Conclusion: an SCR controller's closed loop comes in more than one form — constant voltage (holds output voltage), constant current (holds output current), constant power (holds output power) and constant power-square (ramps power by the slope of P², for resistive loads with a large cold-state inrush). Choose by the feedback quantity and the load: voltage-sensitive → constant voltage, current-sensitive → constant current, heating-power-critical → constant power.
1. What closed loop means
Open-loop control is "give and take" without watching the load; closed loop samples the output (voltage, current, or the calculated power), compares it with the target and corrects automatically so the controlled quantity stays near the setpoint. For temperature and process-quality control, the loop is what delivers accuracy. For the basics of firing and power regulation, see the SCR power controller working principle.
2. The four modes
| Mode | Feedback | Suits what |
|---|---|---|
| Constant voltage | Feedback: output voltage | Load impedance stable, voltage must hold |
| Constant current | Feedback: output current | Transformer primary, fluctuating impedance |
| Constant power | Feedback: voltage × current | Most heating where power must hold |
| Constant power-square | Feedback: slope of P² | Resistive load, large cold-state inrush |
3. How to choose each
Constant voltage fits loads whose impedance is itself stable and where the process cares about voltage accuracy; constant current fits transformer primaries and other loads whose impedance jumps and where current must be locked; constant power is the workhorse for heating because the heat comes from power; constant power-square targets resistive furnaces with very low cold resistance, ramping power smoothly by the slope of P² to avoid cold inrush and give a smooth heat-up curve.
4. Pairing with the firing method
Constant voltage / current / power, which need continuous correction, mostly use phase-angle; loads with large thermal inertia that tolerate whole-cycle switching can pair with zero-crossing. A single controller often supports several closed-loop modes; which one is active is set at selection and commissioning.
5. What to confirm at selection
- Which closed-loop mode the process truly needs (constant V / I / P / P²).
- Where the feedback comes from and its range; external voltage / current transformers may be needed.
- Whether limiting is needed: voltage limit, current limit, power limit, to prevent overshoot.
- Firing method and load-inertia match.
6. FAQ
Which controls temperature better, constant power or constant voltage?
For resistive heating, constant power is more direct — heat is set by power, so holding power locks the heating effect; constant voltage lets power drift as resistance changes, so it is weaker on accuracy.
Where is constant power-square used?
For resistive loads with very low cold resistance (some resistance furnaces cold), it avoids a large inrush when power is first applied and gives a smooth power ramp, gentler on devices and the supply, and a smoother heat-up.
Does closed loop need extra sensors?
Usually the feedback comes from voltage / current transformer signals processed internally; no extra sensor is needed per loop. The exact sampling point and isolation follow the model's manual.
7. Timeliness
Timeliness: compiled from KULUN Electric SCR controller product documentation and technical articles, September 2026. The closed-loop modes, feedback interfaces and limiting configuration for a specific model are subject to the datasheet and project confirmation.
