TB2 Terminal Block and SPDT relay output - PENN Controls - A421ABD - A421AED - Electronic Temperature Control - A421 Series Electronic Temperature Control

The terminals LC, LNO, and LNC on the TB2 terminal block connect to an SPDT dry-contact relay in the A421 Control (Figure 1). The control does not provide any internal power to the TB2 terminals or relay contacts. The A421 Control energizes and de-energizes the relay to open and close the contacts based on the On/Off temperature values.

Relay De-energized (Off) = LC open to LNO as shown in Figure 1, and the relay status LED is off
Relay Energized (On) = LC closed to LNO and the relay status LED is on

Figure 2 shows how to wire the A421 Control to use the same power source that powers the controlled equipment to also power the A421 Control.

Figure 1. TB2 Terminal Block showing connections to the internal SPDT relay

You can also provide an independent power source for the A421 Control on the TB1 terminals and then wire the TB2 relay terminals to a separate power source to switch and power the controlled equipment circuit.

Figure 2. Wiring the A421 Series Controls using the same power source to power the control operation and power the controlled equipment

Table 1. A421 Control wiring terminals and wire size information
Terminal block	Label	Description, function, and requirements	Wire sizes
TB1 120/240 VAC Models	LN	Line-voltage power source (common): Connect the neutral wire for 120 VAC supply power applications. Connect the L1 supply power lead for all 208/240 VAC supply power applications.	28 AWG to 12 AWG 0.08 mm² to 4.0 mm²
	120 V	Line-voltage 120 VAC control power (hot): Connect the 120 VAC supply power (hot) for 120 VAC supply power applications (via jumper from LC in Figure 2).
	240 V	Line-voltage 240 VAC control power (L2) terminal: Connect the L2 supply power connection for 208/240 VAC supply power applications through the jumper from LC. (Figure 2).
TB2	LC	Line-voltage SPDT relay common contact: Connect power supply to power the controlled load. Connect 120 VAC (hot) for 120 VAC applications and L2 for 208/240 VAC applications.	28 AWG to 12 AWG 0.08 mm² to 4.0 mm²
	LNO	Line-voltage SPDT relay normally open contact: Connects controlled equipment to the line-voltage normally open (LNO) contact on the SPDT relay. When LC is closed to LNO, the relay is energized and the green LED is on. The LNO terminal typically provides power to the controlled equipment in cooling and heating applications.
	LNC	Line-voltage SPDT relay normally closed contact: Connects controlled equipment to the line-voltage normally closed (LNC) contact on the SPDT relay. When LC is closed to LNC, the relay is de-energized and the green LED is off. The LNC terminal is not typically wired to the controlled equipment.
TB3	BIN	Detects a switch closure between the BIN and COM terminals and manually starts or stops a defrost cycle. This is now the default behavior for the binary input.	22 AWG (0.34 mm²) stranded, shielded cable
	COM	Connects the low-voltage common from the sensor and binary input.
	SEN	Connects the low-voltage input signal wire from control sensors.