The CCR thyristor
How thyristors (SCRs) regulate a constant current regulator’s output. Move the firing angle, or let the CCR regulate automatically, and watch the waveform, output current and load response.
Many CCRs use a pair of thyristors (SCRs) connected in anti-parallel in the supply to the output transformer. Each thyristor is switched on by a gate pulse at a chosen firing angle in every half-cycle and turns off at the next zero crossing (shown here for a simple resistive load; the transformer’s inductance makes conduction run slightly past the zero crossing in a real CCR). A later firing angle passes less of the waveform and lowers the output; an earlier one passes more and raises it.
A current sensor on the output feeds the control board, which adjusts the firing angle continuously to hold the current at the selected step, whether the load changes or the supply varies. Because the output waveform is chopped, a true-RMS meter should be used to measure it, and filtering may be fitted to limit harmonic effects.
Frequently Asked Questions
Q1What does the thyristor do in a CCR?
A pair of thyristors (SCRs) switches the AC supply to the output transformer. By choosing when in each half-cycle they turn on, the CCR controls how much power reaches the series circuit.
Q2What is the firing angle?
The point in each half-cycle, measured in degrees from the zero crossing, where the gate pulse turns the thyristor on. A later angle passes less of the waveform and lowers the output current.
Q3How does the CCR hold the current constant?
A current sensor on the output feeds the control board, which adjusts the firing angle continuously so the measured current matches the selected intensity step, whatever the load or supply variation.
Q4Why is a true-RMS meter needed?
Phase-angle control chops the waveform, so an average-responding meter reads wrongly. A calibrated true-RMS meter measures the real effective current.
Q5Do all CCRs use thyristors?
No. Thyristor phase control is common, but other designs exist, including saturable-reactor types and other solid-state topologies. Check the manufacturer's data for the unit in question.
Q6What happens if the load is too large?
Even at full conduction the current cannot reach the target. The CCR is overloaded for that step, which is why the circuit load must stay within the CCR rating.
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