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best way to measure current?

K

Ken Williams

Jan 1, 1970
0
I have a coil. I want to accurately measure the amount of
energy/current that it uses when pulsed with one single square wave. I
know I can take a voltage and current measurement and multiply them
together to get a wattage. is that the only way? is there such as
thing as a wattmeter? that just simply tells me how much energy was
just used? I guess its a stupid question.

a side question - if I set my power supply to "constant current mode"
(say at .3A) and at 12V the most my coil can draw is 3.6 Watts correct?
so this would happen when I simply short the power supply with my coil.
 
D

Don Stauffer

Jan 1, 1970
0
Ken said:
I have a coil. I want to accurately measure the amount of
energy/current that it uses when pulsed with one single square wave. I
know I can take a voltage and current measurement and multiply them
together to get a wattage. is that the only way? is there such as
thing as a wattmeter? that just simply tells me how much energy was
just used? I guess its a stupid question.

a side question - if I set my power supply to "constant current mode"
(say at .3A) and at 12V the most my coil can draw is 3.6 Watts correct?
so this would happen when I simply short the power supply with my coil.

Not a stupid question. In fact, it is a very involved problem in AC
measurement. One needs to consider the phase of the current and
voltage. Google "AC current" and phase. The current through, and
voltage across an inductance (and a coil has a LOT of inductance) occur
at different points in the cycle. So the determination of power in an
AC cicuit containing a big inductor or capacitor is not a trivial problem.

And pulsed power IS AC. There are simple power meters (wattmeters) and
complex ones. The later, which do what you want, are not cheap.
 
C

Charles

Jan 1, 1970
0
Ken Williams said:
I have a coil. I want to accurately measure the amount of energy/current
that it uses when pulsed with one single square wave. I know I can take a
voltage and current measurement and multiply them together to get a
wattage. is that the only way? is there such as thing as a wattmeter?
that just simply tells me how much energy was just used? I guess its a
stupid question.

A sqaure wave (I'll assume a constant voltage source) across a coil will
produce an exponential current response with a time constant equal to L/R.

The coil will store the energy and give it back and the resistance of the
coil will convert some of it to heat.

Your question can only be completely answered if you provide more
information. Circuit analysis is often divided into transient and
steady-state and average categories. It all depends ..............
a side question - if I set my power supply to "constant current mode" (say
at .3A) and at 12V the most my coil can draw is 3.6 Watts correct? so this
would happen when I simply short the power supply with my coil.

Watts are not always the same as volt-amperes. Assuming a DC supply, the
power will be dissipated in the resistance of the coil.
 

Resqueline

Jul 31, 2009
2,848
Joined
Jul 31, 2009
Messages
2,848
You have to use an oscilloscope and a boosted pulse generator.
Measure the voltage across a 0.01 Ohm series resistor for instance to read the current.
You'll find that the current starts at zero and rises linearly with time, until the coil core saturates - at which point the current starts to rise more rapidly. The current will eventually level off at U/R (if you pulse that far) but the stored energy won't increase over the saturation level mentioned..
It's easy to boost a pulse generator to 50A capability using a darlington or a MOSFET (and a big cap for current storage). A snubber circuit is required if using a MOSFET. You only need a 100mA 12V supply if you use low repetition rate (1-2Hz).
The inductance of the coil would be: delta time * pulse voltage / delta current
 
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