[Amps] SSRs for main turnon

[John T. M. Lyles]

We used Crydom SSRs and other second sources (gordos, and others) in 
the Broadcast Electronics FM transmitters in early 1980s as a cost 
savings measure, and 'reliability enhancement' over typical 
mechanical contactors. The big rigs still used a massive contactor 
(Furnas) but the 3-5 kW range used a combination of contactors and 
SSRs. We always felt that a solid contact was needed in front of any 
silicon switches, so that when the transmitter was switched off, it 
was not sitting there with SSR on the power line ready to false on. 
Also, sort of gave the end user that satisfying 'kerchunk' sound when 
first turned on.

Also, having microprocessor and CMOS logic control, these rigs would 
have required additional buffers to drive contactors (10-100 mA coils 
usually), whereas the SSR could be logic driven with only a single 
transistor. However, when the contactor switched in front of the SSR, 
sometimes the SSR would turnon for a short time and then off again. 
This plagued us for a long time during the development cycle, and we 
read all the books from Crydom and IR and tried every type we could 
get our hands on, zero crossing and non zero crossing switches. MOVs 
across the output terminals didn't help. RC circuits did lower their 
susceptability. They just don't like to see fast DV/DT transients on 
the triac pins.In some smaller transmitters we even had a small load 
resistor across the output that drew a minimum current when they were 
off. I never liked to kludge things like that, but we never could get 
them quieted down when switching on 240VAC or 480 VAC lines with 
30-100 Amp currents in the same power supply. Three phase systems 
really are best suited with 3 pole relays in my opinion.

Maybe they have improved since then, but I still treat them with some 
disdain and use contactors for the ultimate line disconnect when I 
press the off button. Call it old fashioned.

John
K5PRO