A Communicator Reprise...
The commercial alternatives are good, but pricey. Here is an option for less than $10
In the September and October 2015 issues of the SARC Communicator [and on this blog], we featured circuits that will provide you with a reliable, robust power source. In September 2015 it was Hiu Yee VE7YXG’s simple Gel-Cell Battery Charger, and in October 2015 John Brodie VA7XB’s low cost Battery Monitor Project. This time we’ll round out this series with a device that will automatically switch your station to battery power if the AC fails, and switch it back when the power comes on. It is both inexpensive and simple, yet reliable, as there is only one part.
First, lets look at the commercial alternatives. There are a number of solutions on the market including one, quite expensive, at US$140, known as the West Mountain PwrGate. This device uses solid state devices to charge and automatically insert a backup battery if there is a power outage, and to switch back to the power supply if it is restored.
You will note that the PwrGate above is housed in a large heat sink. This device used Schottky diodes which can generate significant heat. Those fins are there to dissipate that heat. Heat is wasted energy, so we look at an alternative device that is more efficient.
The low-loss PWRGate is billed as being simple, safe, and reliable, and easily able to add backup battery power to your home station or go-kit. The Low Loss PWRgate uses MOSFET power transistors to switch the load between power sources with less than a 20 miliVolt drop, much smaller than systems that use Schottky diodes. This keeps the power losses to a minimum and delivers full battery power to the load. The device is rated at 25 Amp total, with 3 power outlet ports, ARES standard Anderson Pole Connectors, 3 ozs, and US$49.95 plus shipping by USPS Priority mail. Note that there is no heat sink here, and it does not charge the battery. The distributor, Flint Hills Radio Inc. will also sell you a solar battery charge controller for US$ 39.95 plus shipping and a Smart Lead-Acid Battery Monitor and alarm for another US$ 29.95 plus shipping.
Makes our projects seem pretty reasonable doesn’t it?
So back to the low cost alternative. This device transfers up to 40 amperes at up to 14 volts DC continuously. It is a safe way to connect both a 12 volt battery and a 13.8 volt power supply to a load, while electrically isolating both from each other. Whenever your power supply is on, the supply feeds the load, and if you add Hiu’s charger, will also charge the battery, keeping it healthy and ready for use when the power supply is off, or loses AC power, all at a cost of about CA$ 10.00
I did some time in the seventies as a service technician while in my early twenties. One of the products I had some exposure to was alarm systems. In those days before PWRgate, a simple single pole double throw (SPDT) relay was used for the same purpose. The relay is the same as used in many automotive systems. In this application, if the magnetic relay coil is activated, when normal power is on, the contacts switch in the power supply. If the power supply loses voltage, as in a power failure, the magnetic coil is no longer activated and releases the contacts, which then switch in the battery backup. The coil, now deactivated does not rob the battery of any current. A very simple solution with no loss through excessive circuitry or heat. The coil uses a bit of current from the power supply to remain activated, but this is minimal.
These relays are commonly available at auto supply dealers but I ordered mine through eBay and received two, with sockets and mounting brackets, for US$ 3 shipped. They are rated for 12-14 Volts DC and 40 Amps, more than enough to handle the current that most transceivers would draw. Wiring is fairly straight forward and I used three sets of Anderson PowerPole connectors. One for the battery, one for the power supply and one set for the load, being my transceiver. A numbered connection diagram was stamped on the top of the relay I received. The relay coil is wired in parallel with the power supply. If the power supply is on, the relay keeps it feeding the supply circuit. If the power supply goes off current is diverted from the battery.
Once I figured out the contact layout, the actual construction took me only about half an hour, definitely something that can be tackled even by a beginner. Pair this with Hiu’s charger and John’s low voltage alarm and you’re good to go uninterrupted if the power goes out.
~ John VE7TI