Just starting out current ramping? Don't
forget about voltage testing
..
Voltage tests easily expose voltage drops
that can be caused by excessive resistance or excessive current. These
dynamics can often be hard to determine in a current waveform especially
if you do not have detailed specs for the current level.
To see this for yourself test a radiator
fan motor. Most will show some level of voltage dropping as the waveform
below. At what point is it a problem?
Current, Voltage and Circuit Energy
Another value to voltage testing along
with current testing is that it provides a more complete picture of
the energy the circuit is using. Mathematically, the energy of the circuit
is calculated (voltage x current).
The voltage waveform qualifies the current
waveform. Consider this: If the current waveform measures 6 amps and
the voltage is at 6 volts that is only 36 watts of energy. 6 amps at
12 volts is 72 watts.
This can be extremely important when dealing
with electrical motors such as fuel pumps. For example, an electrical
motor can spin slowly due to some mechanical problem or due to a low
circuit voltage. When the motor spins slow there is more time that any
pair of brushes / commutator bars make contact allowing more current
to flow! If you are testing for a specific current level it should be
qualified by doing a voltage test.
A voltage test qualifies the circuit and
the voltage source integrity.
Qualify
There will be times that the waveform does not look as you would expect
or is simply unfamiliar. For most circuits there are other ways to qualify
and those other methods should always be considered. For example, a
current waveform of a new fuel pump can look bad but the pump can still
be doing its job. The waveform looks bad because the new brushes have
not had time to seat in. The lesson learned is that the final qualifier
is fuel pressure and volume - not the current waveform!
Ask yourself:
Ultimately, what is this circuit suppose to be doing and can I relate
what it is doing to what I see on the scope screen?
Keep in Mind
Keep in mind that a current waveform is just a step in the process.
Sometimes it is enough to determine good or bad. Sometimes it is not.
When it is not, qualify with voltage and with what you hear, see, and
sometimes smell and feel. And don't forget the traditional tests when
applicable.
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