Vocademy
Safety

Safety

An oscilloscope is not inherently dangerous. However, you may find yourself measuring dangerous voltages with one.

Oscilloscope ground

The ground lead on an oscilloscope (as with bench-type function generators and multimeters) is a true earth ground. Keep this in mind if you are measuring voltages using other equipment that is connected to the earth. You can connect a typical multimeter to the power outlet any way you want to with reasonable precaution. However, if you connect the ground lead of an oscilloscope to the hot side of a power outlet you will get smoke, sparks and a melted alligator clip.

When connecting equip to a circuit, remember that the ground clip on a function generator, oscilloscope, etc., is connected to a true earth ground through the wall plug. If your circuit is not connected to a true earth ground (battery-powered for example) you can put the ground lead of an instrument anywhere in the circuit. However, now that circuit is connected to a true earth ground at that point. Now if you connect another instrument you must make sure that both grounds go to the same point in the circuit.

For example, if you connect a function generator to a circuit, with the function generator's ground connected to the circuit ground, there is no problem. Now, if you connect an oscilloscope, remember that the oscilloscope ground is also connected to a true earth ground. Wherever you connect the oscilloscope ground is shorted to the same true earth ground provided by the function generator. You cannot now put the oscilloscope ground anywhere in the circuit, only to the circuit ground. Another way to look at it is, both instrument grounds are the same true earth ground. You cannot have two different true earth grounds in a circuit. Those points will be shorted together.
OK

The function generator is grounded through the wall plug. Putting the ground lead at the circuit ground only puts the circuit ground at a true earth ground. A multimeter is isolated so you can place either lead anywhere in a circuit.


NOT OK

An oscilloscope is grounded to a true earth ground through the wall plug. With the function generator already providing a true earth ground at the circuit ground, putting the ground clip of the oscilloscope probe to anywhere except the circuit ground will short that point to ground. Here we are shorting the emitter of the transistor to ground.


OK

Here the oscilloscope probe ground is connected to the circuit ground. This is the proper way to connect the probe.


OK

Here two probes of a dual trace oscilloscope are connected so at to get a differential measurement as the multimeter will. (How to do this will be discussed in the section on using an osilloscope).







How Does Current Know to Take the Shortest Path - A to Q


Vocademy