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Convenient Power with Diode Combining
I am just finishing a new transmitter facility at the Empire State Building and there were a couple of times that I needed power supplies to keep various relays energized (like remote interlocks) or to signal a coaxial relay to move. I'm for keeping things simple and minimizing the actual number of items that are plugged in (especially if a blown fuse can take the station off). Many times there are already power supplies built in to equipment with the right voltage and plenty of current source capability. Why not use those? And it seems to make even more sense when there are multiple power supplies just sitting there waiting to be used.
Here's a specific example. I needed to energize a relay through an interlock chain that included the coaxial relay and the combiner interlock from the 85th floor combiner room. I had three power supplies already built and powered inside the two transmitters (main and backup). The main is a Nautel NV20, and it has a +15V power supply that can source 600ma. Likewise, our backup (a BE FM10B) has two +15V power supplies, each of which has a usable output. By diode-combining those from the BE, and further diode-combining their output with that of the Nautel, I have a rock-solid power source to energize my interlock relay. Figure 1 shows what was done in detail; it's nothing complicated. The idea is actually simplification.
Another example is used for our Dielectric coaxial relays. This facility has two stations (WWPR and WLTW) so of course there are two relays. Each needs a voltage send to activate the relay, so I needed a voltage source for that. Using the power source already shown in Figure 1 seemed like having too many eggs in one basket, so I opted to add special power supplies this time. I used one per station, and again, diode-combined, so that if one fails, the other can still handle both stations.
These power supplies rarely get used; so how does one know if they are still working or not? It would be bad if I went to use one and found it dead. Well, the solution there is simple: We send a sample of its voltage output to our remote control. If the remote control senses a loss in that voltage, it emails me. That sample comes out ahead of the diode-combining, so that potential failures are isolated from one another. Figure 2 shows this.
I added a couple of UPSs to our racks and anyone who has used a UPS for any period of time knows that they sometimes fail. There is little worse than having your transmitter rack go dark because the UPS failed. Our five stations at ESB share the LAN and our air chains come over that LAN; four pieces of equipment are shared for these purposes. Two of the four have dual ac inputs. That's handy, because one can plug in to raw power and the other in to UPS power. However, the other two only have a single ac power input. Both of them run on -48Vdc as well. I wanted each of those to be powered from raw and UPS power. This is where diode-combining came in again. Of course this time, the diodes are turned around, as shown in Figure 3. I used two outboard -48Vdc supplies combined. The output powers the devices. One -48Vdc power supply plugs in to raw ac, and the other in to UPS power.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.
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