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.