Designing with SCR Power Controllers - Part 2: Fusing and Transformers

SCR Design Checklist IconDownload the complete checklist here: Control Concepts SCR Design Checklist.PDF

Read Part 1: Design and Installation


Use High Speed Class J or Class T fuses to avoid nuisance tripping
Size fuses with the formula: 1.25 x SCR frame rating (Amps)
  • Phase Angle: Select closest standard fuse size.
  • Zero-Cross: Select next largest size.
Fuse all legs
For single phase loads, fuse both legs. For three phase loads, fuse all three legs, regardless of whether it is 2-leg or 3-leg control.


Use contactors upstream of the SCR
If the SCR is being used to control the primary of a transformer, there should be no contactors between SCR and the primary of the transformer as well as no contactors on the secondary.
SCRs do not parallel or load share.
i.e. You cannot use two 50A controllers in parallel to power a 100A load.  Either split the load into two 50A loads, or use a single 100A power controller.
Isolation transformers with electrostatic shields are recommended.
An electrostatic shield will shunt the capacitive coupling across the transformer and prevent common mode line voltage to ground on the transformer outputs.  Without the shield, you may see line voltage potential with up to several hundred milliamps of current on the output taps.
Maintain a minimum load
When firing into a transformer, maintain a minimum load of 1 amp on the primary to prevent a misfire and saturation of the transformer. A dummy load may be required to maintain proper operation for inductive loads or magnetrons.