Technical Guides · 2025-04-12
SCR Voltage Regulator Circuit Diagram and Wiring Guide
How to read and apply SCR voltage regulator circuit diagrams for single-phase and three-phase four-wire resistive heating, using the current SCR-100 controller and M1DK pulse-triggered power module, including terminal identification, control wiring, fusing and commissioning.
An SCR voltage regulator controls how much power reaches a resistive heater by adjusting the firing of an SCR power module. This guide explains how to read and apply the circuit diagrams for a regulation system built with the SCR-100 heating controller and the M1DK pulse-triggered SCR power module — the current designations for the legacy ZKG-2000 controller and SSR-DV module — in both single-phase and three-phase four-wire installations. It covers the role of every terminal, the control signal path, fusing, commissioning checks and the wiring mistakes most often seen in the field.
1. Electrical safety
G means gate trigger — not ground. PE (green/yellow) is a separate protective conductor.
Always wire with the supply isolated and locked off — mains voltage is present on the power and load terminals. Before energizing, confirm that the supply voltage, the control signal and the load rating match the controller and module you are installing. Never touch exposed terminals while the circuit is live, and leave any live testing or measurement to qualified personnel. Bond the cabinet, heater shell and exposed metal parts to protective earth — never to the G terminal — and fit a fast-acting semiconductor fuse in series with each power module. Wiring must be carried out by a qualified electrician in accordance with local regulations.
2. How an SCR voltage regulator works
Three different device roles appear in heating control panels, and they should not be confused:
- An SCR voltage regulator continuously adjusts the power delivered to the load by changing the firing of an SCR power module — it is a power modulation device.
- A switching solid state relay (SSR) only turns a load fully on or fully off in response to a control signal — it does not modulate power by itself.
- A PID temperature controller measures the process temperature, runs the control algorithm and produces the control signal or command that drives a regulator or SSR — it makes decisions but carries no load current.
In the circuit covered by this guide, the SCR-100 controller decides how much power the load needs and sends gate trigger pulses to the M1DK power module. The controller never carries the heater current — the full load current passes through the SCR power module and its series fuse, which is why correct power wiring and fusing matter so much.
Signal flow in the system: the controller takes its supply on the L/N terminals, the G/T1 pair delivers gate trigger pulses to the power module, and the full heater current flows only through the module and its series fuse FU.
3. Circuit components
The diagrams below use the same four components throughout:
- SCR-100 heating controller (legacy designation: ZKG-2000) — generates the synchronized trigger signal.
- M1DK pulse-triggered SCR power module (legacy designation: SSR-DV) — carries the full load current.
- FU — a fast-acting semiconductor fuse in series with the module on the live conductor.
- LOAD — the resistive heater being controlled.
4. Diagram-by-diagram wiring explanation
Identifying the controller terminals
Before wiring, identify the four controller terminals on the housing: L and N supply the controller (and provide phase synchronization), while G and T1 carry the trigger signal to the power module. The labels are printed on the top and side of the controller:
Top view: the terminal row is labelled T1, G, N, L. G and T1 form the trigger pair to the module; L and N supply the controller.
The wiring diagram printed on the controller housing shows the same four-terminal arrangement — use it as the on-site reference when identifying conductors.
Single-phase wiring diagram
In the single-phase circuit, live runs through the fast fuse FU into power terminal 1 of the M1DK module; power terminal 2 feeds the resistive heater; and the heater returns to neutral. The controller L/N pair supplies the controller, and the G/T1 pair carries the trigger signal to the module.
In this diagram the live conductor passes through fast fuse FU into module power terminal 1, terminal 2 feeds the resistive load, and the load returns to neutral. The G/T1 pair connects only to the module trigger pins.
Connection steps:
- Connect the controller G terminal to the module gate pin (trigger +).
- Connect the controller T1 terminal to the module trigger pin (trigger −).
- Run live through the fast fuse FU to module power terminal 1.
- Connect module power terminal 2 to the heater, and the heater return to neutral.
- Supply the controller L and N terminals from the same live and neutral.
Three-phase four-wire wiring diagram
For three-phase heating systems, one synchronized controller and one M1DK power module are used per phase-to-neutral heater bank. Each phase is fused separately, each G/T1 pair connects only to its own module, and the heaters return to the common neutral. Keep the three banks balanced so the neutral current stays low.
Each phase-to-neutral heater bank has its own synchronized controller and M1DK module, with a separate fast fuse per phase and a common neutral return for the three heaters.
This circuit is only for three independent line-to-neutral heater banks on a three-phase four-wire supply. Do not use it for delta loads, three-wire star loads without neutral, motors, or other coupled three-phase loads.
5. Control signal connection
The trigger path is a dedicated point-to-point pair: G (trigger +) and T1 (trigger −) run from one controller to one module only. Never share a G/T1 pair between modules, and never connect G to earth or neutral. In three-phase systems, each controller must take its L supply from the same phase as the module it triggers — the L/N supply also gives the controller its synchronization reference for firing the SCR at the correct point in the AC cycle.
6. Load and cooling considerations
This circuit is intended for resistive heating loads only — ovens, hot plates, heating elements and drying equipment. Do not connect motors, transformers or other inductive or coupled three-phase loads. Because the module carries the full heater current it generates heat in operation: mount it on a suitable heat sink and follow the current rating marked on the module and the installation guidance in its product documentation. In three-phase systems, keep the phase-to-neutral heater banks balanced and fuse each phase separately.
7. Commissioning checklist
- Protective earth bonds the cabinet, heater shell and exposed metal parts — never the G terminal.
- A fast-acting semiconductor fuse is fitted in series with each power module.
- Controller G goes to the module gate pin and T1 to the trigger pin — not swapped.
- In three-phase systems, each controller is supplied from the same phase as the module it triggers.
- Each phase-to-neutral heater bank is independent; no delta or three-wire star load is connected.
- All terminal screws are torqued and every conductor is labelled.
- With the controller at low output, the heater warms gradually; at 100% output it reaches full power.
- Each phase has been tested independently before the full system is energised.
8. Common wiring problems
G connected to earth or neutral
G is the gate trigger output. Bonding it to protective earth or neutral destroys the trigger circuit. Only PE (green/yellow) bonds the cabinet, heater shell and exposed metal parts.
G and T1 swapped
If the trigger pair is reversed the module never fires and the heater stays cold even at 100% output. G goes to the module gate pin (trigger +) and T1 to the module trigger pin (trigger −).
Controller fed from a different phase
In three-phase systems each controller must take its L supply from the same phase as the module it triggers, otherwise synchronization is lost and output becomes erratic.
Neutral omitted on a four-wire system
The controller needs its N connection for supply and synchronization. Without neutral the trigger timing reference is missing and the module cannot fire correctly.
Blown fast fuse after a load fault
A shorted heater or wiring fault clears the fast-acting semiconductor fuse to protect the SCR die. Find and fix the cause before replacing the fuse.
Unbalanced three-phase heater banks
Very unequal phase-to-neutral banks raise neutral current and can overheat the neutral conductor. Keep the three banks as evenly matched as practical.
9. FAQ
10. Related products & technical support
Products referenced in this guide:
- SCR-100 Digital SCR Voltage Regulator — heating controller that generates the synchronized trigger signal.
- M1DK Pulse-Triggered SCR Power Module — the power module wired in the diagrams above.
- BDTY Single Phase SCR Voltage Regulator — an alternative regulator with 4–20 mA, 0–10 V, 1–5 V or potentiometer control input.
Browse the full solid state relay and SCR power module range or our Technical Guides for more wiring and setup articles. For selection help or a wiring review of your application, contact the Strong Electric engineering team.