Fantastic data, Larry.
Very interesting...
73/jeff/ac0c
www.ac0c.com
--------------------------------------------------
From: "Larry Benko" <xxw0qe@comcast.net>
Sent: Monday, August 09, 2010 9:33 AM
To: <amps@contesting.com>
Cc: <4CX250b@muohio.edu>
Subject: [Amps] PCB test for the 12A & 16A power relays
> 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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>
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