Keep Communication Equipment Powered in an Emergency, Part 2

Many hubs, routers, and Wi-Fi base stations today are inexpensive and easy to configure, which makes them ideal for rapidly deploying a communications network. However, these devices usually receive their power from "brick" style power adapters and don't offer any easy way to connect a battery pack. This makes it difficult to use these devices in disaster situations, when the electrical power grid is likely to be out.

As discussed in part one of this series, gas-powered generators and power inverters are good first solutions. But when those options fail or aren't available, organizations that need to provide communications access to their constituents or employees must come up with their own solution for keeping equipment powered. More importantly, these organizations need to be able to rapidly design such a solution on-site at any time.

One way to do this is to keep an emergency kit of common electrical components and to use those components to assemble a custom power adapter that allows you to power your devices with batteries instead of the electrical grid. Readers with some background in building electrical circuits -- even if just in a college lab course -- should have little trouble with this project, though we recommend that you take the time to familiarize yourself with the parts and how they work before a disaster strikes.

(Please note that powering any electrical device via a method not recommended by the manufacturer could void your warranty and possibly damage your equipment. We recommend that you weigh this against the situation at hand before deciding to proceed.)

Project Details

If you were to open a standard "brick" power adapter that comes in the box with your Wi-Fi base station, you'd find a transformer, which scales the 110 or 220 Volts coming from the electrical grid down to a level that your communications device can handle. Commercial Wi-Fi gear usually runs on 5 Volts, but 6 V, 9V, and 12V are not unusual, depending on the device.

The power adapter also contains a diode bridge that converts the alternating current, or AC, voltage from the main electrical grid to direct current, or DC, which is more suitable for electronics devices. A capacitor helps smooth the ripples in the power supply so that the electrical current going to the device is constant.

Some adapters will also contain a voltage regulator, which allows the adapter to offer a constant output voltage. Without the regulator, the output could rise and fall sharply depending on which devices are attached to the regulator and how much power they need.

In order to mimic such a power adapter (but allow it to take input from a battery), we have to build an electrical circuit that provides the same kind of low-voltage DC power for the devices. There are four crucial steps here:

1. Finding and preparing a battery that will provide more Volts and amps than you need to run your device;
2. using common electrical components to convert the power from the battery to the correct voltage and amperage for your device;
3. double-checking the circuit to make sure it works as you expect it to;
4. and finally, getting that converted power into your device.

Assembling the Circuit

This circuit diagram shows what electrical components to connect in which spots to create your ad-hoc power adapter. If you haven't had experience reading circuit diagrams, don't worry -- I'll walk you through the assembly process. You will, however, want to practice using your voltmeter or multimeter first, since you'll need that to test the battery and the circuit.

There's just one more refrain before you dive in: When connecting cables together, whether by twisting the wire ends, by soldering, or by using wire connectors, the joints becomes a weakness in your circuit. They are very sensitive to various strains imposed by moving or pulling the wires.

Consider tying the wires together in a knot so that the knot bears the strain and protects the joint. This trick greatly enhances the reliability of connections.

However, wire connectors may be a better solution if you want to disassemble the circuit quickly -- or if you suspect that someone might stumble over long wires running from the battery to the adapter. In this case, the connectors are likely to disconnect instead of pulling the adapter off the table. Similarly, tying the wire around the leg of the table before connecting it to a device on the table will transfer strain to the table leg instead of the device. This could prevent the circuit from getting scattered all over the place.

Set Up the Battery

1. Check the label or printed text on your communications device to find the voltage and amperage need to power the device. As mentioned in part one of this series, Wi-Fi gear usually runs on 5V, 6V, 9V, or 12V. The required amperage usually ranges between 0.5 and 2 amps. (Remember, Volts are like the water pressure in a pipe, while amps are like the amount of water that can get through a certain section of the pipe.) Both voltage and amperage will most likely be printed on the device somewhere.

2. Find a battery that can provide more Volts and amps than the device you want to run. It's a good idea to use a voltmeter to check that the battery is not empty. Let's assume for the purposes of this article that you have a 12V car battery.

   Remember that car batteries can give you quite a shock if they're mishandled. Never touch both terminals on the battery at the same time. Likewise, never touch the wires or cables that you've attached to each terminal at the same time. Finally, connect the battery to the circuit after you've completed building it -- not while you're still adding wires and components.

3. Retrieve a suitable pair of battery clamps from your emergency power kit. (For a car battery, "alligator"-style clamps are best.) Then connect one end of a resettable fuse to the positive side of the clamp. Polyswitch brand fuses tend to be the easiest to find where I live, so you'll see POLYSWITCH labeled on the circuit diagram to indicate where this fuse should go.
4. Connect several inches of wire to the free end of the fuse. Do the same to the negative side of the battery clamp. (You could use longer wires if you wanted to, but you don't want it all getting tangled.) This will leave you with two dangling wires.
5. If you've got wire connectors, like the fast-on type that allow you to connect different parts of a circuit by plugging them together, connect one to each of the two dangling wires. The connectors aren't vital -- you could just twist wires together as you build the circuit. But twisted wires are prone to coming apart, and connectors will make it easier to disassemble or reconfigure the circuit later. Note that if you use fast-on connectors, it's a good idea to make the dangling fuse wire noticeably shorter than the dangling negative clamp wire so you don't accidentally plug these into the wrong ends of the circuit later.
6. If you've got an inexpensive voltmeter that you want to leave connected to the circuit to act as an instant status indicator, connect it now between the two dangling wires as shown in the circuit diagram. The voltmeter is not mandatory, but it can prevent panic by telling you ahead of time when to think about changing the battery.

Wire the Circuit Together

1. Decide what you want to use to hold down the various electrical components in your ad-hoc power adapter. If you're assembling this adapter in advance of an impending disaster, such as a hurricane, it's good to use something that will keep the circuit together when you need to move it. For instance, a small box with wire connectors attached inside would work, as would an actual electrical breadboard designed for building and testing circuits. In a worst-case-scenario, you could even use a wooden shingle and just tape wires and components to it.

2. Pick a voltage regulator that will convert the voltage from your input (the battery) to the level needed for your output (your communications device.) In this case, I'm using a 78T05 regulator, which outputs 5 Volts at 3 amps or less. Use the 78S05 for 2 amps or less. The number of amps depend on what your device requires as printed on its side, bottom, or on its power connector.

   Once you add a heatsink (as described in the next step) you're going to mount the voltage regulator in the middle of your workspace and build the circuit out to the left and right. The battery will be on the far left and the communications device will be on the far right, as shown in the diagram.

3. Select a suitable heatsink from your emergency power kit. This is crucial -- if it's too small, the circuit will overheat easily and that could destroy the equipment. A big heatsink normally used for high-power CPU chips is a minimum. And if that starts overheating, add a 12V cooling fan and power it by connecting wires right to the battery.

   You'll want to attach the heatsink to your voltage regulator. To do this, drill a hole in the heatsink, in a position that will allow you to put an M3 bolt through the hole. Then use the bolt to attach the regulator case to the heatsink. Make sure you use thermal grease and that there's a good contact between the two.

4. Now connect the regulator-heatsink combo to your workspace, whether a box, an electrical breadboard, or just a table. Remember that, with the regulator case facing toward you and the pins pointing down, the middle pin is ground, the left is the unstabilized side (input), and the right is the stabilized side (output).

5. Select a 100 nanoFarad ceramic capacitor (CERAM1) and a 100 microFarad electrolytic capacitor (ELYT1) and attach them to the left of the regulator, as shown in the circuit diagram. These will help filter instabilities in the current.

   Make sure the positive side of the electrolytic capacitor (ELYT1) is connected to the wire going to the voltage regulator, while the negative side is connected to the wire coming from the negative terminal of the battery. Also, for this capacitor, make sure you use models rated to at least 15 Volts; 35 Volts is a safe optimum, but anything above that is superfluous.

6. Select another 100 nanoFarad ceramic capacitor (CERAM2) and a 470 microFarad electrolytic capacitor (ELYT2) and connect them to the right side of the regulator, as shown in the circuit diagram. These will filter instabilities in the output from the regulator.

   For the electrolytic capacitor here (ELYT2), use models rated to at least 6.3 Volts. Though the regulator outputs 5V, you want a capacitor that's rated slightly higher in case of spikes.

7. Select any silicon diode with a current rating of 2-3 amps, depending on the current requirements printed on your communications device. The role of the diode is to protect the equipment from damage if the battery is connected the wrong way.

   Connect the diode wire dangling from the resettable fuse (POLYSWITCH) and the wire going to the input side of the regulator, as shown in the diagram. Mind the polarity here -- we need the diode's cathode (usually marked with a ribbon of paint) to be connected to the input side of the regulator and its anode (the other side) to the battery. A diode connected backward will prevent the circuit from functioning.

8. The transil (TRANSIL) provides additional protection from voltage spikes. If you choose to use it, attach it to the right, or output, side of the circuit. Mind the polarity. As with diodes, the cathode side of the transil is usually marked with a stripe and should be connected to the wire coming from the regulator output. The anode side of the transil should be connected to the wires, then connect the negative side of the battery to the ground side of your communications device.
9. Attach to the two rightmost wires in the circuit -- the stable ( STAB) and ground ( GND) wires -- a power plug connector that fits the device you want to power. But do not connect the device yet.
10. If you've got a device that needs more than 5V to operate, you can raise the output voltage of your adapter by connecting different components between the points A and B in the diagram. If you just run a wire directly between the two points, your adapter will output the nominal voltage from the regulator. But if you insert three silicon diodes or one yellow-green LED, you can raise the output voltage by approximately 2V. Likewise, two yellow-green LEDs can be used to add a total of 4V. For other voltages, you can use a Zener diode.
11. Wrap up the work. Make sure all the electrical components are well affixed, and make sure the wires aren't so tight that they're tugging on the components.

Check the Circuit

1. Set your multimeter to its ohmmeter mode and make sure there is no short circuit anywhere in your ad-hoc power adapter. For instance, the meter should indicate there is some resistance (measured in Ohms) between the circuit's ground and input wires, and also between its ground and output wires. But that resistance shouldn't be too small: zero to a few Ohms is wrong. Also, the measurement between the input and ground should show a higher value than the measurement between the output and ground.

2. The DIODE should be conductive in the right direction -- meaning that when the multimeter's positive wire is connected to the diode's free end and the multimeter's negative wire is connected to the input side of the regulator, the multimeter should show some small resistance on the order of tens to hundreds of Ohms.

   Alternately, when you swap the wires, there should be no conduction. This should appear as either an "infinite resistance" measurement or values in the megaohms.

3. Make sure all the parts that should be connected to the ground (GND) are connected to it, as shown in the diagram. A visual check is usually enough if you trust your wiring skills. But a check with a multimeter can't hurt.
4. The TRANSIL should be conductive -- like the diode -- when the multimeter's positive lead is connected to the transil's ground side and the multimeter's negative lead is connected to the output of the regulator. When you swap the leads, the transil should become nonconductive.

Do the "Smoke Test"

1. Connect the end wires from the battery clamps to the input wires of the circuit. The positive wire from the clamp connects to the free end of the diode, and the negative wire connects to the common ground.
2. Connect the negative clamp to the negative (-) side of the battery.
3. Touch the positive clamp to the positive (+) side of the battery. If you did everything right, nothing visible should happen. If smoke appears, review the circuit setup and replace the part that started smoking. This probably won't happen, though, since the diode prevents you from connecting the circuit the wrong way and the resettable fuse protects you in case of short circuit. A tripped fuse will be warm or hot to the touch and almost nonconductive if tested with the multimeter (in ohmmeter mode.)
4. Check the voltage at the output of the circuit. It should be almost exactly 5V (if using a 78S05 or 78T05 regulator with nothing but a wire connecting points A and B on the diagram. Anything between 4.8 and 5.2V is generally acceptable. Devices are typically tolerant to variants of plus or minus 10 percentof the nominal voltage.
5. If everything seems to be okay so far, connect the device itself to the plug on the right side of the adapter. Check the voltage at the output of the circuit -- it should not drop significantly. The device should come alive. If so, congratulations!

Keep the device operating for a while. Monitor the temperature of the heatsink. If it heats up too fast, or you can't hold your hand on it even for a short while, disconnect the whole unit from the battery and remount it to a bigger heatsink, or add a fan.

In Conclusion

There's no doubt that working with electrical components for the first time can be daunting. Certainly, you have to take your time, do your research, and stay safe. But, hopefully, the instructions here (and in part one of this series) have given you the inspiration to begin planning your organization's emergency power needs. As you get more comfortable with the different methods of keeping communications equipment powered, you'll soon find that you've got the skills to handle most any power emergency in stride.