The reference design is tested for short-circuit conditions with an external fuse. Figure 3-17 shows the wiring diagram during testing. The short circuit is created by shorting the relay coil (COIL-1).

Figure 3-17 Overcurrent Protection Circuit With External Fuse

Test Condition:

• The COIL-1 is short-circuited
• The short-circuit current at the secondary of the transformer is limited by the secondary short-circuit impedance of transformer
• Specification of the transformer used for testing:
  • Manufacturer part number: 4000-09C02K999 (TE connectivity - laminated 20VA transformer)
  • Secondary winding resistance = 2 Ω
• Specification of the fuse used in series with the transformer secondary line RC:
  • Manufacturer part number: 046601.5NR (Littelfuse Inc.)
  • Rating: 1.5 A, 63-V AC/DC board mount fuse, 1206
  • Response time: Fast blow. Melting I²t: 0.1103

Observations during testing:

• During the short-circuit test, the fuse was blown within one electrical cycle (< 20 ms) with peak short-circuit current of approximately 7 A. The SSRs were functional after testing.
• Figure 3-18 shows the test result waveforms.

Figure 3-18 Short-Circuit Test Waveforms With External Fuse