16mA but want to make 100% sure.

Then, how do I actually work out how much current something is using? Can I use a voltmeter to test?

And how do I know where the current is coming from? So lets use the ping))) as an example. It's data sheet says it uses 20mA. So, does that 20 count towards the Digital pin thats sending it signals or towards the 5v/3.3v power pin?

Let's say I wanted to work out how many ping)))'s I could attach - how would I do that?

Thanks!

I make Windows Phone stuff for beer money and rent (in order of priority). I've got a blog and am on the Twitter machine.

Back to top

#2 NooM

Advanced Member

Members
490 posts

LocationAustria

Posted 13 February 2013 - 05:12 PM

http://www.netduino....duino/specs.htm

16ma only on 3 pins.

its somehow sad that there are no good/visible links to the specs for the gen1 boards -.-

My PlayGround and some Code

Back to top

#3 Paul Newton

Advanced Member

Members
724 posts

LocationBerkshire, UK

Posted 13 February 2013 - 05:52 PM

Hi there,

The current that can be drawn differs with each pin. CW2 put this wiki page together which has the v1 limits: http://wiki.netduino...m-Netduino.ashx

If your "voltmeter" is a multi-meter, then yes you will be able to use it to measure the current.

To do this, you would select a current range, and attach the leads so that the supply current flows from the supply through the meter and then into the Netduino. e.g. connect the ground directly to the Netduino, but put the meter in series with the positive supply rail.

Start with a high current range on the meter and turn it down if you are happy you will not exceed the range of the lower settings. Beware than when you change setting, the current may be cut momentarily - this will cause the Netduino to reboot.

When you are using a module, the power normally comes from the supply rails. The IO pins should only be delivering tiny currents to send data to the module. If you have lots of modules, or modules that need lots of current, then you can overload the regulators in the Netduino.

When you are driving other things, like LEDs, then the current will be coming from the IO pin so you need to make sure you will not exceed the current in CW2's table. You may then need to use a transistor. (Especially for driving relays or motors.)

Hope this clears up a few things - keep the questions coming - Paul

Back to top

#4 CW2

Advanced Member

Members
1592 posts

LocationCzech Republic

Posted 13 February 2013 - 06:09 PM Best Answer

Then, how do I actually work out how much current something is using? Can I use a voltmeter to test?

An ampere-meter is much easier to use, as it can measure the current directly. If by 'voltmeter' you mean only voltmeter (and not multimeter), then you'd need to convert the current to voltage, which it can measure (for example a resistor can do that, although there will be rather low voltage for current in mA range; technically ampere-meter works exactly like that, it measures voltage over special low-resistance resistor).

And how do I know where the current is coming from? So lets use the ping))) as an example. It's data sheet says it uses 20mA. So, does that 20 count towards the Digital pin thats sending it signals or towards the 5v/3.3v power pin?

It depends on the connection, but at the end the current will be drawn from the power supply, the sensor current consumption counts towards its 5V pin.

Let's say I wanted to work out how many ping)))'s I could attach - how would I do that?

You can connect as many sensors as their total current consumption does not exceed power supply limit, if you decide to power them all from Netduino 5V pin then there is about 800 mA "available" current, minus what is consumed by Netduino. You can increase number of sensors, if you don't use them simultaneously, but switch them on demand (multiplexing).