Powering the radio. Relay vs Transistor vs Other?

#380837

I'm reinstalling my Cb in my truck. My goal is to get rid of the whine in my radio (I think caused by my alternator). I was going use a fuse tap so my CB would turn on and off with my truck and try to figure out how to use RF chokes. But then I came across k0bg.com. It's a ham operator, but the entire website seems to be the ins and outs of mobile radio setups. My conclusion after much reading is to run power wires directly to the battery. The problem with that is my radio won't turn on and off with my truck like I want. Two possible options. 1) Run a "signal wire" via fuse tap to a relay to connect and disconnect the radio from the battery. I've never done much with relays, but I'm concerned that it will be it's own source of RF interference. 2) Use a transistor for a similar set up. Problem with this is it appears to allow voltage from the "signal wire" to connect with the main power line, thus introducing electricity from the truck's electrical system including any whine I'm currently experiencing.

Any thoughts or experiences with this type of set up?

Also I want to run thick enough guage wire so I can eventually run an amp around 100 watts or so. I'm thinking 10 gauge wire???
Thanks

PS
Truck: 2001 Ford F150 Supercrue
Radios: Usually Cobra 148, Currently Cobra 29
Antenna: Mag Mount Wilson 1000
Radio Mount: Plastic hump mount with external speaker

#380839

you can use a high amp relay like a horn relay that would turn the power on and off to the radio. very simple to install it this way. just have a power wire ran to the ignition switch power to activate the relay and you can have power straight from the battery. very simple to do. forgot to tell you to do what you are wanting all you need is a 40 or 50 amp SPST relay. it has 4 legs. 2 goes to the activation coil. one to ground and one to 12 volts to close the other 2 so you can get full power to the radio from the battery.

#380840

Will the electromagnet in the relay cause rf interfearence? Where I want to mount it is basically right next to the radio.

#380841

I would make sure I mounted it to metal to help just in case. I have put relays like this inside of some old 858 PLL radios that had a weak on off switch from the factory and never had a problem. if the relay has a plastic case it will not matter where you mount it.

#380842

Very cool. Thank you. As a matter of duscusion, have you ever used a transistor to accomplish the same job?

#380844

I would used a high amperage light relay, the same type used for off road lights, they are more toward longer "on" times then the horn relay sonoma suggested.

3's

Greg

#380845

The spotlight relays and most horn relays are excellent for your application but there's no guarantee they wont produce interference. Many automotive relays are housed in steel enclosures and rated for heat which is great for putting them under the hood, near the battery. I like to think the grounded steel enclosure as well as the firewall serve as shields against radio interference so I wouldn't recommend mounting near the radio. I've got service trucks with 7 or 8 relays for various external lights, isolation circuits, and hydraulic controls. They're all mounted (and grounded) to the fenders, most near the batteries. I've always run CB radios in these trucks and never had a problem with interference. I'd also suggest you run a toggle switch in parallel with the key switch so you can use your radio without having the ignition (and other accessories) on. One thing a lot of folks overlook (as was mentioned above) is the absolute necessity of fusing all the new circuits, both primary and secondary. If you short an un-fused wire you can, no, will cause major damage. I've seen a shorted wire heat to 'red hot' and burn off all the insulation in a matter of seconds and that red hot wire will burn through other wires, hoses, plastic clips and even fuel lines. You must fuse!! Another common problem in automotive wiring is bad terminals and connections. Twisting two wires together and wrapping tape around them is a no-no as are cheap crimp connectors. A #10 wire will handle your 100 watt amp but not if it's got a bad or cheap crimp connection somewhere in the circuit. By "cheap", I mean the ones they sell at auto parts stores and big box hardware stores. Go to an electrical supply house or good electronics store and don't be afraid to pay 50 cents a piece for good crimp connectors. They're worth their weight in gold (especially the gold ones!).

On the transistor issue, I'm not smart enough to make something but there are real good Solid state (transistor) relays out there and they work just as well as the typical electro-mechanical ones we've been talking about. They usually cost more too. I'll bet you can find them on **Non Sponsor**.

(edited by MDY, removed **Non Sponsor**)

#380847

WOW mdyoungblood, you must be thinking about the old horn relays from the 70's and 80's. the new relays now are the same for the horns, fog lights, fuel pumps etc. the new relays made today will control any thing all day long. my 99 f150 pickup has numerous relays in the center under the hood and the relays are used for the computer, airbags, fuel pumps and so on. they are all made now to power on about every item on the vehicle now days, and yes the old horn relays were not made to power on full time back then.

#380849

Your right sonoma, I was thinking just that, I used old Ford starter relay's to power up my off road lights cause at the time they didn't make anything that could handle the amperage, even those had a short life span. All the new stuff out now I haven't even checked out, I'll leave it to the younger generation.

3's

Greg

#380850

Yeah, those "starter relays" Ford and a lot of others used were great for starting or jumping dual batteries or glow plugs on Diesels but the coils get hot after a while. But the driving light relays, about the same size, they can stay on all day or until the battery goes dead and keep on tickin'.

#380861

Wow. I really appreciate all the response. Scipio Kid, here is my overall plan. In my pick-up truck, I have a plastic hump mount. I am going to mount on the back of it wire binding posts/terminals (the kind you frequently see on the back of base station power supplies). I want to do that because I'm frequently changing radios, testing, comparing etc. BUT I want 3 binding posts. 1 positive always on, 1 positive on/off with the truck, and 1 negative. (Sometimes when I'm working on my main radio, I run a cheap Cobra 18. It has 3 power wires. The extra is designed to keep power so it saves your channel when you turn the truck off. (Otherwise, it will default to ch. 9). On the front of the hump mount, next to the speaker, I am going to include a rocker switch that will give power to the on/off terminal so I can do just as you said, run the CB without anything else on.

So I'm faced with a decision. Do I run 1 positive wire to the underside of the hump mount, split it there (with relay and signal wire there as well), or do I mount the relay in the dash or on the other side of the firewall and run 2 positive wires to the hump-mount?

Also, don't worry, I'm planning on including fuses. How many fuses do you think I need. I'm going to do one for sure on the positive line before it splits. Is that enough? Should I put a fuse on the split line as well? What about the negative wire?

-- Wed Oct 26, 2016 8:14 am --

Also Scipio Kid. There is a Scipio here in Utah. Do you have a connection to that Scipio? Just curious. I'm up here in northern Utah.

#380865

make sure you put a fuse at the battery so if you get a short from the battery to the radio the fuse will blow and not burn some thing up.I would put a fuse right at the radio just for a problem at each piece of equipment you are running. this way the power from the battery is fused then each radio you are running is also fused. you can put a large fuse at the battery to handle all you have inside the cab.

CB Radio Talk

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