EC-8.1 The Instruments on the Bench

What this is and why it exists

Every laboratory session in this area starts with an instrument already switched on and a reading already on the screen. That hides the two things that actually go wrong. The instrument was set up for a different measurement. And connecting it changed the circuit you were trying to measure.

This topic is the missing first hour. Five instruments, what each one is doing internally, and what each one does to the circuit while it reads.

None of this is difficult. It is skipped because it looks obvious from the outside. Skipping it is why a great many student results are wrong by a factor of two.

The vocabulary

• Input impedance — how much the instrument resists drawing current from what it measures. Higher is less disturbing.
• Burden voltage — the voltage a current meter drops across itself, which is subtracted from the circuit.
• Timebase — the oscilloscope setting that decides how much time the screen width represents.
• Trigger — the condition that decides which moment sits in the middle of the screen.
• Probe compensation — an adjustment that makes a ten-to-one probe respond equally at all frequencies.
• Source impedance — the resistance a generator appears to have in series with its output.
• Current limit — a supply setting that caps the current and lets the voltage fall instead.
• True root mean square — a measurement of an alternating quantity that is correct for any shape, not only a sinusoid.

The mental model

Start with what a multimeter actually does. It measures voltage, and everything else is derived from a voltage measurement. Current is found by passing the current through a small internal resistor and reading the voltage across it. Resistance is found by pushing a known current through the unknown and reading the voltage that appears.

That one sentence explains the behaviour of all three ranges. The voltage range has a very high input impedance, so it disturbs little. The current range has a small resistance in the path, so it drops a little voltage and slightly starves the circuit. The resistance range injects current, so it gives nonsense on a circuit that is still powered.

It also explains the classic failure. A meter left on the current range and connected across a supply is a short circuit through a small resistor. The fuse is there for exactly that.

One more multimeter warning. A cheap meter measuring an alternating voltage assumes the shape is a sinusoid and scales a rectified average. Give it a square wave or a chopped waveform and the reading is wrong. A meter marked true root mean square does the calculation properly.

Now the oscilloscope. Three settings decide whether you see anything at all. The vertical scale sets how many volts one division represents. The timebase sets how much time the width represents. The trigger picks the moment.

Most of the time a beginner sees nothing, the trigger is the reason. The trigger needs a level that the signal actually crosses and an edge to look for. Set the level above the peak and nothing ever triggers, so nothing is drawn.

Probes matter more than people expect. A probe is not a wire. It adds capacitance and resistance to the node it touches, and at high frequency the capacitance is what dominates. A ten-to-one probe divides the signal by ten and adds far less capacitance, which is why it is the default for anything fast.

Compensation is the adjustment that makes a ten-to-one probe divide equally at every frequency. An uncompensated probe rounds off or overshoots every edge. A square wave that does not look square on the calibrator output means the probe, not the circuit. Check it before believing any edge you see.

The function generator has a trap built into its display. Most generators assume the output is loaded with fifty ohms, and the display shows the voltage under that assumption. Connect one to an oscilloscope input, which is a megohm, and there is almost no load. The actual voltage is then twice what the display says. Setting the generator to a high-impedance output mode fixes it, or you can remember the factor of two.

The bench supply has two knobs and gives you whichever limit it reaches first. If you set five volts and one hundred milliamps, it delivers five volts until the circuit tries to take more than one hundred milliamps. Then the voltage falls to whatever keeps the current at the limit.

Set the current limit low before powering a new circuit for the first time. A wiring error then shows up as a supply sitting at half a volt in current limit, rather than as a burnt part.

Finally the logic analyser. It records many lines at once, but records each one only as high or low. That makes it the right instrument when the question is about the timing relationship between signals. It is the wrong one when the question is about the shape of an edge. Choosing between it and an oscilloscope is choosing between many channels with no detail and few channels with all of it.

What you should now be able to explain or do

• Explain why a multimeter measuring current disturbs a circuit more than one measuring voltage.
• Set up an oscilloscope to display an unknown periodic signal, and say what to change when nothing appears.
• Compensate a ten-to-one probe and say what an uncompensated one does to an edge.
• Predict the actual output of a function generator connected to a high-impedance input.
• Use the current limit on a bench supply as protection for a circuit powered for the first time.
• Choose between an oscilloscope and a logic analyser for a given question.

Check yourself