Using a Multimeter

 

DMMEvery ham should have a multimeter and know how to use it.  Multimeter use is a practical skill not only for ham radio but also around the house for general power and wiring work.  As with many topics on this site, details of the subject are too extensive to cover in a simple post so we will give you just basic info along with some resources for further study on your own.  Even with minimal detail this is still a long, involved post.

By definition multimeters measure more than one thing.  In electrical work a multimeter typically measures voltage, current, and resistance.  Sometimes it is called a volt-ohm-milliammeter or simply a volt ohm meter (VOM).  Note that these are the three fundamental electrical parameters as described by Ohm’s law.

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In addition to measuring the three core electrical parameters, multimeters may also read other things such as temperature, frequency, capacitance, plus provide quick checks of diodes and continuity.   At minimum they will measure voltage and resistance, since these are the two most commonly read values.

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To measure voltage  you connect the meter leads between (across) two points. This parallel connection allows real-time undisturbed readings in live circuits or power sources.

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Note that resistance measurements are also made with test lead parallel connections but never on live circuits; more on that below.

Measuring current with a common multimeter is more disruptive because it requires the circuit to be broken somewhere and have the meter leads inserted in-line to read amperage, and that is not often very convenient. 

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This series connection requirement for measuring current is the main reason it is less commonly used, and why clamp-on (non-contact) current meters are sometimes favored at the expense of accuracy and greater cost.

So what would the ham or handy homeowner need a multimeter for?  The possibilities are endless but common scenarios are testing batteries, locating blown fuses, verifying DC and AC power supply voltages, and checking cables for undesired opens or shorts.  For kit or DIY circuit builds it is also useful to verify resistor values and test active circuit voltages.  Here is a link to a list of Ten uses for a multimeter from Ham Radio School.


There are a few safety considerations to note when using a multimeter.  Two involve the test leads which connect the meter terminals to the circuit being measured.  First, the leads (probes, wires, and connectors) should be rated for at least the voltage being measured. The meter itself must also be rated to exceed this voltage.

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Good meter leads will have shrouded plugs, high quality flexible insulation, and finger flanges at the probe tips:

DMM LeadsThey will also be rated at least 600V as marked on the probe for general measurement below that voltage.  Special Continue reading

Impedance

Impedance is an important subject in amateur radio so we want to spend a little time discussing it here.  Several topics on this site will involve impedance so it’s good to have this basic concept well understood.

In ham radio work we deal with impedance in transmission lines, antennas, transmitter outputs, receiver inputs, microphones, speakers, headphones, and other devices.  Impedance matters everywhere a signal couples to something different.

Basic resistance (R) is what opposes current in a DC circuit, and all components have measurable resistance.

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But things get more complicated in AC circuits.  Capacitors and inductors (coils) oppose change.  This includes alternating current, a characteristic of audio, video and radio frequencies.   The properties of capacitance and inductance have well-defined opposition to AC which varies by signal frequency.

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All components have measurable capacitance and inductance so there is always some reactance (X) in a circuit.  There are two flavors of reactance: capacitive and inductive.  Interestingly, they respond oppositely to signal frequency.  Inductive reactance (XL) goes up with frequency while capacitive reactance (XC) goes down.

When you add the constant resistance in a circuit to the capacitive and inductive reactance, the result is impedance (Z=R+jX).  In broad terms, it can be considered “AC resistance”, which is legitimate when we don’t care about the complex phase angle part of the equation.  Resistance plus reactance equals impedance (Z).

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Like DC resistance, impedance (AC resistance) is measured in ohms.

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OK so far?  Click on the many hyperlinks in this article for more detail, along with helpful links below.  Don’t worry, you only need to grasp the basics here; high-level math is not necessary for a working knowledge of impedance.

Now that you know what impedance is, the next important thing to understand is that when an AC signal interfaces with a new circuit, the impedances should match.

When impedance of a source (ZS) equals the load impedance (ZL), the best possible signal coupling occurs. Conversely, when impedances are not the same, signals couple poorly.

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The maximum power-transfer theorem says that to transfer the maximum amount of power from a source to a load, the load impedance should match the source impedance (ZS=ZL).

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Good examples of impedance matching are: audio amplifier output to speaker (8Ω); transceiver RF circuits to antenna feed line (50Ω); microphone to audio input (2000Ω).

Impedance matching can be accomplished by Continue reading