This unit is an ARINC 429 bus analyzer, manufacturer "AIT" (Aircraft Interface Technologies), model "429A". Yes, it's confusing. Rated 115VAC/3A/60-440Hz. It has a 250W SMPS inside with several 3A fuses integrated. The power switch is a C&K 7201J1ZQE2 and claims 100k actuations @250vAC, 5A. How it switches on and off has left me scratching my head however.
Mains power comes in, across a typical switch-mode supply, and to a front-panel board. There, is a DPDT rocker switch, two resistors, and two SCRs. Closing the switch connects the circuit as shown:
- What is the reasoning behind using these SCR's, as opposed to just using the DPDT switch across A/B directly? Is it due to "skin effect" on contacts at 400Hz?
- What determines the values of R1 and R2?
As far as I can tell from simulation, anything from 0-1k٠will "work" as R1/R2 with varying success. Lower values seem better because the A-B voltage drop decreases. And above 1k��, the firing angle lags, reducing the RMS power delivered to the PSU.
The max gate current for the 2N6404 is 2A for 1µs, so that means 165vp/2A = 83Ω minimum (assuming the 72Ω intrinsic gate resistance does not factor in.)
R1/R2 seem to dissipate very little power in sim, but both failed open (1/4W), and their values are not readable. Any hint on a value, failure mode, and/or reasoning behind this bizarre circuit would be appreciated.