 # how can i measure mA if it is running on PWM?

i want to mesure how many mA a resister & LED have at differant level's of pwm.. what should i do to do this? AC mA? DC mA? something else?

THANKS

Why not just calculate it? Say you are driving a resistor to the anode, cathode to ground. You know the output is 5V (the pulse width may vary but the output is 0 or 5V), you know the forwad voltage drop of the LED, you know the resistor value. Thus (5v- Vled)/R = current.

The current will be the same every time the pulse goes high, the only difference is how long the current is conducted, and that is determined by the pulse width.

i don't think i can calculate it because i know i can bearlyunderstand what u said...

Well if your meter is one that measures true RMS values, then reading it as a DC milliamp current will give you the correct answer. However most inexpensive digital multi-meters don't have that feature so you will get a reading but it won't be an accurate measurement. You can easily calculate the average current flow by multiplying the 100% current flow value by the % duty cycle that the pwm signal is outputting. That is (output pwm value / 255) X full on current value.

Lefty

how can i know for sure if my meter has RMS? this is my meter: (link)

[quote author=TECH GEEK link=topic=51560.msg367655#msg367655 date=1297195278] how can i know for sure if my meter has RMS? this is my meter: (link) [/quote]

Typically such a high end feature will be in the title description of the meter or at least a bullet point in it's feature list, like here: http://www.myflukestore.com/p1393/fluke_87v.php

The odds of your meter having the feature without mentioning is around 0% chance.

Lefty

ok thanks also how do i wire up my meter to measure Hz?

Thanks

[quote author=TECH GEEK link=topic=51560.msg367691#msg367691 date=1297196581] ok thanks also how do i wire up my meter to measure Hz?

Thanks [/quote]

As if you were measuring an AC voltage

Tech Geek? Really? If you had the PWM output set to 255, that would be 100% on. Use your meter in mA mode. Connect arduino pin to the red lead connect the black lead to one side of a resistor, connect the other side of the resistor to the long leg of the LED, connect the short leg of the LED to ground.

Take a reading with PWM at 255 (or whatever the highest setting allowed is, I have never used it). For any other value of PWM, the Average current will be (PWM output number)/255 * full current reading.

Does your meter have a Frequency, or Hz, setting?

yes

Does your meter have a manual? It's time to open it. Also it's time to do some google and look for "ohms law".

Kari