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Current draw on a pulsed circuit

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Burnt

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Hi all,

I have a Coil driving circuit & i want to see how many Amps it draws through it under full load.
I don't seem to be able to get a handle on this as it is a Pulsing dc circuit so the normal amp meter doesn't seem to be the correct way to check this out.

Any advise would be appreciated.

Thank You
 
Hi all,

I have a Coil driving circuit & i want to see how many Amps it draws through it under full load.
I don't seem to be able to get a handle on this as it is a Pulsing dc circuit so the normal amp meter doesn't seem to be the correct way to check this out.

Any advise would be appreciated.

Thank You

hi,
A low value, high wattage resistor in series with the coils 0V connection.
Use a scope to measure the voltage pulse across the resistor and use Ohms law.

Ipulse= Vpulse/Resistor.
 
Hi all,

I have a Coil driving circuit & i want to see how many Amps it draws through it under full load.
I don't seem to be able to get a handle on this as it is a Pulsing dc circuit so the normal amp meter doesn't seem to be the correct way to check this out.

Any advise would be appreciated.

Thank You
Amps rms or average amps?
 
Hello,


The units of measurement depend on what the end use will be.
For example, if battery run time then one measurement unit like
average current would be used, but if power input is to be measured
then other units would be used and probably even a different technique.
This means it would help if we knew what you intend to do with
the measurement, as to what you are going to use it for.
 
Thanks Eric, that is what i was looking for.

MrAl, basically i just wanted to see how many Amps the circuit was drawing so if i make changes i can get real world figures on the current draw through the circuit, just learning.
I want to make a power supply for it as well & needed to get some real figures also.
 
Thanks Eric, that is what i was looking for.

MrAl, basically i just wanted to see how many Amps the circuit was drawing so if i make changes i can get real world figures on the current draw through the circuit, just learning.
I want to make a power supply for it as well & needed to get some real figures also.


Hi again,


Oh ok, well in that case you can measure average current, which you
can get by either placing the meter in series with the load (switched
to an appropriate current scale of course) or connect a small value
resistor in series with the load and measure the voltage across that
resistor and use Ohm's Law to calculate the current.

Using a 0.1 ohm resistor (make sure the power rating is sufficient also)
Ohm's Law comes out to this:

I=E/R
I=E/0.1
or
I=10*E

so whatever you measure across the resistor (volts) you just
multiply by 10 to calculate the current through that resistor.

Example...

Using a 0.1 ohm resistor and a 2.000v full scale digital meter, we
measure 0.1 volts across the 0.1 ohm resistor. What is the current?
The current I is:
I=E*10
I=0.1*10
so
I=1 amp average.

Most meters by default measure average current.
Sometimes you can use a lower range for the meter, such as
0.200 volts full scale.


PITFALLS OF A RESISTOR IN SERIES TO MEASURE CURRENT

1. The resistor has to be able to handle the full load current and
that means a resistor with a good power rating at least equal to:
P=2*I*I*R. Keeping R low helps with this problem and this also
helps to reduce heating which changes the resistance of the resistor
which affects measurement accuracy.

2. The resistor causes a voltage drop in the circuit which affects
measurements. This means the voltage drop must be kept low.
Since the voltage drop is E=I*R, we want to keep R as low as
possible so that E is also low.
 
If Burnt will forgive me for putting a bit different spin on the pulsed current question.

I am thinking about the case where PWM is used to dim a LED. From a pure math viewpoint it would be the duty-cyle-on-time times the current for continous operation In the real world things are never quite that simple. Is the simple percentage idea close or does the extra current draw each time the LED goes on make it invalid?
 
Hi Burnt,

There is a small capacitance associated with every LED, but it usually doesnt
seem to bother anything at somewhat 'normal' pulsing frequencies under
around 20kHz.
The first approximation is to do what you said, multiply current times the
duty cycle, and this approximation has been used for years and years with
all kinds of LEDs.
The deviations from that approximation are caused mostly by the curve for
the light intensity vs current, of which it is not a straight line and and
light intensity falls slightly as current increases, which also means the
efficiency falls a bit. How bad is this effect?
For a small white LED relative to 20ma, at 10ma
we would expect to get 50 percent intensity, but we actually get about
60 percent intensity, which is slightly higher than what we would expect
to get with 1/2 the current. At 40ma, we get 180 percent intensity, when
we would expect to get twice as that from 20ma or 200 percent. This means
the white LED is more efficient at 10ma than at 20ma, and that it is less
efficient at twice the rated current, pulsed or not.
 
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