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Solid States Devices => solid state devices => Topic started by: magnetman12003 on October 26, 2015, 02:08:44 AM

Title: Measuring current
Post by: magnetman12003 on October 26, 2015, 02:08:44 AM
What is the most accurate way to measure current using a true RMS multimeter?
I am talking DC current.  Plan to use some value resistor in series with the positive lead.
Is this the way to go for an accurate reading?
Title: Re: Measuring current
Post by: poynt99 on October 26, 2015, 02:19:58 AM
It's very simple.

First, you don't need a true rms meter. Obtain a non-inductive current viewing resistor, say 1 Ohm and place it in series with your positive lead. Use your meter set on DC volts (or millivolts) to measure the voltage across that resistor and compute your average current.
Title: Re: Measuring current
Post by: magnetman12003 on October 26, 2015, 06:29:41 AM
It's very simple.

First, you don't need a true rms meter. Obtain a non-inductive current viewing resistor, say 1 Ohm and place it in series with your positive lead. Use your meter set on DC volts (or millivolts) to measure the voltage across that resistor and compute your average current.

If my voltmeter is in series how can I measure the voltage "ACROSS" the resistor?  Can you show a small diagram so I get this straight?  I always had a hard time measuring current.  Thanks for your reply.
Title: Re: Measuring current
Post by: TinselKoala on October 26, 2015, 08:38:44 AM
I'll jump in here with a diagram. If the system is really and truly straight, nonvarying DC, (like a battery powering a simple light bulb) you don't even need your resistor to be non-inductive. But for general purpose use you should use non-inductive resistors, and your voltmeter (or oscilloscope) should be connected as close to the body of the resistor as possible.

Then you simply measure voltage drop across the resistor and use Ohm's law to calculate the current. With a 1 ohm resistor the calculation is easy: what you read on your voltmeter in volts = the current in amps.
Title: Re: Measuring current
Post by: memoryman on October 26, 2015, 03:02:26 PM
Just to add: the 1 Ohm value for the current sense resistor may be too high (or even too low); I would pick a value of ~1% of the total load resistance.
Title: Re: Measuring current
Post by: TinselKoala on October 26, 2015, 04:52:20 PM
Just to add: the 1 Ohm value for the current sense resistor may be too high (or even too low); I would pick a value of ~1% of the total load resistance.

Of course you can use whatever value you like. Just put the actual value into the Ohm's Law equation to find your current. A one-ohm resistor is convenient because then you don't need to do any math: a voltmeter reading of 100 millivolts will be indicating 100 milliamps of current, for example. But depending on the load, other values of the CSR may be more appropriate.
Title: Re: Measuring current
Post by: memoryman on October 26, 2015, 05:03:44 PM
TK, I added that not for your (or any other qualified person) benefit, but for people who ask the question in the first place.