Your Lab Kit and Your First Measurements
Unit 1: Welcome to Healthcare Technology Management. Lesson 3 of 53 in BMET Academy.
Reading about electricity teaches you the words. Putting probes on a part teaches you the job. This course has a virtual multimeter built in, and if you also buy about forty dollars of parts you can do every lab for real on your own bench.
The kit
- Digital multimeter with test leads. The one tool a biomed carries every day.
- Solderless breadboard and jumper wires: a board full of spring clips that lets you build a circuit without soldering.
- Resistor assortment: parts that limit current. Get at least 100, 220, 470, 1k, 2.2k, and 10k ohm values.
- A few capacitors: parts that store charge.
- 9 V battery and a battery clip: your power source, small enough to be harmless.
- LEDs, red and green: little lights that only work one way round.
- A slide switch: makes and breaks a path.
- A fuse holder with a 1 A fuse: the sacrificial part that opens before something expensive does.
Resistors carry their value as color bands; the memorization sheet's band table decodes them (brown, black, red, gold is 1,000 ohm at 5%), and the lab asks you to read one.
The meter, one control at a time
- The dial picks what the meter measures: V DC for steady voltage, V AC for wall voltage, Ohms for resistance, continuity for "is this one piece of wire", A DC for current, and on many meters a diode test.
- The jacks: the black lead always goes in COM, the common jack. The red lead goes in the V/ohm jack for voltage, resistance, continuity, and the diode test, and moves to the amps jack only when you measure current. Moving that one lead is the step people forget.
- Auto range means the meter picks the scale itself; you just read the number and the unit on the display.
- OL on the display means over limit: the value is beyond the range, which is what an open circuit or a broken wire looks like on ohms.
- A minus sign is not an error. It means the red probe is on the more negative point. Swap the probes and the sign goes away.
Two rules that save meters and fingers
- Never measure ohms on a powered circuit. The meter measures resistance by pushing its own tiny current through the part. Any outside voltage makes the reading meaningless and can damage the meter.
- Never put the meter on amps across a source. On the amps range the meter is almost a plain piece of wire. Across a battery or an outlet that is a dead short, and the best case is a blown fuse inside your meter.
How a lab card works
The virtual meter does exactly what the kit does. Each lab shows a drawing with named test points, a power switch for the circuit, and a list of tasks. You click the red probe's point, then the black probe's, read the display, type the number, and press Record. Check grades the card, tells you why each answer is right, and lets you retry a task or redo the whole lab. Using real parts instead? Tick the box and type what your own meter shows; the accepted ranges allow for real-world tolerance. Your first lab measures a 9 V battery and one loose resistor, which is the smallest possible version of everything that follows.
This lesson includes an interactive lab in the course.
Your Lab Kit and Your First Measurements, explained slowly
The dial, one position at a time
Every handheld meter has the same handful of functions under different symbols.
- V DC (a straight line, sometimes with a dashed line under it). Steady voltage: batteries, power supply rails, board test points. This is where the dial lives most of the time.
- V AC (a wavy line). Alternating voltage: the wall, the primary side of a transformer, a motor winding. Reading AC on a DC range gives you a meaningless number near zero, which fools people into calling an outlet dead.
- Ohms. Resistance. The meter supplies its own small current and measures what comes back, which is why the circuit must be dead.
- Continuity (a speaker symbol). The same measurement with a beeper on it. You use it to ask one question: is this path unbroken?
- A DC (amps). Current. The red lead moves to the amps jack and the meter goes into the circuit.
- Diode test. Pushes a known small current and shows the forward voltage.
- Some meters add capacitance, frequency, and temperature. Nice to have, rarely the fault finder.
Jacks, and the one lead that moves
The black lead lives in COM and never moves. The red lead sits in the V/ohm jack for voltage, resistance, continuity, and the diode test. For current it moves to the amps jack, often two jacks: one for milliamps and microamps and one for up to ten amps. Leaving the red lead in the amps jack and then probing a live circuit on the voltage range is the classic destructive mistake, because the meter is almost a wire on that path.
That amps path is protected by a fuse inside the meter. Blow it and the amps ranges read zero or OL forever while volts and ohms still work perfectly, which is a confusing failure until you have seen it once. A meter that will not read current is usually a meter with a blown internal fuse.
Reading the display
Display Meaning 0.000 a real reading of zero: connected, nothing there OL or 1 alone over limit: beyond the range, or an open path minus sign the red probe is on the more negative point; swap and it goes away mV, mA, k, M the prefix is part of the reading: 1.2 k on ohms is 1,200 ohm a drifting value an unsettled reading: wait, or the probes are not making contact blank or dim dead meter battery
Auto range means the meter picks the scale for you and tells you the unit on the display. Read the unit every time; the difference between 470 and 470 k is the whole answer.
Looking after the leads
Test leads fail more often than meters. The break hides inside the insulation where the lead meets the plug or the probe, so a lead can look perfect and read OL. Test a suspicious pair against themselves: on continuity, touch the probe tips together and the meter should beep and read a fraction of an ohm. Store them loosely coiled rather than wrapped tight around the meter, keep the tips clean, and keep your fingers behind the finger guards.
The two rules, restated
Ohms and continuity are power off measurements, always, and ideally with one end of the part disconnected so parallel paths cannot pull the reading low. Current is an in-series measurement, never across a source. Those two sentences cover nearly every ruined meter and every impossible reading a beginner reports.