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Author Topic: Homebrew power supply q's  (Read 1703 times)

Posts: 6

« on: January 21, 2002, 01:14:20 AM »

I'm planning on running my mobile radio as a base in the house, and I want to power it with a 12 volt automotive battery outside the house. My radio uses 6.7 amps transmitting at 25 watts. How thick a wire do I need to use to reliably carry this load? The distance from the battery to the radio will be about ten to fifteen feet in all. Thanks,

Jason Kendrick (call sign supposed to be here next week!!)

Posts: 1435

« Reply #1 on: January 21, 2002, 04:38:41 AM »

Congrats on getting your first ticket.

What wire do you have haging around?  Are you going to fuse the wire?  How are you going to tap into your car's electrical system?

Posts: 135

« Reply #2 on: January 21, 2002, 07:24:08 AM »

Hello and congrats on your new ticket! For 6.7 amps DC, with a 100% safety factor, you should
use at least 16 gauge copper wire, or even 14 gauge.. As you may know, as the gauge number
gets smaller, the wire size/diameter gets larger. 15' is not  a particularly long wire run. 16 gauge
copper wire (stranded) should only present a resistance (DC) of 4 ohms per 1000'. Both positive
and negative lines should be fused as close to the battery as possible, with the same amperage
fuse as is probably already in your radio's positive power lead.

I strongly recommend the 'Electronics Pocket Handbook' by Metzger, or similar. It costs only a
few dollars and is a wealth of information, i.e. wiring practice.

Posts: 56

« Reply #3 on: January 21, 2002, 10:13:45 AM »

A few suggestions:

1) Use #14 or #12 gage stranded wire to avoid voltage drop. You can go to Home Depot, Lowes or Ace and buy it by the foot.

2) Fuse the hot lead AT THE BATTERY. A typical auto battery is capable of producing 500-1000 amps of current, which would make the wire hot enough to start a fire. The fact that your radio has a fuse will NOT protect your house if the supply wire shorts as it enters the house.

Have fun!

Posts: 21825

« Reply #4 on: January 21, 2002, 01:44:12 PM »

Jason, your question creates more questions, at least for me.

It's a good idea, if you're using a car battery for power, to keep it outdoors.  I like that.  Much safer.  However, how will you charge it?  Via the same wires that power your rig, or differently?  And what will it be sitting on?  I assume you know it's not a great idea to have a car battery sitting on the ground, it should be on something insulated.

Also, wherever it is, the battery should be protected from having anything accidentally fall across its terminals.  While 12Vdc is not a hazardous voltage, the sparks and burning that can occur if an accidental short-circuit happens are extremely dangerous.

Let us know more about what you're doing!

73 de Steve, WB2WIK/6

Posts: 550


« Reply #5 on: January 21, 2002, 02:04:49 PM »

An automotive battery is not a good choice for your application. If you must use a 12 volt battery, use a deep cycle type RV or Marine 12 volt battery. At 6.5 amps, you can get a long life out of one new battery if you keep it fully charged between usage. I did this for 5 years with a 100 watt radio and had no trouble keeping the battery in the shack at room temps where it will charge and discharge efficiantly. (Use lots of good common sense when protecting the battery and the wiring.) Better yet... Get a low cost Astron RS-20A supply and you will avoid the issues that you will have with battery power. That's what I finally did! ;-)

Posts: 6

« Reply #6 on: January 21, 2002, 05:24:56 PM »

Ok, let me answer a few questions and mabye clear things up a bit.

I'm planning on keeping the battery outside the house on my deck, with about 15' to run the wiring. The battery will be stored in a ventilated plastic crate (enclosed plastic tote with hand holes), so there will be no danger of anything falling on the battery.

Fuses were planned, but I'd not thought of where I was going to install them... Good point!

I'll charge the battery with an old 12 volt twelve amp charger. I'm thinking about installing insulated spade connectors between the radio and battery. When I need to charge the battery, I'll just un-plug the radio, and plug in the charger. This way I can leave the battery alone for the most part, and I won't have to go out in the cold to hook it up to the charger. My old charger has 12 guage wires with aligator clips that run to the battery.

As a backup for the 110 volt charger, I've also built a 12 volt 60 amp charger using a lawnmower engine and a Delco alternator (internally regulated, of course). This small 12 volt generator also powers my camper during such power outages when heat is needed (for the wife and kids!)

For the time being, an auto battery is all I've got. I plan on upgrading to a deep cycle marine battery when finances allow. My reason for going with a battery supply instead of an ac/dc power supply is that when the utility power goes out, I don't plan on sitting around waiting for them to get it fixed so I can use my radio. :-D

Thanks for the suggestions guys, and if you can think of anything else, just post!

Jason Kendrick

Posts: 21825

« Reply #7 on: January 21, 2002, 07:07:33 PM »

Fuse the leads right at the battery, and use it in good health!

Remember most amateur gear that runs on low voltage DC is actually optimized by design to run at ~13.8Vdc, not 12Vdc.  Reason is: 13.8V is the nominal voltage floating across a car battery when the engine's running and the alternator's charging it.  As such, the companies who design and manufacture power transistors and RF amplifier hybrid modules optimize their designs to center on 13.8Vdc, not 12Vdc.  And so, most ham gear won't perform to full specifications when used at 12Vdc, which is all you're going to get from a car battery.

But, that usually only means that a "50W output" VHF rig will only run 35-40W output when used at 12Vdc.  It's not a deal-killer, just a sacrifice in output power.  Likewise, the "100W output" HF transceivers, when used at 12Vdc, typically run in the 75W range.  But they still work.

Not being in an area subject to power outages, I can't relate well to the problem you describe.  The last time our power went out was immediately after the huge earthquake on January 17, 1994, and it was almost as immediately restored.  But if you live in an area subject to frequent blackouts, I guess a battery's not a bad idea!

73 de Steve, WB2WIK/6

Posts: 9930

« Reply #8 on: January 21, 2002, 09:48:45 PM »

Remember to keep the battery from getting too hot or too cold, and think about getting a small solar panel to set out and charge the battery during the day, you can pick up the " keep your battery charged " panels for $30-50 and it charges them all the time the sun is out. keep the batter out of the house, hydrogen fumes are explosive.. 73 tom N6AJR

Posts: 18057

« Reply #9 on: January 22, 2002, 12:07:25 PM »

I keep a 5A power supply attached to the batteries that power
my rig, with the voltage set  for float charge.  This is enough to
keep the battery up, even while running 100 watts (though usually
I run much less than that) and there is no danger of exceeding the
charge current limit on the battery if it is discharged when the
power comes back on.

Remember the current draw on the battery is a function of your
power output - switching to low power can really extend how long
you can operate on a backup battery without recharging.

Posts: 70

« Reply #10 on: January 31, 2002, 03:38:33 PM »

I'd like to jump in here with a question of my own if I may. I have been contemplating a similar setup. In anticipation, I have attached Anderson Powerpole connectors to my factory 706 power cable, beyond the fuses. From reading the suggestions here, I am confused as to whether I should/need to add fuses near the battery as well.

Posts: 2

« Reply #11 on: February 01, 2002, 02:15:29 PM »

By all means, install fuses in both positive and negative leads as close to the battery as possible. Remember, any wiring or equipment between the fuses and battery is totally unprotected and just begging for trouble.....A shorted wire between your fuses and the battery has no trouble at all burning up a $40,000 automobile in a heartbeat!!

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