Connecting two low power SMPS output with series diodes

Thread Starter

Willen

Joined Nov 13, 2015
294
Condition is: I have a TV satellite Dish antenna and its LNB needs power which come from TV setup box through coax. The LNB of dish antenna takes power from the coax and sends UHF singals through the same coax. I am going to use two setup box in two rooms. Two watch TV, I need to power LNB from both setup box, so LNB gets power from box A or B (both became 'master'.

In this condition, I am planning to put diodes in series so power from one box do not kick back to another. And to pass UHF signal from LNB to the boxes, I an thinking to use 1nF capacitor parallel with these series diodes. Does this work?
 

Thread Starter

Willen

Joined Nov 13, 2015
294
It should.
You might want to use Schottky diodes to minimize the voltage drop.
Thank you, happy to hear it works. And maybe voltage drop would not be a critical point because setup box feeds 25V DC to the coax and LNB takes just 5V with its 5.0V regulator chip.
 

AlbertHall

Joined Jun 4, 2014
10,400
Here in UK, the polarity of the DC voltage going to the LNB is used to switch the polarisation of the LNB. Clearly this would be disastrous if your system works this way. You need to check whether this system is used in your receivers.
 

AlbertHall

Joined Jun 4, 2014
10,400
You can't split the feed from an LNB.

http://www.satcure.co.uk/tech/lnb.htm#nosplit
You can't split the output feed. In order to tell the LNB output which of the four frequency bands it needs to provide, the receiver "communicates" with the LNB as follows (this gets technical):-

A modern "Universal" LNB requires a 22 kHz signal at 0.5v p-p to switch its Local Oscillator to 10.6 GHz ("high band"). Otherwise it uses its 9.75 GHz oscillator.

Polarisation switching is controlled by DC voltage supplied by the receiver. 12.5v to 14.5v gives vertical and 15.5 to 18v gives horizontal polarisation. A higher voltage than that can damage the LNB (although I have run it up to 24 volts, briefly, without damage). A voltage that is too low will prevent the LNB from working correctly.

This switching voltage, fed up one or more cables, also powers the internal electronics of the LNB. A modern LNb requires 125mA or more. Older LNBs and LNBs with several outputs may take more current.

Under normal circumstances, one cable can carry only one LNB feed (because of the voltage and tone communication method), although it is sometimes possible to use a "stacker-destacker" to carry two.
 
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