ECE 220 Common Mistakes

Measurement of Voltages and Currents:

Voltages are measured with the DMM probes in parallel with the circuit element,
but Currents are measured with the DMM probes in series with the circuit element.

Remember,
Voltages occur across
Current flows through

Hence, while measuring Currents, it becomes necessary to break the circuit and let the current pass through the measuring device.
Measuring Voltage is straightforward. Just place the two measuring probes on the two points between which the potential difference is to be read.

While using the DMM:
A DMM can measure Voltages, Currents and Resistances. Be sure to use it in the right mode, i.e., I/V/Ohm. While measuring V or I, ascertain whether the AC/DC switch is in the appropriate position. Attempting to measure AC in DC mode will read out a zero (or a very small ) value. Never plug the DMM in Ohm mode into a live circuit.

While using the DC power supply:
The DC power supply in your lab is equipped with two independent power sources, which may be used together if necessary. But often you will need only one of them. You may use source A or source B, but make sure you vary the right voltage. (Avoid plugging into source A and varying source B).

Modeling ideal devices:
We know ideal devices have an internal resistance of zero ohms, but it is not the case in a practical ones, e.g., the function generator in your lab or the D'Arsonval meter you use in Lab 2. We represent this as a series combination of an ideal device and a resistor on paper, i.e., circuit diagrams. But while building the circuit in the lab, it must be remembered that this 'Resistor' is already in the device. It is not a separate component. Building the circuit with shown internal resistance as a separate external component doubles it!