Do I need an AC contactor with a REX controller?

With the relay output, loads below 800 W can be connected directly. For loads above 800 W, use an AC contactor before connecting the load: the controller relay switches only the contactor coil (A1/A2) and the contactor main contacts carry the heater current.

How do I wire a K type thermocouple to a REX-C100?

On the REX-C100 the K type thermocouple (2-wire) connects to terminals 9 and 10. The blue lead goes to terminal 9 and the red lead goes to terminal 10.

How do I connect a 3-wire PT100 RTD?

On the REX-C100 connect the red RTD A wire to terminal 8 and the other two blue B/B wires to terminals 9 and 10. On the REX-C700 use terminals 12, 13 and 14, and on the REX-C400/C900 use terminals 14, 15 and 16, with the red RTD A wire on the first terminal of the group.

Which SSR is shown in the wiring diagrams?

The diagrams use the Strong Electric BSSR-25DA single-phase solid-state relay. Its output terminals 1 and 2 switch the 24–480 VAC load circuit, and its control terminals 3 and 4 accept the 3–32 VDC signal from the controller SSR output.

Where does the alarm lamp connect?

The alarm lamp or buzzer output uses terminals 6 and 7 on the REX-C100, terminals 8 and 9 on the REX-C700, and terminals 9 and 10 on the REX-C400/C900.

Does the BSSR-25DA need a heat sink?

Yes. A single-phase SSR dissipates roughly 1 to 1.5 W per ampere of load current, so a matched aluminium SSR heat sink with thermal paste and free vertical airflow is required for continuous heating loads.

Technical Guides · 2026-08-23

How to Wire REX-C100, REX-C700 and REX-C400/C900 Temperature Controllers

English wiring diagrams for REX-series temperature controllers, including relay, AC contactor, SSR, K-type thermocouple and 3-wire PT100 connections.

1. REX series terminal layouts

The REX series PID temperature controller is supplied in three case sizes. The wiring principle is identical — 220 VAC supply, a jumper on the output block, a heater circuit and a sensor input — but the terminal numbers differ, so identify the model on the housing label before you start.

Item	REX-C100	REX-C700	REX-C400 / C900
220 VAC power input	Terminals 1 and 2	Terminals 2 and 3	Terminals 2 and 3
Output block jumper	Between terminals 1/2 and 6/7 block	Between terminals 2/3 and 9/10	Between terminals 3 and 10
Alarm lamp / buzzer output	Terminals 6 and 7	Terminals 8 and 9	Terminals 9 and 10
Control output to contactor / SSR	Terminals 4 and 5	Terminals 6 and 7	Terminals 7 and 8
K type thermocouple (2-wire)	Terminals 9 and 10	Terminals 13 and 14	Terminals 15 and 16
3-wire PT100 RTD	Terminals 8, 9 and 10	Terminals 12, 13 and 14	Terminals 14, 15 and 16

2. Safety notice

Wire one output method only: either the relay contact output or the SSR drive output — never both at the same time.

Isolate and lock off the supply before wiring. 220 VAC is present on the power, output and load terminals. Connect loads below 800 W directly; for loads above 800 W use an AC contactor before connecting the load. Wiring must be carried out by a qualified electrician in accordance with local regulations.

3. REX-C100 wiring

3.1 REX-C100 relay output with AC contactor

REX-C100 relay output and AC contactor wiring diagram — REX-C100 relay output driving an AC contactor, with K type thermocouple and 3-wire PT100 sensor options. Click to enlarge.

Connect the 220 VAC supply to terminals 1 and 2 and fit the jumpers shown on the output block. The relay output on terminals 4 and 5 drives the AC contactor coil: A1 to the supply neutral and A2 to the controller output. The contactor main contacts L1/L2 take the supply and T1/T2 feed the heater. Terminals 6 and 7 drive the alarm lamp or buzzer. Two sensor wiring methods are shown: for a 3-wire PT100 RTD connect the red wire to terminal 8 and the other two wires to terminals 9 and 10.

3.2 REX-C100 with BSSR-25DA solid-state relay

REX-C100 and Strong Electric BSSR-25DA solid-state relay wiring diagram — REX-C100 SSR output driving a Strong Electric BSSR-25DA single-phase solid-state relay. Click to enlarge.

With the SSR drive output the controller switches a solid-state relay instead of a contactor. Connect the 220 VAC supply to terminals 1 and 2, then run the DC control pair from the controller output terminals to the BSSR-25DA control terminals 3 and 4 (3–32 VDC). The SSR output terminals 1 and 2 switch the 24–480 VAC heater circuit. Terminals 6 and 7 remain available for the alarm lamp or buzzer, and the sensor connections are unchanged.

4. REX-C700 wiring

4.1 REX-C700 relay output with AC contactor

REX-C700 relay output and AC contactor wiring diagram — REX-C700 relay output driving an AC contactor for 220 VAC load control. Click to enlarge.

Connect the 220 VAC supply to terminals 2 and 3 and fit the jumper across the output block. The relay output on terminals 6 and 7 drives the contactor coil A1/A2; the contactor main contacts T1/T2 feed the heater. Terminals 8 and 9 drive the alarm lamp or buzzer. The K type thermocouple lands on terminals 13 and 14, and a 3-wire PT100 RTD uses terminals 12, 13 and 14 with the red RTD A wire on terminal 12.

4.2 REX-C700 with BSSR-25DA solid-state relay

REX-C700 and Strong Electric BSSR-25DA solid-state relay wiring diagram — REX-C700 SSR output driving a Strong Electric BSSR-25DA solid-state relay. Click to enlarge.

Keep the 220 VAC supply on terminals 2 and 3. Run the DC control pair from the controller output terminals 6 and 7 to the BSSR-25DA control terminals (3–32 VDC), and wire the SSR output terminals into the heater circuit. Sensor and alarm connections are identical to the relay-output version.

5. REX-C400 / C900 wiring

5.1 REX-C400 / C900 relay output with AC contactor

REX-C400 and REX-C900 relay output wiring diagram — REX-C400/C900 relay output with AC contactor for 220 VAC load control. Click to enlarge.

Connect the 220 VAC supply to terminals 2 and 3 and fit the jumper to terminal 10. The relay output on terminals 7 and 8 drives the contactor coil A1/A2, and the contactor contacts T1/T2 feed the heater. Connect loads below 800 W directly; for loads above 800 W use a contactor before connecting the load. Terminals 9 and 10 drive the alarm lamp or buzzer. The K type thermocouple lands on terminals 15 and 16, and a 3-wire PT100 RTD uses terminals 14, 15 and 16 with the red RTD A wire on terminal 14.

5.2 REX-C400 / C900 with BSSR-25DA solid-state relay

REX-C400 and REX-C900 with Strong Electric BSSR-25DA wiring diagram — REX-C400/C900 SSR output driving a Strong Electric BSSR-25DA solid-state relay. Click to enlarge.

With the SSR output, the 220 VAC supply stays on terminals 2 and 3 and the controller output terminals 7 and 8 provide the 3–32 VDC control signal to the BSSR-25DA. The SSR output terminals 1 and 2 switch the 24–480 VAC heater circuit. Alarm and sensor terminals are unchanged.

6. Sensor wiring: K type and 3-wire PT100

Every diagram shows two sensor wiring methods for the same controller: a 2-wire K type thermocouple and a 3-wire PT100 RTD. Observe polarity — reversed thermocouple leads make the reading fall while the process heats up.

REX-C100: K type on terminals 9 and 10; 3-wire PT100 with RTD A (red) on terminal 8 and B/B (blue) on terminals 9 and 10.
REX-C700: K type on terminals 13 and 14; 3-wire PT100 with RTD A (red) on terminal 12 and B/B (blue) on terminals 13 and 14.
REX-C400 / C900: K type on terminals 15 and 16; 3-wire PT100 with RTD A (red) on terminal 14 and B/B (blue) on terminals 15 and 16.

7. Choosing between a contactor and an SSR

Criterion	AC contactor (relay output)	BSSR-25DA SSR (SSR output)
Switching frequency	Slow, seconds to minutes per cycle	Fast, suitable for continuous PID cycling
Wear parts	Mechanical contacts wear and pit	No moving parts
Temperature stability	Wider overshoot band	Tighter control around setpoint
Heat dissipation	Negligible	Heat sink required
Typical use	Loads above 800 W with slow thermal response	Ovens, extruders, packaging machines, tight PID control

When you choose the SSR method, size the relay at roughly twice the steady-state load current and fit a matched SSR heat sink with thermal paste and vertical fins.

8. Pre-power checklist

Only one output method is wired: relay/contactor or SSR — never both.
220 VAC supply is on terminals 1/2 (REX-C100) or 2/3 (REX-C700, REX-C400/C900).
The output block jumper is fitted exactly as shown for your model.
Contactor coil A1 goes to neutral and A2 to the controller relay output.
Loads above 800 W are switched through an AC contactor, not directly.
BSSR-25DA control terminals see 3–32 VDC; output terminals carry the 24–480 VAC load.
K type thermocouple polarity is correct: blue and red on the sensor terminal pair.
3-wire PT100 red RTD A wire is on the first terminal of the sensor group.
The controller input type parameter matches the fitted sensor (K type or PT100).
The output type parameter matches the wiring method actually used.
The SSR is mounted on a heat sink with thermal paste and unobstructed airflow.
All terminal screws are torqued and the reading rises when the process heats up.

9. FAQ

10. Technical support

Beijing Strong Electric manufactures PID temperature controllers, solid state relays, SCR power modules, thermocouples and RTD sensors. More wiring and setup articles are collected in our Technical Guides.

Email: sales@strong-ele.com
Website: strong-ele.com