Multimeter
| Multimeter | |
|---|---|
| Also called | DMM (digital multimeter), VOM (volt-ohm-milliammeter), multitester |
| Basic functions | DC and AC voltage, current, resistance |
| Common extras | Continuity beeper, diode test, capacitance, frequency, temperature |
| Main types | Digital and analog |
| Buying | |
| Safety standard | IEC 61010 measurement categories (CAT II, CAT III, CAT IV) |
| Typical input resistance | About 10 megohms on the voltage ranges of a digital meter |
| Features worth having | True RMS, fused current inputs, an independently tested CAT rating |
A multimeter is a test instrument that combines a voltmeter, an ammeter and an ohmmeter in a single case. It is usually the first piece of test equipment a ham buys, and it answers the questions that come up most often in the shack: Is the power supply really putting out 13.8 volts? Is that fuse blown? Is the coax shorted? Even an inexpensive digital model is enough for most station and homebrew work.
What it measures
Every multimeter handles three basic quantities, which are tied together by Ohm's law.
- Voltage is measured across a part or circuit, with the probes connected in parallel with it. Meters read DC volts, for batteries and power supplies, and AC volts, for household power.
- Current is measured through a circuit. The circuit must be broken and the meter placed in series, usually with the red lead moved to a separate jack. Most hand-held meters have a milliamp input and a 10-amp input, each protected by a fuse.
- Resistance is measured with the circuit switched off. The meter passes a small current from its own battery and measures the resulting voltage. A resistor still soldered into a circuit may read low, because other parts are connected in parallel with it.
Two related functions are just as useful. Continuity mode beeps when the resistance between the probes is very low, so connections can be checked without looking at the display. Diode test shows the forward voltage of a diode or transistor junction, about 0.6 to 0.7 volts for silicon.
A multimeter can't measure radio-frequency signals directly; most are specified for AC only at power-line and low audio frequencies.
Digital and analog meters
An analog meter, often called a VOM, uses a needle moved by a coil in a magnetic field. Its loading on a circuit is rated in ohms per volt, around 20,000 for a good one, so on low ranges it can upset high-resistance circuits. The needle's great advantage is that it shows trends at a glance, which makes an analog meter pleasant for peaking a tuned circuit or watching a capacitor charge.
A digital multimeter, or DMM, shows a number. Its resolution is given in counts: a 6,000-count meter displays up to 5,999 before switching to the next range. Most are autoranging, choosing the range by themselves, and on voltage ranges they present a high input resistance, typically about 10 megohms, so they barely disturb the circuit under test.
True RMS is worth paying for. Cheaper meters measure the average of an AC waveform and scale it as if it were a pure sine wave, so they misread distorted waveforms such as the output of a modified-sine-wave inverter. A true-RMS meter gives the correct RMS value for any waveform within its frequency range.
Safety ratings
Meters meant for household and building wiring are rated under the international IEC 61010 safety standards, which define measurement categories by how much energy a voltage spike, or transient, could deliver at the point being measured. The closer to the utility supply, the higher the category needed.
| Category | Where it applies |
|---|---|
| CAT II | Wall outlets and equipment plugged into them, such as appliances, tools and the line side of a power supply |
| CAT III | Building wiring: distribution panels, breakers, fixed wiring and permanently installed equipment |
| CAT IV | The service entrance, the electric meter and utility lines outside the building |
Each category is paired with a voltage, such as CAT III 600 V, and a meter rated for a higher category may be used in a lower one. Ratings printed on very cheap meters are not always backed by testing, so a mark from an independent laboratory such as UL, CSA or TÜV is reassuring. Good meters also have high-energy fuses and shrouded test leads.
For low-voltage DC work on radios, almost any meter will do; the ratings matter once the probes go near mains wiring.
Common uses in the shack
- Power and batteries. Checking power supply output, battery voltage at rest and under load, and the voltage drop along a DC power cable while transmitting.
- Coax and cables. With both ends disconnected, the center conductor and the shield should each show continuity end to end and no connection to each other. With an antenna attached, a short is not always a fault: some antennas and matching transformers are a direct short at DC by design. Continuity mode also checks fuses, switches and microphone cables.
- Dummy loads. A dummy load should read close to 50 ohms.
- Current draw. Knowing how much current a radio takes on receive helps size a battery for portable operating. A 100-watt transceiver draws around 20 amps on transmit, beyond most meters' current inputs; a clamp meter that reads DC, sensing the magnetic field around a single wire, is the safer tool.