When hooking or unhooking or doing maintenance on the coax feeding the LNB, be very careful!
The circuitry in the
C band LNB that switches between the C/Ku LNB’s is a bit anemic. It can’t stand reverse polarity/current flow!!!!
Reverse current flow can be applied to it by stray AC and DC voltages that exist between the house wiring and the ground that the dish is placed on.
Stray voltages can be applied to the LNB RCVR port (power input side) when you disconnect and connect the coax to the LNB system. On most coax assemblies, the center conductor usually extends past the barrel of the connector. The center conductor will make contact before the ground conductor will. So standing AC/DC voltages will be applied to the center conductor for a momentary time.
This probably applies to the
BSC621-2 DiSEqC 1.2 LNB also, if it uses the same LNB switching system components.
The switch in the LNB uses a combination of signal switching diodes and a couple of transistors to switch power between the two LNB’s It cuts the power to the one it’s not using to prevent bleed through because it doesn’t use a two stage diode switch. So the only way that current gets through to the LNB’s is through active components. The problem is a result of the fact that they don’t have any reverse polarity protection diodes on the power input (RECV side) of the switch. If the input goes negative there is nothing to shunt the current to ground on the power input side of the switch.
Each of the LNB’s themselves have a voltage regulator running off the input to each LNB. It is standard design practice for regulators to have an internal reverse voltage protection diode on the input to ground, output to ground, and between the input and output. As both the regulators in the LNB’s have. So there is reverse voltage protection diodes in the LNB’s themselves (internal to the voltage regulators) So, when the input voltage goes negative, the only current sink is the input to the LNB’s. (diodes in the regulators)
Now you should notice the problem by now. When the input goes negative, it is shunted by the voltage regulators in each of the LNB’s in the assembly. But the regulators are AFTER the switch. The internal switch components are not designed to handle the reverse current flow. It lets the magic smoke out of the transistors that turn on and off the power to each LNB.
Basically. There is a place for the reverse current to go, but it must go through the switch to get there. And the switch can’t take it.
The surface mount transistors can handle a good amount of current in the forward direction. That is when the are switched on and there is zero volts drop across them. But in reverse, the circuitry biases them in the on direction just enough to let enough voltage and current stand across the transistors that it overheats them.
A simple diode connected to the DC takeoff inductor on the power input side (rcvr side) would solve that problem.
The only other danger threatening the LNB is positive over voltage. Technically, the maximum positive input that most regulators are designed to handle is +30V. But normal voltage regulators are relatively tolerant to momentary over voltage, so in the real world it isn’t much of a problem when hooking and unhooking the coax. But still, if they wanted to get fancy, they could put an over voltage crowbar on the input by using a SCR and a 30V zener diode. That would protect it from almost all stray voltage conditions, excluding catastrophic incidents.
But I, myself, would be happy if the head people on this forum would forward it to the LNB manufacturer that it would be a good idea to put a reverse voltage protection diode on the power input side (RCVR side) of the switch.
You may ask how I figured this out. I had bought two C/KU FTA systems for a couple C band dishes that was already in service. One for me, and one for a friend. With each system, I got the LNB of the type listed. One started acting funny after I had been doing some coax work. I could tell it was the internal switch. I could get C band, and KU band when hooked directly from the KU LNB. But I couldn’t get anything through the switch on the KU side. I verified it by moving the LNB to the other system.
Being that it was just after the saudon return period. One week after to be exact. (It always works that way) And I didn’t want to take the time shipping it out to the manufacture. And considering my extensive electronic repair experience. (And my never ending curiosity!!!) I opened it up and fixed it myself.
It took out the transistor feeding power to the KU side in the switch. The transistor feeding the C band side was bloated but still functioning. I replaced the surface mount transistors and added the reverse protection diode into the system to stop it from happening again.
The reason I am posting this now, is because the second LNB I had bought had the same thing happen to it at my friends place. A lose coax connection took out both transistors totally. I just finished repairing it, and doing the diode modification.
The absence of the diode levees this LNB particularly susceptible to stray voltage. So if you have one I would suggest that you have a
grounding wire between the receiver and the LNB before you hook it up. That would shunt any stray current and voltages between the house wiring and the dish mount.
Considering that the sales website has a warning about hooking up external DiSEqC switches with the power on. (or to that effect) If the same manufacture that is making the switches is also the ones making the LNB, then it may be highly likely that it is also the reason for the switches dieing when hooking them up.
So Please ask the manufacture to think about putting a diode on the inputs and output of their switch products (this LNB in particular) to protect against stray reverse voltage..
And on external switches, have diodes between the inputs and outputs. That way, if the LNB is at a higher voltage than the RCVR side, then it don’t’ reverse the power switching element (transistor.) That can happen when you are hooking up an ungrounded LNB’s to the switch.
That is why linear regulator designers put them in all the 78** and 79** series regulators. Just for those reasons.
It is a moderate design oversight in this product, and it needs to be corrected badly!!!!!!!!!!!!