Jim, W8ZR recently mentioned his preference for the RTB140XX 12A relay
over the RTD140XX 16A relay for switching legal limit RF. His
preference was because the RTB relay only uses 1 pin per contact
connection and staggers them resulting in a slightly larger pin spacing
than the RTD relay which uses 2 pins per contact connection. The
assertion was based not on the relay itself but on the PCB traces
needing to be closer together to connect to the 16A relay. I believe I
have paraphrased Jim's assertion correctly.
I decided to do a quick PCB test. I made a small PCB with various
trace widths and hi-potted them. The traces were about 2 inches long and
parallel to each other. Also I live at 5400' above sea level and the
humidity is low here.
Trace spacing and voltage breakdown:
0.115" = 3000VAC rms = 4243V pk
0.085" = 2300VAC rms = 3253V pk
0.055" = 1800VAC rms = 2546V pk
0.040" = 1600VAC rms = 2262V pk
0.025" = 1100VAC rms = 1556V pk
The voltages are very hard to measure accurately so the voltages shown
are slightly below the breakdown points. Also the traces did not have
bends but slightly curved to the hi-pot connection points.
Using 0.200" trace widths with the center trace centered on the relay
center contact pins and the outside 2 traces offset by 1/4 of the trace
width away from the center trace (to increase the trace spacing) would
result in a trace spacing of 0.050". By offsetting the outside 2 traces
so that the hole is .03" from the trace edge would yield PCB trace
spacings of 0.070". Using this offset and increasing the PCB trace
width to 0.240" would still allow a PCB trace spacing of 0.05".
Running legal limit into a 4:1 SWR will produce a maximum peak voltage
of sqrt( 2 * 1500 * 50 * SWR ) = 775V pk
Even running 3000W into a 6:1 SWR is still a maximum peak voltage of
sqrt( 2 * 3000 * 50 * SWR ) = 1342V pk
Obviously the above maximum peak voltages occur at only 1 impedance for
the given maximum SWR but we need to be sure about the voltage
breakdown. I haven't done any tests to see if 0.200" or 0.240" traces
on either one or both sides of the PCB can handle the current necessary
but this discussion was intended to be about voltage breakdown. Has
anyone done/seen an analysis on RF trace currrent capacity in the HF region?
From this analysis it appears that there is plenty of PCB trace spacing
for all amateur powers provided the relays are not between an antenna
tuner and the antenna where the voltages could be even higher. Comments
as to any problems with the analysis?
73,
Larry, W0QE
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