# Measure Power Consumption Through 1 Ohm Resistor

Hi guys, so I remember reading on the forums here that measuring power consumption through a 1 ohm resistor is bad but I don't remember why, and I can't seem to find the forum post again. I have a image below of how I was hoping to measure the power of the arduino through a 1 ohm resistor by using V=IR to obtain the current.

Would anyone know why this is the reason? Apologies for the scummy picture.

http://www.ebay.com/itm/171389065881

what is the voltage drop ?

this would have to be on the raw power in or the USB so you do not include the voltage regulator.

how sensitive is your voltage sensor ? what is the full voltage span you expect to see ?

As posted that picture has the scope measuring a voltage above the -ve. Most scopes have one input grounded so you are best to put the sensing resistor the other side of the load.

Depends on the load, more load more wasted energy in the form of heat.

If you wanted to monitor an led's current say a 5mm led 20ma sure go for it. A 1/4watt 1 ohm would be fine.

A 3watt led on the other hand not so good, eg 700ma would fry a 1/4 1ohm from dissipation

0.01 shunt is ideal except your adc resolution shrinks with it when reading the voltage a 1 ohm gives a nice resolution

trustnoone: Hi guys, so I remember reading on the forums here that measuring power consumption through a 1 ohm resistor is bad but I don't remember why, and I can't seem to find the forum post again. I have a image below of how I was hoping to measure the power of the arduino through a 1 ohm resistor by using V=IR to obtain the current.

Would anyone know why this is the reason? Apologies for the scummy picture.

It can be done that way, but there's two dangers:

a) Your resistor can easily catch fire. Remember: V=IR ... AND ... Watts = Volts x Amps

b) As Mike said: The crocodile clip on your 'scope lead is 99.9% likely to be connected directly to mains ground and so is your USB cable (...and therefore your Arduino). Connecting the crocodile clip anywhere other than Arduino ground will let the magic smoke escape.

Thanks guys, I really appreciate it. In terms of watts I'm hoping it won't be too bad, I got a tad cautious and ended up buying a resistor rated for 10watt just in case.

I remade the picture below, but was a bit confused about the oscilloscope leads, did you mean that ground should be connected to the negative terminal?

Yes that's better, assuming -5V is ground.

If you have a 'scope, you probably have a multimeter too. Both leads on a meter are floating, so there's no need to ground the negative lead. Use a meter instead of the 'scope and your 1st schematic will work fine (as long as you have enough current to get a measurable voltage across the resistor).

Or, you can connect the meter in series (in place of the resistor) and measure current directly.

Or, if you have a dual-trace 'scope there is usually a differential mode where you can connect one probe to each side of the resistor and measure the difference between each end.