This page last updated 2-4-2001
How to build a battery back-up circuit
Did you ever wish you could trickle charge your 12 volt lead acid or gel cell batteries and have them instantly ready to power your radio equipment if the AC power failed without having to buy a special power supply? Well, now you can! With a bit of work, and only 4 parts! Here's how it's done.
First you'll need the parts:
2 high current diodes rated to handle the current you need.
1 garden variety 2.5 amp diode
1 10 ohm 10 watt resistor OR a #1157 tail light for a car.
1 REGULATED power supply
Just mix all ingredients in a bowl, and bake at 450 degrees for...
NO, I'M JUST KIDDING! (Don't do that or you'll need more parts!)
Here's how to wire the parts:
D1 and D2 are the high current diodes and should be rated to EASILY handle total current drain of the radios or load. They need to be mounted on a heat sink in most cases. As an alternative, you can use 1/2 of a bridge diode assembly. D1 will get warm when using AC power, D2 will get warm when using battery power.
D3 is the low current (2.5 amp) blocking diode. It's job is to keep the battery from draining through the power supply when the AC power fails.
R1 limits the charging current to the batteries when they are low and the AC power is restored. If you use the #1157 tail light, use the low wattage filament! This will limit the charging current to about 1 amp max. If you want faster recovery time and the batteries can handle the charge rate, the high wattage filament can be used... but you will need a higher current rating for D3. This will limit the charging current to about 3 amps max.
Now for the magic!... You'll need to adjust the power supply output voltage. Most WELL REGULATED supplies have their output set to 13.8 volts. Each diode will drop about .6 to .8 volts when conducting, so the power supply should be adjusted for an output of 14.4 volts. This will give you about 13.8 volts to the radios AND the batteries when they are fully charged. When the AC power fails the output will drop to about 13.2 volts with fully charged batteries, and quickly to 12.6 volts (nominal battery voltage). This should not cause any problems with the equipment and is better than 0 volts. Keeping the batteries at 13.8 volts allows them to remain fully charged without boiling the water out causing shortened battery life and explosive gasses!
Here's a good idea... put fuses in the positive leads of the batteries AND the power supply, and be VERY CAREFUL not to short the output of this circuit! For example, if you use 30 amp fuses, the output current will be on the order of 60 amps before they blow! Fuses should be used for everything connected to this circuit. Even 10 gauge wire can only handle 30 amps safely before things go up in smoke!
I hope you find this little circuit useful. I've been using it for years now with great results. My packet BBS computer that runs on 12 volts hasn't needed rebooting for years, and my station is always operational when the lights in the house go out! Kind-a gives me that warm and fuzzy feeling. I hope it does the same for you!
Hope to hear you on the air the next time the power goes out!
Tom Bert [email protected]
Cheap homebrew charger for HT batteries
Here's a real neat solution to the ever present problem of keeping those HT batteries charged up where ever you go. Virtually all of them slide or clip on to the bottom of the radio using a couple of contacts to provide power to the radio. With a bit of ingenuity you can take an old credit card (see, they don't have to be cut up any more!) and with a hair drier carefully heat and bend the card so that it fits snuggly on to the battery where the radio normally connects. Using a very fine point soldering iron, make a pair of slits in the card on each side of each of the two power connectors. Take a strip of tin (used solder wick works great for this part) and slide it through the slits for each connector and bend it into an oval with the ends of the strip on the top side of the card. Carefully solder the ends of the strips together. These are the charger contacts. I use the contacts from the top of an old 9 volt battery for a charging connector so the circuit can be moved from one battery type to another. Next add the following circuit to the top of the mating 9 volt connector. The 9 volt connector can be eliminated if the charger will only be used with one battery type:
The two lamps are to limit the charging current to a safe level for charging. I've found that a pair of small bi-pin 5 volt 100 ma or so lamps give about the right charging current for most 7.2 volt NiCad packs. A small switch can be added across one of the lamps to raise the charging current to fast charge the battery pack when the switch is turned on. The diode is for safety and to prevent the battery from discharging back through the power source if it's turned off. The small lamps, switch and connectors can be held together with hot glue. Use a light gauge wire to a 12 volt connector and fuse of your choice. Using this charger means you should check the charging current and pick the lamps that give the right value for slow (battery capacity divided by ten for 14 to 16 hours) or fast charging (battery capacity divided by four for 3 to 4 hours). The important thing to check is the battery voltage. It should be down to 1 volt per cell before recharging and 1.4 volts per cell when done. The normal resting voltage for a charged cell after charging is 1.25 volts. You can get the most life out of your NiCads if you follow these guidelines. After you've charged and discharged your pack a few times, you'll be able to approximate the condition of your batteries by looking at the brightness of the lamps. When charging starts they'll be rather bright, getting dimmer as charging finishes. They can also be used to make sure you have a good power source and connections to the pack!
Happy charging,
Tom Bert [email protected]
Quarter wave telescopic HT antenna
Here's a slick antenna for your HT that works MUCH better than the original rubber duck antenna that came with it. It's easy to build and it works great. The only parts needed are a telescopic antenna rod (preferably one with a moveable elbow), a PL-259 connector and adapter, a short piece of stiff wire and 1.5 inches of the outer jacket of your favorite piece of RG-8 cable (strike that... a SPARE piece of cable!). File the plating from the bottom of the antenna and tin it. Solder the stiff wire to the bottom of the antenna. slip the RG-8 jacket over the bottom of the antenna and slide it into the back of the PL-259. Depending on the antenna you've chosen, you may or may not have to add a few wraps of electrical tape to the bottom of the antenna to get a snug fit in the coax jacket. Once you have everything just right, solder the wire to the center pin of the connector and you're all set. You'll want to add an adapter in most cases to fit the connector on your radio. Now for the neat part... it's multi band! Extended to 19 inches your on 2 meters, 12 inches works fine on 220 MHz and 6 inches is good for 440 MHz. Most rubber ducks have a gain of -6 db... that means your radio would work 4 times better at the same power level with an antenna like this. Not a bad deal for a whip and a connector, eh?
Happy portable operating,
Tom Bert [email protected]
A repeater of your own that uses zero power!
Here's an interesting concept I've tried and it actually works! It involves the principle that an antenna will radiate signals from a feed line, and couple them into it. It's handy if you have a HT for mobile operation in a vehicle that has an outside antenna, or if you're using your HT around the house to work a repeater that's just a bit too far over yonder hill to work with a HT and you don't like repeatedly connecting and disconnecting the external antenna. Being tied to a wire is no fun... especially since ham radio is wireless! The solution is to simply connect a wire dipole to the radio end of the feed line that goes to the outside antenna... THAT'S IT! At 2 meters for example, connect a 19 inch piece of wire to the center pin of a connector that will plug into the feed line, and another equal length piece of wire to the shield. Orient the wire so it will be out of the way near where you plan to operate the HT. As an alternative, you can just connect another rubber duck to the feed line going to the outside antenna. As long as you keep your HT near the inside antenna, the signal will be coupled to the outside antenna when you transmit, and from the outside antenna when you receive! While this is not extremely efficient, in areas where you need just a bit more signal than a HT and a rubber duck will provide, it works! To make this arrangement work as good as possible, the inside antenna should be out in the open away from everything as much as you can. Hanging it from the ceiling the middle of a room or automobile works best. Try different arrangements for the inside antenna until you find one that works best.
Look ma... no wires!
Tom Bert [email protected]
A 12 volt DC power connection for everyone
A standardized DC connector that's cheap, easily available, handles up to 25 amps, can't be plugged in backwards, can be hooked up with no tools in total darkness... Sounds like the perfect solution to portable/emergency operations right? Well, I think I've found a logical standard for high and low current 12 volt distribution, the grounded AC receptacle and the RCA phono plug. This may sound strange, but follow my logic and I think you'll agree that this is a common sense fix for a problem most hams will encounter sooner or later.
My criteria for connector selection was this:
1. They must be readily available in all areas.
2. They must handle 15 to 25 amps. RCA plugs 5 amps or less.
3. They must be inexpensive.
4. They must be reverse polarity protected.
5. They must be foolproof.
Here's the problem. W8ABC has a radio he wants to use at field
day with W8XYZ's power supply. With some tools we can hook the wires
to the power supply with no shorts and hopefully the right polarity.
You need tools, good lighting and time. In a real emergency, time may
be crucial, tools and lighting may not be available.
Here's the solution. W8ABC puts a male grounded AC plug on his
radio power cable wiring negative to the ground and positive to both
blade terminals. W8XYZ puts an outlet box and female receptacle on his
power supply wiring it the same way. Now hooking up to the supply takes
only seconds. If the radio or power supply fails, it can be unhooked
and replaced easily.
Taking this to the next step, they can also put the receptacles
under the dash of the car, on their emergency batteries, generators and
solar panels and put the male plugs on other rigs, lights and junction
boxes. If two cars have receptacles installed, getting a "jump" is as
easy as two male plugs on a heavy gauge cable. Now everyone can plug
into everyone else... isn't that part of what ham radio is all about?
The solution for low current (up to about 5 amp) devices like
TNC's, computers, HTs and chargers for HT batteries is the RCA
phono plug. Positive to the pin and negative to the shell. A 1/4 inch
hole in the side of the outlet box will mount a female RCA jack that
you can plug into with a short jumper to a multi RCA jack outlet box.
Now you can plug in all your accessories.
Now for the cautions and warnings:
1. BE ABSOLUTELY SURE TO PLAINLY LABEL ALL GROUNDED AC CONNECTORS
FOR 12 VOLTS DC ONLY. It's good insurance.
2. Wire a 10 gauge jumper inside the AC plug and across the back of
the receptacle. This and all connections should be well
soldered. If the cable is inadvertently plugged into 120 volts
AC, the jumper will blow the fuse/circuit breaker. Note: I have
not tried this and I don't recommend that you do! Refer to
number one above.
3. Be sure to install a 20 to 40 amp fuse/circuit breaker AT THE
VEHICLE BATTERY. A circuit breaker is better here if you plan
to do a lot of jump starting or heavy charging/discharging.
Use #10 wire or heavier. Find a good chassis ground to mount
the metal outlet box in a vehicle.
4. Be sure to install the right value fuse in each power cable for
each device (radio, TNC, keyer... etc).
5. Use caution when using a double male jumper cable to connect one
battery/vehicle to another. Don't short across the plug.
Of course it's a good idea to make up adapters to go to and from that OTHER 12 standard connector that is usually intermittent and burns up at less than ten amps... the cigarette lighter plug/socket. I've installed the above distribution system in most all of my vehicles, radios and portable equipment. A few years ago I built a 100 watt repeater that also uses this type of power distribution. It has worked flawlessly. It's handy in case of a failure or to remove and upgrade the equipment. The more of us that build to the same standards the better off we all are. I hope you find this idea useful. If you have comments, suggestions or are using this system please let me know.
73 and more power to ya,
Tom Bert [email protected]