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Guest Message by DevFuse

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2 SWM16 with SWM-E2


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42 replies to this topic

#41 OFFLINE   slice1900

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Posted 30 March 2013 - 09:20 PM

The document states that "LNBs employ current management to minimize the current carried per coax." The LNB needs power for itself, which it can only draw from the four coaxes connected to it. For reasons having to do with Ohm's Law, it would be preferable that it draw power from more than one coax at a lower amperage versus drawing all operational power from a single coax at a higher amperage.

The document doesn't state that the LNB draws a constant 200ma from the 18v lines, but it does imply that. If it does so, it is likely utilizing constant current diodes, which would always draw the current they are designed for, never more, never less, independent of voltage. This would allow it to limit the draw on any one coax to a set value. It would then use a linear regulator to step down to whatever voltage it requires to power itself, since one would expect that introducing as little noise as possible is one of the primary design goals of an LNB.

The document describes a 16v minimum on the 18v lines but does not list a minimum for for the 13v lines. If Sonora's document is correct about the 13v lines drawing only 50ma, they probably aren't used for power, but for signaling only. When the 18v lines fall below 16v presumably there isn't enough power at the linear regulator's input to produce the output power required for the device's operation.

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#42 OFFLINE   veryoldschool

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Posted 30 March 2013 - 09:43 PM

If Sonora's document is correct about the 13v lines drawing only 50ma, they probably aren't used for power, but for signaling only. When the 18v lines fall below 16v presumably there isn't enough power at the linear regulator's input to produce the output power required for the device's operation.

Given that the SL5 must work with only one coax and using only 13 volts, Sonora's testing isn't correct "for all cases".
This is sort of Sonora's problem with their testing/spec sheets.
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#43 OFFLINE   slice1900

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Posted 31 March 2013 - 12:31 AM

Perhaps this is the source of the discrepancy between the figures Sonora provides and what you said with all four LNB outputs drawing 200ma. They mention that it is current limited to a total of 500ma, so could be that any of them are capable of drawing 200ma if they're the only one (or two?) in use, but perhaps when there are four it arranges things so the 18v lines draw 200ma and the 13v are limited to 50ma to keep the total draw at 500ma.

I don't know enough about how it works to guess whether it would need less power if there is only one output active. It seems reasonable, but I have no real idea. The document discusses a single LNB 95* dish and says "LNB current is typically less than 200ma", so I guess those aren't current limited like the SL5. If someone had a few multimeters lying around to hook up to the lines running to an SL5, they could do a little testing and figure out what it draws under what circumstances, and what the minimum and maximum total draw is. Not that it really matters to anyone but the terminally curious such as ourselves.

Sure would be nice if there was a technical manual available for LNBs and switches, so it wouldn't be down to a bunch of speculation and guessing :confused:




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