OK - embarrassing as it is I need to confirm this - so my multimeter is set to 20m to read AMPS and the reading is 0.07 - is this 7mA?
How do I interpret the meaning of 20m - does that mean it will go to mA or A of 2 decimal places?
|
OK - embarrassing as it is I need to confirm this - so my multimeter is set to 20m to read AMPS and the reading is 0.07 - is this 7mA?
How do I interpret the meaning of 20m - does that mean it will go to mA or A of 2 decimal places?
Without being familiar with your meter, I would assume 20m means 20mA. And I would further assume that if it was measuring 20mA on that setting, it would display 20. Which suggests that 0.07 means 0.07mA (70 uA). Does it have a 200 uA scale?
Thanks for the reply - yes it does, it has (in order) 200u, 2000u, 20m and 200m...
How does it work then, surely if it was measuring 20mA then it would max out on 20.00 and 7mA would be 7.0, no? Or even measuring 20mA would mean that 1.0 is one lot of 20mA (can you see the logic in that one?)...?
I imagine on the 20mA setting, it actually maxes out at 19.99 - this allows them to use a single segment display for the leading digit but still go up to 2 (almost). The ranges will just set the max that can be displayed, but the display will still show the actual current measured (within that range).
Scaling it to mean that 1.0 was 20mA would be too much mental arithmetic for most engineers!
So, if you set to to 200u then it should read about 70.0 for the same current.
One approach I used to take was to set the meter to the least sensitive setting (2A or whatever) and then gradually turn it down until you got the most accurate reading. For example, on 20m it will read 0.07, but on 200u it could display the current more accurately, which could be anywhere between 65.0 and 74.9.
Thanks! I had to make sure as I just got an arduino and am measuring the consumption of a 12v 120mm fan...@ 5V which is what the arduino puts out it seems to be consuming 7mA at both free spin and stall...I wonder if the microprocessor is doing that...?
One more thing though (after thought) - if it is set to 20m would not 7mA be 7.0 rather than 0.07? It made sense when you said it was putting out the correct value, so would the max on 20m be 0.2?
I'm confused. I am not familiar with the Arduino ... does it not have separate 12V and 5V supplies?
Maybe you could look up data on the fan and see how much current it is supposed to draw (probably not much). And it might be the roughly same if it is stalled (rather than disconnected) - depends on the type of motor.
Last edited by fatman57; March 1st, 2013 at 04:12 PM.
So the 12V supply will be driving the fan. I assume a 5V supply for the electronics is derived from that. So if you measure the 5V supply it will not include the current for the fan (unless I have misunderstood how it works - maybe I should go and look up a schematic online...)
« Networking question- making segment wireless midway. | resistor calculation » |