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Author Topic: Power supply for mobile radio.  (Read 6861 times)
KK4IKO
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Posts: 67




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« on: October 11, 2012, 12:51:33 PM »

Hi,

I've read some comments about connecting the power source for a mobile rig directly to the car battery, using very large conductors (#8@40A, or #6@50A).  My radio is rated for 20 amps, #12 wire, so I removed the cigarette lighter and connected #12 directly to its wires.  My cigarette lighter is fed by a 20 or 25 amp relay, through a 25 amp fuse.  The ignition switch has to be in the ACC position, or the engine running to hot up the radio, which means I can't accidentally leave the radio on when I remove the key.  I have CRS these days, and that sort of stuff happens more and more. 

I'm only setup to use the radio on VHF, and UHF in my vehicle, for which my radio restricts the power to 50 watts or less.  I don't see the  advantage of going to larger conductors and connecting directly to the battery in this case. I am a retired industrial electrician, so a detailed technical answer will be understood.

My rig for HF is in the house.

Bruce, KK4IKO

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M6GOM
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Posts: 988




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« Reply #1 on: October 11, 2012, 02:04:25 PM »

Basically it would seem that in America, auto manufacturers scrimp on the cable so whilst the fuse may be rated at 25A and the manual says it is rated at 25A, the wiring may not be. But if you've connected to the wiring already and confirmed it is a sufficient diameter, you should be OK.

Alan K0BG will inform you that common mode current can end up in the wiring and screw with the electronics. Whilst that is true if you use a magmount or similar, if you use a proper antenna mount - hole drilled through roof type such as NMO body mount - then it isn't likely to be a problem especially if you're only operating V/UHF. Can't say I've ever experienced it myself even though I once tried to use a triple magmount and had common mode on 80m so bad it rebooted the radio when TXing with more than 50W.

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K0BG
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« Reply #2 on: October 11, 2012, 02:30:07 PM »

Using the accessory socket to power any amateur radio gear, is the number one sin of mobile installing!

Any given fuse rating has nothing to do with the continuous capacity of any given circuit. That is to say.... Fuses are sized to protect the wiring, but they do not directly correlate to the size of the wire. Nor do they correlate to voltage drop in the circuit in question.

And, every single automobile manufacturer on the planet suggests wiring directly to the battery or jump points as the case may be. Further, the big three (Icom, Kenwood, and Yaesu) all recommend direct to battery connections in their respective manuals. This said, vehicles with battery monitoring systems, should always be wired before any BMS. While wiring to the accessory socket satisfies that requirement, it is still not safe, if for no other reason than the wire size feeding the socket. If you believe it is, then you'd best read the Wiring article. on my web site.
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KK4IKO
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« Reply #3 on: October 11, 2012, 03:35:34 PM »

I am using a drilled NMO mount in the center of the roof.

K0BG, I understand what you are saying.  How would you suggest connect directly to the battery, so that I still have peace of mind about my radio not being on when the key is off?  Like I said, my CRS is kicking in so I'm tending to go with more foolproof stuff.  Should I install a relay, controlled by the ACC circuit, in the line, or would I need an electronic switching circuit of some sort to avoid interaction with the vehicle's electronics?  In the 6 months since installation, I have detected no issues. 

My mobile unit's (2000 Jeep Cherokee Sport) battery monitoring system is the old fashioned kind...battery dies, no workee.  No advance warning or protection like on my new car. 

Bruce, KK4IKO
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WB6BYU
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« Reply #4 on: October 11, 2012, 04:46:34 PM »

I can tell you what happened in our car:  I had an IC-207 mobile rig connected
to the lighter socket "temporarily".   It turns out that, if the input voltage drops
below a certain level when the rig is in transmit mode, it doesn't come out of
transmit when you let up the mic.

With the engine running the voltage drop in the wiring was enough to do this at
50 watts, so we never used the high power position.  25 watts seemed to work
OK with the engine running, but with the engine off we saw the same problem.
We discovered this when my wife turned off the engine, signed off the radio, and
left the car...   with the rig stuck in transmit mode.  Took me about 20 minutes to
find the car (I only knew what town she was in), unlock it with the spare key, and
turn off the radio.


So, while the fuse might be rated at 20A, the voltage drop in the wiring at 10A
was enough to cause the radio not to work properly.

And I've learned to watch how much the panel lamps dim when I transmit as
a gauge of the quality of the wiring.
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KK4IKO
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« Reply #5 on: October 11, 2012, 05:15:22 PM »

K0BG,

Just checked out your website, and I must say you have a lot going on there.  It's guys like me who really appreciate good solid info made available by good folks like you.

Anyway, with your help, my dilemma has been solved.  I'm going to order one of those Timer/Switch units from Powerwerx, and run out to the battery.

One question, though...does the radio require a separate ground connection to the vehicle for mobile use, in addition to the power ground?  It doesn't show one in the radio manual.  My meter indicates that all points which should be at ground potential, are grounded to the vehicle chassis and sheet metal.

Bruce, KK4IKO

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WN2C
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« Reply #6 on: October 12, 2012, 01:04:22 AM »

KK4IKO, does your radio have an auto timer to turn it off?  Most newer radios do.  Check your manual. What radio are you running?

wn2c Rick
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K1CJS
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« Reply #7 on: October 12, 2012, 04:34:17 AM »

In today's cars (and in yesterday's, for that matter) the fuse that protects the wiring for the cigarette lighter (or the power socket) also protects other wiring circuits.  That fuse has to be large enough to handle all of those circuits.  The requirement of having a certain size wire for a certain size fuse holds true for house wiring, not for wiring in cars--especially in foreign cars.

The wire(s) connecting that power socket to the fuse in about 95% of the cars are nowhere near the ten or twelve gauge wire that a transceiver needs to work correctly.  Wiring that can handle the 8 to 10 amps that a cigarette lighter needs to heat just can't handle the increased amperage needed to power the usual transceiver.  Remember that the lighter heats up in a few seconds--a much shorter time than transmitting on a rig usually takes.

The usual power socket in newer cars labelled 12 volt 20 amp is so badly misleading that I'm surprised that there hasn't been lawsuits out the kazoo yet about it!

For safety sake, do NOT use the existing wiring in any car to power a transceiver you're going to use to transmit at full power.  It simply isn't large enough to handle the needed power safely.  I believe that K0BG has pictures of the results of doing so on his website--please take those pictures seriously.

Added:  Wiring directly to the battery--or to the appropriate terminal under the hood for cars with battery charge sensors--is always the best way of connecting power to any transceiver.  Using the existing wiring in the passenger compartment is ALWAYS the WORST!  
« Last Edit: October 12, 2012, 04:37:11 AM by K1CJS » Logged
KK4IKO
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Posts: 67




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« Reply #8 on: October 12, 2012, 01:55:33 PM »

KK4IKO, does your radio have an auto timer to turn it off?  Most newer radios do.  Check your manual. What radio are you running?

wn2c Rick


Rick, I have an IC706MKIIG radio.  A Powerwerx Timer/Switch unit will allow me to program it for up to 12 hours, so that should work nicely for my needs.  When I'm operating stationary, it's only for 30-45 min before I restart the engine.  Since I'm only operating on VHF and sometimes UHF, the maximum power my radio provides is 50W, which draws significantly less than the 20A needed for full power (100W).

I can see the advantages of connecting to the battery, and I'll feel much better knowing I can't accidently leave the radio on when I'm not there.

Bruce, KK4IKO
« Last Edit: October 12, 2012, 02:16:59 PM by KK4IKO » Logged
AA4PB
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« Reply #9 on: October 12, 2012, 02:18:22 PM »

On the IC706mkIIg the timer shuts the radio off after a set period of inactivity, regardless of whether the engine is running or not. Inactivity is defined as making any changes like moving the VFO, changing filter settings or using the PTT on the microphone.
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M6GOM
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« Reply #10 on: October 13, 2012, 03:56:16 PM »


K0BG, I understand what you are saying.  How would you suggest connect directly to the battery, so that I still have peace of mind about my radio not being on when the key is off?  Like I said, my CRS is kicking in so I'm tending to go with more foolproof stuff.  Should I install a relay, controlled by the ACC circuit, in the line, or would I need an electronic switching circuit of some sort to avoid interaction with the vehicle's electronics?  In the 6 months since installation, I have detected no issues.  


Easiest way as you mention is to use a relay controlled by the ACC circuit.

Alan likes to over-exaggerate powering radios with accessory sockets somewhat. I'm surprised he hasn't posted the picture he likes to do where he claims a 5W handheld transceiver with no faults that draws less than an amp on keydown powered by a cig lighter socket melted the car wiring and set the car alight.

When it comes to powering radios, some of the stuff Alan posts needs to be taken with caution. Whilst none of the solutions he suggests causes issues, some of the things he says you shouldn't/mustn't do are, shall we say, not always correct.
« Last Edit: October 13, 2012, 04:01:07 PM by M6GOM » Logged
M6GOM
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« Reply #11 on: October 13, 2012, 03:59:07 PM »


One question, though...does the radio require a separate ground connection to the vehicle for mobile use, in addition to the power ground?  It doesn't show one in the radio manual.  My meter indicates that all points which should be at ground potential, are grounded to the vehicle chassis and sheet metal.


No you shouldn't use one. Some people do to cure RFI issues but if it cures a problem, something else is amiss and this is just masking it.
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K0BG
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« Reply #12 on: October 13, 2012, 06:27:45 PM »

I can not speak for all makes and models. However.... Most newer models of amateur radio gear have an automatic off feature. Set it for about 30 minutes, and all is well. If yours doesn't have the feature, then here is a solution: A PowerWerx APO3. http://www.powerwerx.com

There is also a bunch of anecdotal information on the net about disconnecting amateur radio gear from the battery (via a relay most of the time) when starting the vehicle. Its PSHAW!
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W9IQ
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Posts: 102




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« Reply #13 on: October 14, 2012, 03:31:26 AM »

The PowerWerx timer sounds nice but before I spent $70 for it and then spent hours fishing power cables through the vehicle, I would get more quantitative by measuring the voltage at the accessory socket during full power transmit. A half hour of field testing could save a lot of unnecessary work and expense.  

Every situation is probably unique but I ran my 50 watt dual band radio off the accessory socket in my Yukon with no problems. The radio body even sat on top of the vehicle computer modules inside of the center console.

- Glenn DJ0IQ and W9IQ
« Last Edit: October 14, 2012, 04:08:04 AM by W9IQ » Logged
K1CJS
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Posts: 6055




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« Reply #14 on: October 14, 2012, 06:20:27 AM »

....I would get more quantitative by measuring the voltage at the accessory socket during full power transmit. A half hour of field testing could save a lot of unnecessary work and expense....  

Not unless you could INSURE that every vehicle device and circuit that had any connection to the wiring that you're connecting that mobile rig to would be on.  There are so many different devices and other items connected that switch on and off BY THEMSELVES that you couldn't be sure of any such quantitative measurements being accurate.  Far better to wire directly to the battery--unless there was a high power tap you could wire to located in the passenger compartment, as there used to be (I'm not sure about the newer cars) in quite a lot of full size cars used for commercial work
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