Wiring diagrams
Wiring diagram is a general term for Schematic, Wiring and Position diagrams. A schematic diagram shows the components and connecting wires in a way that eases determining how the unit sequences when it is energized. Below is a simple schematic diagram.
Below is a simple wiring diagram. It shows how the wires are connected to the components of the circuit.
The position diagram shows the location of the components in the control panel.
The schematic diagram is the most valuable one for the service tech. With this diagram, one can quickly determine how the unit should cycle. If you know how it should cycle, you can determine where the problem is. Once you understand the schematic diagram, you can troubleshoot almost any electric appliance.
The schematic diagram is separated into different circuits and voltages. When we look at the schematic below, we can see the power source at the top. L1 and Neutral is the power source. The power at L1 then passes down thru a manual switch at left to a load. The power then passes back to the source thru neutral. This is the simplest of diagrams but it contains all a circuit needs. A source, a switch, and a load.
The schematic diagram is separated into different circuits and voltages. When we look at the schematic below, we can see the power source at the top. L1 and Neutral is the power source. The power at L1 then passes down thru a manual switch at left to a load. The power then passes back to the source thru neutral. This is the simplest of diagrams but it contains all a circuit needs. A source, a switch, and a load.
When using schematic diagrams it is useful to sequence operation of the unit. That is to follow the power from the source thru all switches and to the load. By sequencing the circuit, we can find the problem where the power stops before energizing the load. The video below describes this circuit.
Any given circuit will have only one load. As with everything electrical, there is no hard and fast rule for every circuit. But under most conditions, there will only be one load on a circuit.
However, the number of switches in a circuit is unlimited. The video below shows a circuit with 2 switches. There is both a manual switch and a temperature controlled switch. In this case, either switch, when opened, will cut power to the load. These switches are wired in series.
However, the number of switches in a circuit is unlimited. The video below shows a circuit with 2 switches. There is both a manual switch and a temperature controlled switch. In this case, either switch, when opened, will cut power to the load. These switches are wired in series.
The video below adds more switches. The manual switch can cut off all power to the circuit. It also includes a normally closed limit switch that will open on an increase in temperature above the normal temperatures for safety. Note all switches are illustrated in the deenergized or power off position.
Below we have added another load and switch. Here we have a complete appliance. This would be a simple gas furnace. The manual switch on the left shuts down the entire appliance. The other switches can either turn on or turn off the 2 loads. There are 2 parallel circuits on this diagram. If the manual switch is closed, power passes thru the normally closed limit switch. If the thermostat is closed, the gas valve is energized. This would allow the burners to light. In the lower circuit, Power passes to the normally open fan switch. If the burners are lit, the furnace will warm and close the fan switch which will energize the fan motor. The motor is an independent (or parallel) circuit from the other, gas valve circuit. If the furnace is warm, the fan motor is energized.
In the diagram below we have added a low voltage circuit. The high voltage circuit consists of a fan motor circuit that energizes the fan motor on an increase in heat exchanger temperature and and the primary of the low voltage transformer. The transformer is energized as long as the manual switch is closed.
In the 24 volt circuit, control voltage is always available as long as the manual switch is closed. The sequence of operation of the low voltage is: Power passes the the normally closed limit. If the thermostat is closed, power passes thru the thermostat to energize the gas valve.
In the 24 volt circuit, control voltage is always available as long as the manual switch is closed. The sequence of operation of the low voltage is: Power passes the the normally closed limit. If the thermostat is closed, power passes thru the thermostat to energize the gas valve.
On the diagram below, the complexity of the diagram has been increased. We have added a low voltage control for the fan. It is relay that can turn on the fan without the heat operated fan switch being closed. Note the high voltage circuit. We have a parallel circuit with the fan motor. If the heat operated fan switch is closed, the fan will run. If the fan switch R-1 is closed, the fan will run. If both are closed, the fan will run. This circuit shows the arrangement of parts of the schematic diagram that differs from the wiring diagram. In the wiring diagram, both the contacts and the actuating coil of the relay are in the same place. In the schematic, the relay R-1 is shown in both the 24 volt circuit and the 120 volt circuit. This is done to make the circuit easier to sequence. The sequencing of the manual fan switch is as follows: When the manual fan switch closes, the 24 volt coil of relay R-1 energizes. When the coil energizes, the contacts of R-1 in the 120 volt circuit close, energizing the fan motor. When troubleshooting the circuit, you need to understand that relays or other components will have their components in located in different positions. They will be linked by a designation such as we see here as R-1.
In the next video below, we have installed a fan motor with 2 speeds. There is no difference in the low voltage circuit. However, the high voltage circuit now includes a 2 speed fan motor. This places more complication to the circuit. The 2 speed fan motor cannot have both speeds on at the same time or the motor will burn out. So we use a single pole double throw switch on the relayR-1. This is what we call an either or switch. When the normally closed portion of the relay is in the deenergized position, the heat operated fan switch can close and the fan motor will energize in low speed. If the manual fan switch is closed, the fan relay R-1 will energize, switching the contacts of the relay. This will open the circuit of the low speed. It will also close the normally open contacts, energizing the fan in high speed. This a common circuit to protect the motor from damage.
In the video below, we have added a low voltage thermostat. This is a typical mechanical type thermostat that is used, in this case, in a heat only system. The high voltage portion of this diagram is no different than the above diagram. Look closely at the wiring of the thermostat. Also note the letters used on the terminals of the thermostat. For more on thermostats, check here. Low voltage power is connected to the R terminal. If the manual fan switch is closed, power passes thru the manual switch and out thru The G terminal. Power then energizes the fan motor. If there is a call for heat, Power passes from R thru the closed thermostat and out W, thru normally closed limit switch to energize the gas valve, starting the furnace.
The diagram below is both a gas furnace and an air conditioner.
In the high voltage side, We have added a compressor with its capacitor, a contactor with its single pole, single throw, normally open contacts, and a condenser fan motor.
In the low voltage circuit, we have added the Y circuit with the coil of the contactor.
We will look at the sequencing of the air conditioner: When the thermostat closes from R to Y, power passes thru Y to energize the contactor 1 coil. In the high voltage, contactor coil contacts will close, passing power to energize both the compressor and the condenser fan motor.
At the same time, note the thermostat fan circuit. Power passes from R thru the thermostat and thru the manual fan switch (single pole, double throw) out thru G to energize
Fan Relay R-1 which reverses the R-1 contacts in the fan motor circuit.
In the high voltage side, We have added a compressor with its capacitor, a contactor with its single pole, single throw, normally open contacts, and a condenser fan motor.
In the low voltage circuit, we have added the Y circuit with the coil of the contactor.
We will look at the sequencing of the air conditioner: When the thermostat closes from R to Y, power passes thru Y to energize the contactor 1 coil. In the high voltage, contactor coil contacts will close, passing power to energize both the compressor and the condenser fan motor.
At the same time, note the thermostat fan circuit. Power passes from R thru the thermostat and thru the manual fan switch (single pole, double throw) out thru G to energize
Fan Relay R-1 which reverses the R-1 contacts in the fan motor circuit.
Below we have added a 2 stage heating system. This includes a thermostat with a rather different switch. Note the temperature actuated switch at the bottom of the thermostat. The top switch closes to W1 at a temperature about 2 degreesF higher than the lower switch. This will add more heat if the first stage cannot keep up with the load.
Power is available at R. If the W1 switch is closed, power will pass thru the normally closed limit switch to energize the gas valve first stage. If the temperature continues to drop, power will pass thru W2 to energize the gas valve 2d stage. The fan continues to operate in low speed.
Power is available at R. If the W1 switch is closed, power will pass thru the normally closed limit switch to energize the gas valve first stage. If the temperature continues to drop, power will pass thru W2 to energize the gas valve 2d stage. The fan continues to operate in low speed.
In the diagram below, We have added high speed on the fan for when the furnace is in high fire. When the 2d stage thermostat closes, power will pass thru and energize the gas valve in 2d stage as on the above diagram. However, we have added another relay, Fan relay R-2. The low voltage coil is energized. In the high voltage circuit, the R-2 relay is similar to R-1 in that the contacts are single pole double throw. This drops out low speed and closes high speed when the furnace is in high fire.
The video below just gives another way to show a schematic diagram. We also have included a legend to tell what the abbreviations mean. We have placed the high voltage circuit horizontally. It is no different than the vertical diagram in its sequencing. It is just drawn differently. Diagrams will be different for different manufacturers, and you must learn all of them.
In the circuit below, we have an air conditioner that is 3 phase. The high voltage circuit uses a discount ct that opens 3 circuits and the contactor also uses 3 sets of contacts. Starting with the low voltage, when the thermostat closes, power passes thru the thermostat to energize contactor C and fan relay coil. This closes the 3, C contacts that pass power to the 3 phase compressor and the single phase condenser fan motor CFM. Note in many 3 phase units the compressor is the only 3 phase load. The CFM is single phase as it only uses 2 of the lines. The indoor fan motor, IFM is also single phase.
The air conditioner below includes a number of components that are not shown on earlier diagrams. Lets go over some of these and what they do. In the high voltage, note CCH on the left over the contactor points. This circuit is interesting because CCH is a crankcase heater for the compressor. When the compressor is running we want to shut off the heater. Lets see how this is done. With the C contacts open, the only way for power to pass thru the circuit is to travel thru the crankcase heater then thru R on the compressor and out C thru the thermal overload OL. There is also a circuit thru CFM but most of the power passes thru the compressor because the windings have far less resistance than CFM. The heater is a small one, using about 50 to 80 watts. With the amperage reduced by the high resistance heater, the power will pass thru the compressor winding without starting the compressor. When the contacts C close, it provides a low resistance path to the compressor and CFM. This reduces the amperage draw of the heater to the point that it is effectively off. Its a simple method of controlling the crankcase heater without a relay. The thermal overload OL cuts power to the compressor if it overheats. There is also a second CFM that only comes on at a preset outdoor temperature. This provides for low ambient operation of the air conditioner. We have also provided a potential relay to provide extra starting torque for the compressor.
In the low voltage circuit, there are several safety switches provided to shut down the unit if certain pressures or temperatures are exceeded. DTS is a normally closed discharge temperature switch that will open if the temperature of the refrigerant leaving the high side of the compressor is too high. HPS is a normally closed high side pressure that shuts the unit down if the pressure gets too high. LPS is a normally closed pressure switch that opens if the suction pressure is too low to operate normally. We have also added W1 and W2 terminals that, when energized, will energize HR1 and HR2 to turn on electric heat in the air handler. The high voltage circuit of the HR1 and HR2 are not shown in this diagram.
The video below does not explain the sequencing of this diagram. You may try sequencing this one to see if you understand how it works. There is another video coming up that will give an explanation of the sequencing.
In the low voltage circuit, there are several safety switches provided to shut down the unit if certain pressures or temperatures are exceeded. DTS is a normally closed discharge temperature switch that will open if the temperature of the refrigerant leaving the high side of the compressor is too high. HPS is a normally closed high side pressure that shuts the unit down if the pressure gets too high. LPS is a normally closed pressure switch that opens if the suction pressure is too low to operate normally. We have also added W1 and W2 terminals that, when energized, will energize HR1 and HR2 to turn on electric heat in the air handler. The high voltage circuit of the HR1 and HR2 are not shown in this diagram.
The video below does not explain the sequencing of this diagram. You may try sequencing this one to see if you understand how it works. There is another video coming up that will give an explanation of the sequencing.
The video below explains the sequencing of this unit.
Below we have a circuit that has been used as a safety circuit to shut down the unit if a safety control opens. It is called a lockout circuit. This circuit is interesting because if the control opens, the controls will be locked out until the power is shut off, then restarted.
Lets look at this circuit. When the thermostat closes, power passes thru the thermostat, thru the normally closed limit switch, thru the normally closed low pressure switch, thru the normally closed LOR switch to energize contactor coil C. In the high voltage, contacts C close energizing both CFM and Comp.
LOR coil is wired in parallel with the safety switches. The coil is a high resistance coil and the power traveling thru the coil is negligible. When a safety switch opens, any of the switches in series, the path to the contactor coil C is opened. However, there is still a circuit thru the Relay coil LOR. Because the LOR coil will energize at a very low amperage, it will energize by passing power thru the higher amperage coil of the contactor coil C without energizing C. When LOR coil energizes, it opens normally closed LOR. Once contacts LOR open it creates a holding circuit keeping contacts LOR open. Even if the safety switch closes, Contactor coil C stays deenergized because LOR coil is energized. The only way to get the circuit back to normal is to shut off the power. Then the unit will reset. This circuit has been used for many years to protect equipment from safety switches opening then closing when the out of normal condition changes. This could cause equipment failures from constant recycling.
Lets look at this circuit. When the thermostat closes, power passes thru the thermostat, thru the normally closed limit switch, thru the normally closed low pressure switch, thru the normally closed LOR switch to energize contactor coil C. In the high voltage, contacts C close energizing both CFM and Comp.
LOR coil is wired in parallel with the safety switches. The coil is a high resistance coil and the power traveling thru the coil is negligible. When a safety switch opens, any of the switches in series, the path to the contactor coil C is opened. However, there is still a circuit thru the Relay coil LOR. Because the LOR coil will energize at a very low amperage, it will energize by passing power thru the higher amperage coil of the contactor coil C without energizing C. When LOR coil energizes, it opens normally closed LOR. Once contacts LOR open it creates a holding circuit keeping contacts LOR open. Even if the safety switch closes, Contactor coil C stays deenergized because LOR coil is energized. The only way to get the circuit back to normal is to shut off the power. Then the unit will reset. This circuit has been used for many years to protect equipment from safety switches opening then closing when the out of normal condition changes. This could cause equipment failures from constant recycling.