Backup Power service
The backbone of the South Carolina Forestry Commission’s radio communications system is a VHF high-band repeater network with associated control stations at strategic points. Some of the repeaters are located at sites that have backup generators on standby. If the power fails, the generator will start and, once stabilized, will supply power for continued operation during the power outage. The other sites have no available backup generator. A question arose as to how we could obtain an alternative (meaning less expensive) source of backup power for these other sites.
Our repeaters, base stations and control stations operate from a 12V power supply. In our particular forestry region, our repeaters, control stations and base stations are scattered over a 19-county region. We also have personnel with pickup trucks, transport trucks and crawler tractors spread out over the same geographical region.At least one employee is stationed near to almost every base station, repeater or control station. So, the idea of using a vehicle’s 12V battery to supply temporary backup power in an emergency was a cheap alternative to installing permanent power generators at these remote locations.
Sure, there would be some downtime associated with a power outage, but the tradeoff of some downtime against the cost of permanent backup generators was considered acceptable. All we needed to do was come up with an arrangement that would allow employees to temporarily hook up the vehicle battery through jumper cables to supply battery power to the repeaters or base stations, as needed. The idea was to make it simple and goof-proof (or should I say, goof-resistant) so that any chance of a calamity was minimized.
The base stations and control stations that we use would be no problem because they have a direct 12V external power supply. The repeater has its own internal power supply, but a study of the power supply schematic indicated that it would be possible to integrate the external 12V supply with this internal supply so that no conflict would occur.
Figure 1 below is a diagram of the basic hookup that is used to facilitate using a vehicle battery as the temporary emergency power source. The plus (1) and minus (2) connecting points shown to the right in the diagram are connected to copper pipes that are inserted through the concrete walls of the communication shelters. The pipes are flattened on one end, and a hole is drilled through the flattened end to accommodate a 0.25″ stainless steel bolt to which the electrical connections are made. On the outside of the building, a clamp is attached to the copper pipe at the entrance to the building to hold the pipe snugly against the building.
It is important to use heavy-duty relays with contacts sufficient to carry the current load. In Figure 1, two sets of contacts are connected in parallel. For an extra measure of safety, two relays can be used with the same points connected in parallel to handle heavier load currents. Photo 1 on page 45 shows a board with two relays connected in parallel for extra current-handling capability. Notice in Figure 1 that only the positive leg is controlled by the relay. Relays could be used for both legs, but connecting only the positive leg through the relays allows us to lower the voltage drop across the contacts. This also helps prolong relay contact life because each set of contacts is only required to carry a small part of the total load current.
Notice in Figure 1 that relay RLY2 is connected between the positive connecting post and the control relay (RLY1). This relay (RLY2) is a heavy-duty solenoid-type relay that is used in automotive applications such as the “starter solenoid.” The positive and negative connecting posts are accessible through the outside of the wall, so that an employee can attach jumper cables from a vehicle to the connecting posts without entering the building. The diode (D1) is inserted in series with the relay coil, so if the jumper cables are connected with reversed polarity, the relay will not be energized and reversed polarity will not be applied to the radio. The relay will energize only when the correct polarity is applied to the connecting posts.
As long as the commercial power is available, the control relay (RLY1) is energized from the 110V ac supply. If a power outage occurs, the control relay is de-energized, and the relay contacts move to the normally closed position. This connects the radio positive lead to the emergency power connecting posts. The radio is dead until personnel arrive and connect a jumper cable from a vehicle to the connecting posts. When commercial power is restored, the control relay is again energized, and power is obtained from the normal power supply.
Heavy-gauge wire should be used for longer runs. The relay wiring can be done with smaller wire for jumpers that are only a few inches long. The voltage at the radio should be checked in both the receive and transmit modes. If the voltage drop in the transmit mode is significant, find out where the voltage drop is occurring. Check across the relay contacts, across the wire between the relay(s) and the radio, and between the relay(s) and the connecting posts on the wall of the shelter.
This backup power “system” is about as simple and inexpensive as it gets. It won’t suit the needs of all agencies or operations, but there are many situations where such an application is a viable option and can save a fair amount of money. Employees must be trained to hook up the vehicle jumper cables properly when needed. Alternatively, a battery (deep cycle, marine lead-acid type) can be used along with a “float” charger. The connections to the battery can be made from the “through-wall” posts without the need for the switching relay circuit. The solenoid relay should still be used to prevent improper polarity. An isolation diode should be connected between the positive lead of the float charger and the positive battery post. A number of variations are possible using this same basic principle.
Until next time-stay tuned!
