# Hot on the trail of an elusive current fluctuation

An engineer relies on first principles to troubleshoot a solar cell testing system

I
was tasked one day to read the fluctuations of a large current DC
source and
sink system. This
happened to be a solar cell testing system that simulated the dynamic
loading
of a satellite system on a solar panel array’s output.(We
produced the load, a large dynamic impedance range, 400 amps at 8 volts
to 400
volts at 15 amps.)

The question was where the current fluctuation was coming from--the changing load or the source? We
needed to prove to our customer that it was not the load or control
system that
was the culprit. This
was the question that needed to be answered, but I
first needed to be able to measure a small AC current level on a large
~200 amp
current.The
problem, of course, being the large difference between the total current and the
~200
milli-amp current fluctuation.

How
was I to measure this? If
I placed the scope and amp probe setting on a setting that displayed
the
waveform, the changing level became too small to even see--let alone
measure with
any accuracy.

Clever
engineers go back to basic physics principles to find a solution. In
this case the solution lie in the fact that the current probe measures amp turns, not
amps.This
may not seem obvious or even true unless you remember that a current
probe
usually just has one turn--so measuring amp turns is measuring amps.

Some
readers may have jumped to the “aha” by the hint dropped so far. My
solution was to wrap 100 turns of small gage wire around (through) the
current
probe along with the high current cable carrying the 200 amps. Hooking
up an
accurate current source with a low impedance output (they go
hand-n-hand ) and running 2 amps through this wire in constant current
mode.

The
2 amp x 100 turns created 200 amp turns of probe signal that canceled
out the
constant DC 200 amp level, leaving just the ripple
current.
Now the scope's resolution could be turned up, revealing the ripple
current in
high resolution.

The
question at hand could be answered based on the data taken with this
technique.

Ron Davison is a staff
electrical engineer at TAVIS
CORPORATION