TESTING A CIRCUIT

Electronic circuits

Whenever you test a electronic circuits, the TEST EQUIPMENT puts "a load" or "a change" on it.
It does not matter if the test equipment is a multimeter, Logic Probe, CRO, Tone
Injector or simply a LED and resistor.
There are two things you need to know.
1. The IMPEDANCE of the electronics circuit at the location you are testing, and
2. The amount of load you are adding to the electronics circuits via the test equipment.
There is also one other hidden factor. The test equipment may be injecting "hum" due
to its leads or the effect of your body at absorbing hum from the surroundings or the
test equipment may be connected to the mains.
These will affect the reading on the test equipment and also any output of the circuit.
Sometimes the test equipment will prevent the circuit from working and sometimes it
will just change the operating conditions slightly. You have to be aware of this.
The point to note here is the fact that the equipment (and the reading) can be upset
by hum and resistance/capacitance effects of test equipment. This is particularly
critical in high impedance and high frequency circuits.