mobile: to bat or fuse box?

Discussion in 'Mobile Radio Systems' started by KD0KKV, Jan 5, 2011.

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  1. N4WRD

    N4WRD Ham Member QRZ Page

    My question is, what relay would I need to do this? Where can I get it? How do I install it?

    (can you tell I'm a newbie?) :rolleyes:

  2. KX0Z

    KX0Z Ham Member QRZ Page

    The relay will have 4 terminals One will be connected to source of 12V switched power that turns on and off as you turn the car on and off. One will be connected to ground. Those two terminals are what switch the relay on and off.

    There will be 2 more terminals. One will connected to the wire that comes from your battery. The other terminal will be connected to your power cable that goes to your radio. Inside the relay is a set of contacts that make and break contact as the relay engerizes and de-energizes. When the relay is engerized a magnetic coil pulls the flexible contact onto the other pole and completes the circuit which will provide power to your radio. When you turn the car off, the relay will de-engerize and the contacts will no longer touch which will cut the power to the radio..

    About any auto parts store will have a relay. Maybe even Radio Shack. :)
  3. KA9VQF

    KA9VQF Ham Member QRZ Page

    I’ve done this and it works fine but I added an additional switch so I could power the radio without turning on the cars ignition.

    I used a double throw single pole switch with no ‘off’ in the middle.

    Set the switch one way and the radio would shut off when I would turn the ignition off. Set the switch in the other position and the radio had power to it until I would flip the switch the other way.

    When I start the car I had the option of flipping the switch again or just leaving it on. It didn’t matter. But I would usually flip it so the rig would shut down with the ignition again.

    Almost any 12 volt relay will do so long as the contacts are rated at, at least the power handling capabilities your radio requires.

    My HTX-242 is a 45 watt rig so it needs the better part of 8 amps to be happy I use a relay with contacts rated at 25 amps just to have some wiggle room.

    I bought the relay at radio shack and put it in a nice little metal project box with tabs on either end so I could screw it to the underside of the dash in my car.
  4. K6CPO

    K6CPO Ham Member QRZ Page

    I hate to say it but, Radio Shack...

    I use one of these to control the auxiliary driving lamps on my motorcycle. The lamps are connected to the battery through the 30A contacts of the relay and the coil of the relay is driven by either the accessory terminal or the high beam lead of the headlight through a SPDT toggle switch. This way I can have the driving lights on all the time (with the acc terminal) or switched with the headlight high beam. Never had any issue with the contacts handling the current of two halogen off-road type lamps.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    That'll work. Scotty has a NAPA near him that may have a "Fog & Driving Lamp Relay" pn GRO 448405 (also rated 30A) in stock for a couple of bucks less ($4.58 each):


    1/4" Faston (aka Quick Connect) terminals can be used to connect it. RS or NAPA may have a connector that'll ensure the relay is connected properly (assuming it is wired correctly). Whatever relay is used, it is best to use put a fuse inline at the point where power is picked up. Any circuit fault downstream from there will blow the fuse. Another good idea is to put a back-EMF diode across the relay coil. Without it, you can have a nasty voltage spike appear when the relay coil is deenergized:

  6. KC9TLP

    KC9TLP Ham Member QRZ Page

    You should see the setup i got going on here, one switch with the car off, that turns on my stereo(jvc avx-706, hardwired bluetooth kit(motorola t605) and another relay that provides 30A of power for a planned yaesu ft-857D. I wired all of this to a second battery in the trunk(100ah agm battery) since trying to fire up a 100 watt HF rig off the starting battery only assures one thing, needing a jump start, esp in cold weather. I owned the nice stereo and bt kit before i got into ham radio and it seemed silly to not let them in the 0 rpm club, esp when i'm sitting it seems an hour a week in it getting on the weekly nets, as the building that i work in is one of the worst rf traps i have ever seen, you can't even get the weather stations on a HT there.
  7. KJ6LFD

    KJ6LFD Ham Member QRZ Page

    Here's my a, maybe excessive.
    I have a Chevrolet pickup with a gas engine. The diesel models have two batteries, I bought the second battery mount and installed a comms battery (Optima deep cycle). It is charged through a device from
    It is basically a voltage sensitive solid state relay. When the charging system gets the vehicle battery up to about 13 volts it turns "on" and sends power to the second battery. There is a 3 position switch on my dash. "Auto" does what I just described. "On" forces the relay on in case I need the comms battery to help the vehicle battery. "Off" keeps the relay from switching on. I use a bolt-in AMG fuse on each side of the crossover. There is room for one in the fuse box for some diesel stuff I don't have. The other one is on the aux battery side.
  8. K0BG

    K0BG Ham Member QRZ Page

    Lots of folks use relays, but they have their own set of problems, some of which aren't so safe. There are a couple of solutions.

    Almost every new transceiver out there has an auto-off feature just in case you forget to do it manually. All considering, I wouldn't pick one with out the feature.

    If you think you just have to have a relay, use a Perfect Switch instead. (

    They're a FET device, and if you exceed their current rating they simply drop out. Their keying current is a few uA, and since there are no contacts, there isn't any of their (contacts) inherent problems.
  9. KR2C

    KR2C Platinum Subscriber Platinum Subscriber QRZ Page

    There was a really cool article in QST a a year or so back that described making a relay box. If you like making stuff this looked simple and fun to build.
  10. K0BG

    K0BG Ham Member QRZ Page

    Let me expand on what I wrote previously.

    The use of relays in powering off & on relatively low current devices, is fine as long as you have adequate protection (a fuse for example) to protect the wiring. All too often, however, the protection scheme is flawed. The aforementioned schematic is an example. It doesn't show any protection for the control line itself.

    All too often, similar schematics show one side of the relay connected to the B+ after the fuse, but then indicate that the control line is much smaller in current carrying capacity, than the fuse. One has to remember, an average 20 amp fuse will carry 40 amps for about 10 seconds, and about 80 amps for about 1 second.

    It pays to design your wiring with the best possible protection by proper fuse selection and where in the circuit those fuses are located; proper wire size to keep voltage drop under control (something almost no one thinks about); and obviously proper routing to prevent possible short circuits.

    In any case, it isn't a 20 minute job to tackle just before you leave on your vacation!
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