20A Cigarette Lighter socket

Discussion in 'Mobile Radio Systems' started by KC3U, Oct 3, 2013.

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  1. KC3U

    KC3U Ham Member QRZ Page

    I know that I *should* run my own DC wires through the firewall, right to the battery, etc. and never to use a cigarette lighter adapter to power my HF mobile radio. Having said that, my '04 Dodge RAM 1500 has a socket in the center console compartment clearly labeled "12V 20 Amps" which tells me that the wiring behind it must be fairly beefy, in terms of gauge.

    Has anybody used this socket to power their radio? If my Ohm's law serves me right, 12v x 20A = 240Watts, which is way more than what I plan to run (I run about 50W max = ~4Amps).

    de Ramon KC3U
  2. KF5FEI

    KF5FEI Ham Member QRZ Page

    Yes and no -- it probably isn't intended for continuous duty at high current.

    If you are a Hamnation viewer, you'd have seen Gordo resolve this issue with one of the Powergates and a secondary battery. Radio runs off the secondary battery, charged by the lighter outlet in the rental car.
  3. K0SPN

    K0SPN Ham Member QRZ Page

    I would suggest that you actually test the power draw on a standard supply or battery first.
    Just because 50W at 12V equals 4.166666A doesn't mean that's how much you'll actually draw.

    There is other overhead in the transmitter and I wouldn't be surprised if it drew an extra amp or two, then you've also got to figure that the final amp is not 100% efficient.
    I'd imagine you'd be closer to 8A-10A than 4A.

    It does vary with different brands and models, so get a current meter and find out what you'll actually draw.

    That said, you'd probably be okay at 50W, but I'd say that's the absolute limit as I wouldn't trust that "20A" rating.
    Be sure to measure the voltage at the radio when you try it the first time and see how much drop there is; also, measure it with all the lights, AC, etc. on and make sure you're not dipping too low.
  4. WA9WVX

    WA9WVX Ham Member QRZ Page

    Hi Ramon,

    I'd run a pair of Red & Black Stranded # 10 or 12 AWG directly to the battery. Make sure to Fuse the + 12 VDC Red Wire at the Battery so if that wire should short to ground, it won't start a Fire or Drain the battery. The 50 W Rating is referring to RF Output, you'll always have a higher DC current draw for the DC Input. It would be at 6 A maybe 7 A depending on the radio. You could probably find your mobile transceiver electrical specifications on the internet for Transmit DC Current Draw.

    I had a buddy that had the same arrangement on one of his vehicles and when he would shut off the ignition the "IR" drop was sooo bad that even using only 5 W Output, the radio wouldn't key up.

  5. K0BG

    K0BG Ham Member QRZ Page

    I don't care what it is rated at, the wire size is number 14. It just isn't large enough to handle the rated load over time. Think about this:

  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I agree.

    It may be fused at 20 but the rated load should be a lot less.

    Have Fun.
  7. KH6AQ

    KH6AQ Ham Member QRZ Page

    Automobile 12 volt Power Outlet design is covered in SAE Standard J563_200902.

    The 2012 Ram Truck owner's manual specifies a 20 amp fuse for each accessory outlet but says not to exceed 13 amps.

    Page 202:

    "Do not exceed the maximum power of 160 Watts
    (13 Amps) at 12 Volts. If the 160 Watt (13 Amp)
    power rating is exceeded the fuse protecting the
    system will need to be replaced."

    There is no mention of wiring damage if the load exceeds 13 amps, only an open fuse. Running SSB a 100 watt HF transceiver draws around 20 amps peak and less than 13 amps RMS.
    Last edited: Oct 4, 2013
  8. KF5FEI

    KF5FEI Ham Member QRZ Page

    ...Which you can buy a copy of for $68. Care to enlighten us with the pertinent details?
  9. KC3U

    KC3U Ham Member QRZ Page

    Thank you everybody for the useful feedback. I have a Kill-a-watt at home, so I'm going to measure the IC-746's current draw at receive and at 50W transmit, and go from there. I can always back off the RF power to a point where I feel "safe" (I'm also trying to buy a Heathkit HW-8 from the Swap forum, so I'm pretty sure that will be QRP enough for the truck wiring :)

    de Ramon KC3U
  10. K0BG

    K0BG Ham Member QRZ Page

    There are several issues which may not be evident.

    Voltage drop is a major consideration, especially with solid state transceivers. The rule of thumb is less than .5 volts at peak load. Average load doesn't count!

    No one makes an accessory socket plug capable of a sustained 20 amp, or ever 13 amp, load. What usually fails is the spring-loaded tip.

    The plugs are held in by spring pressure along. Since they're subjected to vibration, they have a tendency to loosen, with predictable results.

    Plugging in your cellphone charger or GPS is one issue, doing so with an amateur transceiver shows a lack of due diligence.
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