I agree, just because something might work doesn't mean it is a good idea let alone should be recommended to others.
Power inserter needed?
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I agree, just because something might work doesn't mean it is a good idea let alone should be recommended to others.
It takes more than that to power the SWM LNB as far as I know ...
13 and 18 volts was the non SWM system.
Why do these PI's list as 21 and 29 volts then ?
The 29volt PI I'm using says on it ... Output 29 volts
I would think that it outputs 29 volts then.
The recvr may only call for 13 or 18 volts, but the PI outputs 29 volts ...
I'm sure the 21 volt PI is 21 as well.
The power inserter outputs either 21 or 29 volts depending upon the model, but the receiver/dvr itself outputs either 13 or 18 volts.
No, that would mean a PI. I'm asking if anyone has measured the voltage o from the receiver without a PI.
yes, probably,but i'm interested if anyone has actually measured it. In SWM mode you don't actually need the 13/18v transponder selection voltage.
They list 21 & 29 volts because that's what they are rated to output. The SWM has a DC to DC converter in it that operates over a wide voltage range. That's why it doesn't matter if you use a 21 or 29 volt PI, whereas some devices expect a particular voltage and won't work if you use too low a voltage and will be damaged if you use too high.
Directv had to design their LNBs and switches to accept a very wide range for two reasons. One, an unregulated switching power supply (which is what the PIs are) have a wide output voltage range. A PI-29 is nominally 29 volts, but depending on the actual load the voltage could vary significantly depending on the load. If you measure the output of a PI-29 with a multimeter, you'll probably measure like 40 volts, because the open circuit voltage of an unregulated power supply is much higher than the rated voltage. As load increases the output power drops to the rated power (or at least in the ballpark) A lot of people in dbstalk/satelliteguys gave/took advice to use PI-29s instead of the Directv recommended PI-21 to power their SWM LNB, and have since switched to a reverse band DSWM LNB. Those only draw 4 or 5 watts, while the PI-29 is able to output almost 10x as much. Undoubtedly those PI-29s are supplying over 30 volts given that the DSWM LNB is such a small load.
Two, the other reason is resistance through coax, especially with longer runs or with coax that isn't solid copper. That resistance will reduce the voltage - the higher the load (the more amps) the greater the voltage loss. So Directv power supplies were designed to supply more voltage than a device actually needs to account for the potential loss through long coax runs (IIRC Directv supports up to 150 feet from PI to LNB) Directv may install PI-21s with LNBs, but that doesn't mean they need 21 volts. They will be able to operate on less - they HAVE to, otherwise they wouldn't work at the far end of a 150 foot run. Between the two factors, that may increase the output voltage in some cases, and decrease it in others, their LNBs and switches must accept a wide voltage range. It can't be like your laptop where you may damage it if it expects 19.5 volts and you give it 24, or it may not charge if you give it 15. This is why Sonora's power inserter outputs 20v for the 18v lines and 14v for the 13v lines - to account for potential voltage loss over a long run to a legacy LNB (I'll bet if you measure it, the SWM16 and DSWM30 do the same)
I'm not sure what the minimum voltage a SWM will operate at, someday I should try hooking up a SWM16 or DSWM30 to a 12 volt 2A power supply...I wouldn't be surprised if it worked. It definitely works with 18 volts, that's easily within spec. In the other direction, I had a DECA connected to 24 volts (off a regulated power supply, so it really was right at 24 volts) for years, which is double what it is spec'ed for.
Very informative and makes sense .... I hadn't thought about the voltage with and without a load on it.
I was just thinking about the output without a load.
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