Measuring current at the led

Which is the best method for measuring the current at the led in a step-up circuit?

I'm testing the ZLT (zetex 300) with a white luxeon 1w.

Trying to figure out the proper balance between runtime and lux cutting the sense wire.

At the battery I have 520mA

Thanks

Pablo

Hi again Pablo,

When we did the initial tests we simply used a 0.1 ohm
resistor in series with the LED. Measuring the
voltage across this resistor and using Ohm's Law makes
it possible to calculate the current as:
current=volts/0.1

or simply:
current=10*volts

where the volts is the voltage measured across the
resistor while the circuit is running.

For example, if you measure 0.035 volts across the
resistor you have 350ma flowing.


The reason the value of the resistor is 0.1 ohm is because
0.010 ohm is too low to get a good reading and
1.00 ohm is too high and introduces too much error in
the measurement.

This assumes of course that you use a digital meter
to measure the dc voltage across the resistor.

For caution, it's best to solder this resistor into
place for the tests because if it becomes disconnected
something blows out like the transistor, diode, cap, etc.
When finished with the tests simply unsolder.

Take care,
Al

Thanks Al,

I have a spare 5w 0.1ohm resistor that fits this purpose just right!!!

I'm a little bit closer to make a reality of my tag line: I will build flashlights /ubbthreads/images/graemlins/smile.gif

Pablo

Hi Pablo,

Oh that's good! So you shouldnt have any problem
measuring the current now.

Yes, now you're building the flashlights for sure
I'm working on something too...it's slow but if it works
it will be very nice.

Let us know how well it turns out?

Take care,
Al

Hi Al,

I usually just stick the meter in series with the Lux as I assumed that doing so would give me an accurate, direct reading of current flow. I also don't have too many low value resistors lying around and didn't feel like ordering them when the above method seemed just as good.

So is the series meter method a bad idea or do you use the resistor because it's mechanically easier to work with?

pb

Hi there pb,

When working on switchers it's a good idea to use a
resistor to measure current because meters can introduce
unknown resistance and additional inductance which can
cause all sorts of problems.
With series resistor LED circuits it's not as bad, and
i've used my meter lots of times but i know it's internal
resistance is only about 0.010 ohms which is very low
so there is no real error introduced.

If you want a more accurate reading you can always
'calibrate' your 0.1 ohm resistor using one of at least
two different methods:

1. You can select a value slightly above 0.1 ohm and
'trim' it down using larger resistors in parallel, such
as 1 ohms, 10 ohms, etc. To check the value while adding
parallel resistors simply connect in series with your
trusted meter and a larger resistor and a dc voltage
source. You can adjust while watching the meter. If
the meter reads one current level the resistor should
show a voltage drop equivalent to that current flowing
through it (Ohm's Law).

2. You can simply determine the 'actual' value of your
particular 0.1 ohm resistor by connecting it in series
with your trusted meter and a larger resistor and a
dc voltage source. Take the reading from the meter and
use it to calculate the 'true' value of the 0.1 ohm
resistor. You might end up with a true value as low as
0.095 ohm or as high as 0.105 ohm for a 5% unit. When
you go to use it to measure current in a switching circuit,
you apply the 'true' value instead of the approximate
value of 0.100 ohms. It's only a matter of about 5% less
or 5% more current. If you are using a 10% unit then
it could be higher of course so you might want to
determine the true resistance before using it.

Take care,
Al