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Author Topic: Measuring Amps out of Voltage Regulator  (Read 1307 times)
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I have attached a serial backpack and lcd screen to an arduino via a breadboard. i am trying to figure out how many amps is being drained out of the regulator? Is there an easy way to do this? If I can read the output out of the regulator is there any way to read the current output out of the arduino?
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What test equipment do you have?  Do you have a multimeter?  If so, then you can select a current range, connect the meter in series with the output of the regulator, and read the current (in Amps) directly.
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yeah I have a digital multimeter. What I did is I set up my project ona a breadboard and made sure it worked. Then I took out the wire connectting the Arduino 5V to the Positive rail on the breadboard. I replace this one wire with 2 wires, one for arduino 5V ad one for the positive rail. I then put the multimeter in series in between. It would not work though. No power seemed to be going through, the project on the bread board would not work. I ahve no clue why it won't work.
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Check the instructions for your multimeter!  You will need to select "DC Amps", and you may have to change the positive (red) plug from the "Volts/Ohms" socket to the "Amps" socket.  This is a safety feature of many modern multimeters.
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Also check your fuse in your multimeter. It is quite common to blow them if you check voltage while set on amps.  :smiley
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I followed all the right steps. And I know my multimeter works. I measured the voltage on the Arduino after to make sure ti was working.

I also know that I am measuring Amps properly because I checked the amps on a parallel port interface board and was able to get a good reading.

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What measurement current ranges does your meter offer? The AVR chip by itself will only draw a few milliamps if it's not driving any I/O output pin loads.

Lefty
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Yeah I was looking up the data sheet of the Atmega and noted the current it normally uses when idle.

But in my first post I mentioned I have hooked it up to my project, which is a LCD screen with a serial backpack. It should be pulling about 250mA of current but I get 0 on my multimeter.

My multimeter's datasheet says it measures accurately from 100 micro amps to 10 amps
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@Anachrocomputer
So what does it mean if the Arduino doesn't power up with the meter in line and set to measure current?

 :-?  
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Couple of things could be wrong if the Arduino won't run with a multimeter in series.  Firstly, make sure the meter is set to a current-measuring range (Amps).  Then, make sure you have the leads in the proper holes for current measurements; many modern meters have a separate positive terminal for current measuing, as a safety feature.  Finally, there's usually a fuse inside the meter to protect it against excessive current, and this may be blown.
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Ahhh, looks like the fuse is blown.  Thx for the help.
smiley Rob
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Even if the fuse in a DMM is blown, it will still read resistance and voltage fine, just not current.

If you're having problems measuring current with a DMM, the only way to tell if the fuse is blown is to check it. Trying another function doesn't help you since they do not rely on the fuse.

Reason: the DMM has a high impedance output when measuring voltage/resistance so very little current flows, not requiring a fuse. This is so the meter doesn't load down whatever it is measuring. i.e. if you're measuring the voltage across a 1k resistor with a meter impedance of 10k (wayyyyy too low), you're going to get an answer 91% of what it should  be. If the meter impedance is, lets say, 10Mohms (10 mega-ohms), your reading will be, for all intents and purposes, accurate. (1k in parallel with 10M is still effectively 1k. Exact number would be 9.999k, but whatever haha) When measuring current, in order to be accurate, the output impedance must be very very low (short circuit) to achieve an accurate reading. (An impedance more than a few ohms would add resistance to the circuit, lowering the current flow.) This requires a fuse as hundreds of milliamps or even amps can flow through the meter's circuitry, and there is only so much it can handle. (Most meters have a scale of a few hundred milliamps and then a 10A scale with a different fuse)

DMMs blow when you measure voltage (across a circuit element) while on the current setting because you are effectively putting a short circuit in parallel with whatever it is you're measuring.
For Example: you accidentally put the meter across a 1k resistor that has a voltage drop of 3V (Typical if you're running an LED off of a 5V supply). When putting the meter in parallel with it, you are basically shorting out the resistor with the current meter. Say the meter has 1 ohm of resistance on the current setting (check the net, I made that up). This would mean that 3V/1ohm or 3A of current flows through the meter, blowing the 250mA fuse (typical value). Even if the meter resistance is 10 ohms, 300mA still flows, blowing the fuse.

I've done this MANY MANY times, I keep like a dozen extra fuses just in case.
« Last Edit: April 16, 2009, 10:41:51 am by minime72706 » Logged

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