Polarity is the direction of current flow in a circuit from the two poles, positive and negative. You require two points of differential charge for any electron movement to take place.

In an AC circuit, each pole alternates between positive and negative 60 times per second. You can see from the image of the AC sine wave, that the change in polarity of AC does not happen instantly, although close to and 60 times per second (Hertz).

(Top positive side of the sine wave) Voltage increases from 0V to its max “peak” voltage, which is about 170V in the above example, while a meter reads 120V RMS. At that point, the voltage then begins to decrease back to 0V.

(Bottom negative side of the sine wave) The voltage then reverses polarity and drops below 0V, again heading for the peak voltage but the negative polarity.

In a DC circuit, it’s between the negative and positive with current flowing from negative to positive.

What this means on a service call

Polarity decides which lead is positive and which lead is negative at the moment you test. Get the leads backwards and your reading, your wiring, and your diagnosis can all point the wrong way.

In a DC circuit, polarity stays put. One side stays positive and the other side stays negative while the circuit is powered. Current flows from negative to positive in the original lesson, and the load, the control, and the meter all expect the same two sides in the same places. Swap the two wires on a DC control or a DC powered part and the part may not work, even though voltage is present.

In an AC circuit, polarity changes. Each pole alternates between positive and negative, as the original lesson states. That does not mean the wires are interchangeable in every case. On HVAC equipment, the hot side, the neutral or common side, and the ground side still have assigned places on the transformer, the control board, and the thermostat connections. Put a wire on the wrong terminal and a control may not call, a safety may not prove, or a meter reading may not make sense.

On a service call, start by naming the two points you are testing. Say which lead is on which terminal, and say whether you expect that point to be positive, negative, hot, or common at that moment. If the names do not match the wiring diagram, stop and sort out the wiring before you replace a part.

Numbers and measurements you will see

Use the numbers that belong to the circuit in front of you. A control circuit is commonly 24V. A line circuit is commonly 120V. AC in the original lesson alternates 60 times per second, which is 60 Hertz. Your meter will show those values in different ways depending on the leads.

• 24V on a control circuit, measured from R to C on a transformer or a control board. A reading near 24V tells you the transformer secondary is powered. A reading of 0V tells you it is not, or that you are on the wrong two points.
• 120V on a line circuit, measured at the disconnect or at the line side of a control. The meter displays 120V when the leads are on the two points that carry line voltage.
• 0V at the crossing point on an AC sine wave. Voltage passes through 0V as it changes from positive to negative and back again. A steady 0V reading on a powered circuit usually means the meter is on two points at the same potential, or the circuit is dead.
• A negative sign on a DC reading when the leads are reversed. If you expect positive 24V and the meter shows negative 24V, the leads are on opposite points from the way you expected. Swap the leads and recheck before you decide the circuit is wrong.
• 60 Hertz on an AC circuit. A meter with a Hertz setting should show 60 Hertz on a standard AC supply. A reading far from 60 Hertz, or no Hertz reading at all, tells you to recheck the meter setting and the test points.
• Positive on one lead and negative on the other in a DC circuit. The two readings should be opposite in sign and equal in size when you swap the leads. If they are not, one test point is not what you think it is.

A common mistake

Treating AC wires as if polarity never matters is the common mistake. A technician swaps two wires, the equipment still gets power, and the technician assumes the swap is fine.

Power at the equipment does not prove the control wiring is right. A thermostat, a control board, and a safety control each expect specific terminals to be hot, common, or switched. A swapped pair can leave a control with no clear call, or with a call that never clears. Put each wire back on the terminal shown on the wiring diagram, and test from the diagram, not from memory.

A second mistake is reading a negative DC voltage and calling the circuit dead. A negative reading is still a reading. It tells you voltage is present and the leads are reversed from the polarity you expected. Swap the leads, note the sign, and use the result. Do not ignore the minus sign.

Check it in the field

Caution: live and powered work can hurt you. Treat a circuit as live until you prove it is dead with your meter. Keep your hands clear of bare conductors and terminals. If you are not sure a test is safe, stop, turn the power off, and get help. Do not bypass a switch or a safety control to force a load to run.

1. Identify the two test points from the wiring diagram before you touch the meter. Name each point and note whether it should be hot, common, positive, or negative.
2. Set the meter for the circuit you are testing, AC voltage for an AC circuit and DC voltage for a DC circuit. Check the meter on a known source first.
3. Place the red lead on the point you expect to be positive or hot, and the black lead on the point you expect to be negative or common. Read the number and the sign.
4. Swap the leads and read again. On DC, the size should stay the same and the sign should reverse. On AC, the size should stay about the same. If the readings do not match that pattern, recheck the test points and the meter setting.
5. On a 24V control circuit, measure R to C. Expect about 24V. Then measure from R to the terminal in question. A missing or wrong reading points to the wiring or the control, not to the meter.
6. Turn the power off before you move a wire. Put each wire on the terminal shown on the diagram, tighten the connection, restore power, and retest with the leads in the same places you used first.

Related posts

• Electrical circuit components
• Direct current
• Alternating current
• Voltage
• Current (amps)
• Resistance
• Series Circuits
• Parallel Circuits
• Ohm’s Law

What is Direct Current?

In a DC (direct current) circuit, all the electrons hop from electron to electron in a single direction, hence the name, direct current. Since electrons only flow in one direction, electrical polarity matters, so you’ll need to place each wire exactly how it’s depicted on a wiring diagram.

Electrons flow from negative to positive. Since one side of the power source is always negative, and the positive side is always positive, the difference is what pushes electrons from one side to the other.

A battery in a flashlight is an example of a source of direct current.

What this means on a service call

Direct current is steady in direction. The positive side stays positive and the negative side stays negative while the source is connected. That steadiness is the point to use on a service call.

When a DC circuit is not working, the first question is simple: is the positive wire on the positive point, and is the negative wire on the negative point. The original lesson states that polarity matters in DC and that each wire must be placed exactly as shown on the wiring diagram. A DC load will not run right if the two sides are swapped, even if the source is good and the wires are tight.

Most HVAC line and control power you test every day is AC, not DC. The 24V control circuit and the 120V line circuit are AC. DC shows up in smaller places: a battery in a thermostat, a battery in a meter, a DC signal on a control board, and a DC output from a power supply or a sensor circuit. Do not assume a circuit is AC or DC. Read the diagram and the markings on the board, then set the meter to match.

On a service call, a steady DC reading that stays on one side of 0V tells you the circuit is holding its polarity. A reading that swings from positive to negative, or that reads 0V on the DC setting while the load is supposed to be powered, tells you to recheck the meter setting, the test points, and the source before you replace a part.

Numbers and measurements you will see

DC numbers are read with the sign included. The size of the reading matters, and so does the plus or minus sign in front of it.

• A steady DC voltage reading. The number should hold steady while you watch the meter, not swing up and down. A reading that will not settle can mean a loose connection, a weak source, or the meter on the wrong setting.
• 0V on a DC circuit that should be powered. A steady 0V reading means there is no difference in charge between the two test points, or the source is not connected. Check the source, the fuse or breaker in that circuit, and the connections before you test the load.
• A negative reading when the leads are reversed. If the meter shows a negative value on DC, the black lead is on the positive point and the red lead is on the negative point. Swap the leads and the reading should show the same size with a positive sign.
• A reading of 0V, or a very low reading, on the AC setting when you test a DC source. A meter set for AC will not read a DC source the way you expect. Set the meter to DC volts, retest the same two points, and compare.
• 24V and 120V as AC readings for comparison. Those are the common AC values on a control circuit and a line circuit. They help you spot the wrong meter setting: if you expect DC and you read a steady AC value instead, you may be on the wrong two points or the wrong source.
• Positive on one terminal and negative on the other, every time you test. In DC, the two terminals do not swap roles. If the sign changes without you moving the leads, the source or the connection is not steady, and you need to find why.

Record the setting, the test points, the sign, and the number together. A note that says DC, red lead on the positive terminal, black lead on the negative terminal, steady positive reading, is useful. A note that says only a number is not.

A common mistake

Testing a DC circuit with the meter set for AC is the common mistake. The meter shows 0V or a low, unstable value, and the technician calls the source dead.

The source may be fine. The meter was on the wrong setting for the circuit in front of it. AC and DC use different meter settings because the current behaves differently. Check the diagram, set the meter to DC volts for a DC test, and retest the same two points before you decide anything is dead.

A second mistake is swapping the DC leads and trusting the first reading. A swapped pair gives a negative reading that looks like a fault. Swap the leads back, confirm the positive terminal is where the diagram says it is, and read again. The sign is information, not a fault by itself.

Check it in the field

Caution: live and powered work can hurt you. Treat a circuit as live until you prove it is dead with your meter. Keep your hands clear of bare conductors and terminals. If you are not sure a test is safe, stop, turn the power off, and get help. Do not bypass a switch or a safety control to force a load to run.

1. Find the DC source and the two terminals on the wiring diagram. Note which terminal is marked positive and which is marked negative.
2. Set the meter to DC volts. Check the meter on a known source first, such as a battery, so you know the meter and the leads are working.
3. Place the red lead on the positive terminal and the black lead on the negative terminal. Read the number and the sign, and watch whether the reading holds steady.
4. Swap the leads and read again. The size should stay the same and the sign should reverse. If it does not, recheck the terminals and the meter setting.
5. If the reading is 0V, check the source, the connections, and any fuse or switch in that DC circuit before you test the load. A dead reading at the source means the load cannot run, no matter how good the load is.
6. Turn the power off, or disconnect the source, before you move a DC wire. Put each wire on the terminal shown on the diagram, restore power, and retest with the leads in the same places you used first.

Related posts

• Electrical circuit components
• Polarity
• Alternating current
• Voltage
• Current (amps)
• Resistance
• Series Circuits
• Parallel Circuits
• Ohm’s Law

What is Alternating Current?

Alternating current, (AC) is the most common form of electricity. AC became the standard due to the ease of distributing power over a long distance. With AC, a simple transformer outside your house steps the voltage down from the power company supply to a usable voltage in your home. Since it’s made up of no moving parts and some coils of wire, it’s a lot less expensive than using DC power as the standard delivery method.

In alternating current, the electrons do not move in just a single direction like they do in a DC circuit. Electrons will travel one path going from atom to atom, happy as can be, then turn around and hop backward atom to atom. They do this many times per second.

The rate at which alternating current reverses direction is called the frequency, and we express this in Hertz. Canada and the US are at 60 Hz whereas Europe is 50 Hz.

Example: It often helps compare the back and forth flow of alternating current to water in a piping system.

With the shaft fully forward, water is pushed in one direction. Water will start to move in this direction as the shaft begins to push outwards from the opposite direction.

As the shaft starts to pull in, water alternates direction and flows the opposite way. Now visualize this taking place 60 times a second.

Regardless of what direction of flow, water still flows throughout the whole piping system.

Each time the shaft in the piping above is fully outwards or fully inwards would be the peaks of the positive and negative sine wave.

What this means on a service call

Alternating current powers almost everything you touch on an HVAC call. The line circuit, the control transformer, the motors, and the controls all run on AC. When AC is missing or wrong, the equipment cannot run, even if every part is good.

AC changes direction many times per second, as the original lesson states. Your meter hides that back and forth movement and shows a steady voltage value instead. That steady value is what you use to decide if power is present. A motor that hums, a control that will not pull in, and a transformer that reads 0V all start with the same first test: is AC voltage present at the right two points.

The transformer is the link between the line circuit and the control circuit. The original lesson notes that a transformer steps voltage down from the power company supply to a usable voltage in the home. On HVAC equipment, that step down gives you the 24V control circuit. If the line side has 120V and the control side has 0V, the transformer, its fuse, or its wiring is the next place to check. If both sides are dead, the problem is further back toward the disconnect or the breaker.

Numbers and measurements you will see

Use the meter on the AC volts setting for AC work. The numbers below are the common field values from the lessons on this site.

• 120V on a line circuit, measured at the disconnect or at the line side of a control. A reading near 120V tells you line power is present at that point.
• 24V on a control circuit, measured from R to C on a transformer or a control board. A reading near 24V tells you the transformer secondary is powered and the control circuit has a source.
• 0V across a closed switch or a closed contact. When a switch or a contact is closed and current can flow, there should be little or no voltage drop across it. A large reading across a closed contact points to a bad contact.
• Full voltage across an open switch or an open contact. When a switch is open, the full circuit voltage appears across it. That reading proves power is reaching the switch, even though the load is dead.
• 60 Hz in Canada and the US, and 50 Hz in Europe, as stated in the original lesson. A meter with a Hertz setting should show 60 Hz on a standard AC supply in Canada and the US. A reading far from 60 Hz, or no Hertz reading, tells you to recheck the meter setting and the test points.
• 0V on a circuit that should be powered. A steady 0V reading means no difference in charge between the two test points, or the circuit is dead at that point. Work back toward the source, one test point at a time, until voltage appears.

A common mistake

Testing an AC circuit with the meter set for DC is the common mistake. The meter shows 0V or a low, unstable value, and the technician calls the circuit dead.

Line and control power on HVAC equipment is AC. Set the meter to AC volts before you test a line or control circuit. Recheck the setting if a reading does not match what the circuit should have. A wrong setting wastes time and can lead to a good part being replaced.

A second mistake is testing only one side of a circuit. A reading of 120V at the disconnect does not prove 120V is reaching the load, and a reading of 24V at the transformer does not prove 24V is reaching the thermostat. Test at the load, at the control, and at the source, and compare the three readings.

Check it in the field

Caution: live and powered work can hurt you. Treat a circuit as live until you prove it is dead with your meter. Keep your hands clear of bare conductors and terminals. If you are not sure a test is safe, stop, turn the power off, and get help. Do not bypass a switch or a safety control to force a load to run.

1. Set the meter to AC volts. Check the meter on a known source first, such as a 120V outlet, so you know the meter and the leads are working.
2. Test line voltage at the disconnect. Expect about 120V on a 120V circuit. If the reading is 0V, check the disconnect, the breaker, and the supply before you test further into the equipment.
3. Test control voltage at the transformer. Measure R to C and expect about 24V. If the line side is powered and the control side is 0V, check the transformer and any fuse in the control circuit.
4. Test across the load with the circuit calling for that load. A load that has full voltage across it and does not run has a load fault or an open inside the load. A load with 0V across it is not being fed.
5. Test across each switch and contact in that circuit. An open switch should show full voltage across it. A closed switch should show about 0V across it.
6. If the meter has a Hertz setting, check the frequency on the AC supply. Expect 60 Hz in Canada and the US. A reading far from 60 Hz tells you to recheck the setting and the test points.

Related posts

• Electrical circuit components
• Polarity
• Direct current
• Voltage
• Current (amps)
• Resistance
• Series Circuits
• Parallel Circuits
• Ohm’s Law

Everything is made up of atoms. Atoms are made up of protons, electrons, and neutrons. The nucleus of the atom contains neutrons which have a neutral charge and protons which have a positive charge. Electrons orbit the nucleus and have a negative charge.

If you have the same amount of protons as electrons, you’d have a neutral charge. If the atom loses an electron, you’d have a positive charge. If the atom has more electrons, the atom would be negatively charged. Electrons can jump from one negatively charged object to a positively charged one and will balance the charge. The energy used when the electrons move is the voltage.

Valence electrons are responsible for the electrical conductivity of an element. Valence electrons are the electrons in the outermost shell of the atom which can be transferred to other atoms.

Conductors have few valence electrons. They’re willing to give them up easily. They’re like the terrible parents of atoms. They’re good conductors. Copper, gold, and silver are examples.

Insulators have outer electrons held very tight. They’re like the protective parents of atoms. They don’t allow for good electrical flow. Glass and plastic are examples.