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Author Topic: Power supply for mobile radio.  (Read 12303 times)

Posts: 67

« Reply #15 on: October 15, 2012, 03:01:59 AM »

Thanks for everyone's help.

I've decided the only practical solution which meets all my requirements will be connecting to the battery through a Timer/Switch, such as the AP03.  I didn't realize such a device existed before looking at Alan's web site, and checking out Powerwerx.  I may not be an electrical newbie (45 years as an industrial electrician), but I admit to being and amateur radio newbie.

As an aside, I believe that in my case, the cigarette lighter fuse is just that, because that fuse had been blown by a faulty lighter unit, and stayed blown for several months before installing the radio...everything else in the vehicle still worked, including the separate 12V accessory outlet.

Bruce, KK4IKO

Posts: 9743


« Reply #16 on: October 18, 2012, 07:51:08 AM »


Most vehicle electrical system designers have enough common sense to not share accesories like a lighter or power plug with systems noticable or required in normal safe operation.

A 50 watt radio is generally only around a 10 amp load.

I'd worry about a 100W HF SSB and CW radio, or 100W FM, but probably not much about a standard size FM mobile.

I stay off the negative post with the negative lead chassis-grounded accessories, and ground to the unibody. It is safer long term, if you ever should develop a negative battery lead resistance issue.

73 Tom

Posts: 1758

« Reply #17 on: October 18, 2012, 11:02:38 AM »

there are two considerations... current capability and noise.

recently had to replace the blower motor on the wife's Taurus, and the wires to the thing were all baked and crumbling.  we had noticed for some time that if the glovebox was moved or opened, the blower went full speed no matter what was selected.  that's function of the wires burning up from inadequate design and shorting to the frame periodically afterwards, bypassing the resistor in the ground lead.

14 amp blower, 20 amp lighter socket, same 16 gauge wire which is rated for 10 amps max.  you do the math.  the automakers are figuring on intermittent or reduced usage in their circuits, so the higher peak load averages out to a lower mean load, and they figure the wiring should cool off.

this augurs for big fat ugly thick wiring straight to the battery all by itself.

on to noise.  all of these bundled wires with computer-control leads to the body module, engine module, security module, money-eating-often-failing module, fuel pump,  etc. create lots and lots of square wave noise.  do you really want to couple that nonsense common-mode into your radio?

when I was a innocent little bairn learning to swear at the engineer's knee in the broadcast control rooms waiting for Dad to take me home, car accessories had vibrators and 0Z4 noise-generator rectifiers.  sounded like bacon frying if you put another radio in those things.  you got past all the line noise going with a #8 or #6 (if you had a 50 watt VHF band business radio in the trunk) back to the battery, and what was left filtered out with a couple big caps.

second reason to go back there.  a car battery is the world's biggest smoothing capacitor.
« Last Edit: October 18, 2012, 11:05:09 AM by KD0REQ » Logged

Posts: 13851

« Reply #18 on: October 18, 2012, 11:17:55 AM »

Your IC706MkIIg already has a built-in timer that will power the radio off. Why do you want to add another external timer? I have my IC706MkIIg is set to power off after 30 minutes of inactivity. A few seconds before it turns off it beeps several times as a warning. All I have to do when I hear the beep is slightly move the VFO dial or "pop" the PTT switch to reset the timer for another 30 minutes.

Posts: 6231

« Reply #19 on: October 19, 2012, 05:02:30 AM »

...I may not be an electrical newbie (45 years as an industrial electrician), but I admit to being and amateur radio newbie.

As an aside, I believe that in my case, the cigarette lighter fuse is just that, because that fuse had been blown by a faulty lighter unit, and stayed blown for several months before installing the radio...everything else in the vehicle still worked, including the separate 12V accessory outlet.

Vehicle wiring is not the same as industrial electrical wiring.  Sorry to say that your experience as an industrial electrician doesn't count for much in this case, not as in you wouldn't do things properly, but as in how the average vehicle wiring harnesses doesn't follow the same strict standards as industrial or even home wiring does.

The fact remains that even at ten amps (and all too many 50 watt transceivers pull MORE than that when transmitting) if you use the harness wiring instead of running directly to the battery, you're asking for trouble.  The modern public service transceiver is similar to the ham rigs in that they also pull between 10 and 15 amps--and the professional installers of those units will NEVER use the existing vehicle wiring.

It may well be true that you may never have a problem and that your rig may work just fine, but is it really worth it to take that chance?  The pros won't--doesn't that tell you something?
« Last Edit: October 19, 2012, 05:07:27 AM by K1CJS » Logged

Posts: 13851

« Reply #20 on: October 19, 2012, 05:35:47 AM »

I don't know what year the tests were actually made, but the latest info from the SAE was published on 1/30/2012 ("Immunity to Conducted Transients on Power Leads").

The following provides an overview of earlier information:

The safest place to connect, in terms of noise and voltage spikes, is at or very close to the battery. The positive lead should always be fused. The negative lead should also be fused IF the connection is made directly to the battery terminals. Some vehicles provide a + and - terminal near the battery for connecting accessory equipment like radios. If this is available, that's the place to connect, primarily so that your wires are not exposed to battery acid and corrosion.

Some radios (for example, the IC-706Mkiig) have DC power always applied to the finals, even when the radio is turned off. I haven't had any equipment failures but I have had the radio run the battery down when the vehicle hadn't been started for a couple of weeks. When the battery gets low, the radio starts cycling on/off which then causes it to draw additional current from the battery. My present solution is to unplug the power cable from the radio if the vehicle will not be used for a week or more. The better (automatic) solution would be to add a power relay that disconnects power when the ignition switch is turned off (i.e. tie the coil to the acc line on the vehicle).

Posts: 399

« Reply #21 on: October 19, 2012, 06:23:42 AM »

There is a nice article in the November 2012 issue of QST on page 43 for an automatic mobile power control that incorporates an adjustable turn off delay and a turn on delay to avoid starting transients. It looks like a simple one afternoon project.

My 50 watt dual band radio takes 12 amps on transmit. Leaving my DSO in-line for several days didn't show any spikes in excess of 15 volts coming off of my Yukon's front accessory socket - but everyone's mileage may vary...

When I would run my 706 radio mobile with 100 watts SSB, I would mount the body of the radio in the back of the Yukon and with a PowerPole block connecting the radio, a separate AGM battery, and the rear accessory socket together. The battery handled the higher current draw, the accessory socket would charge the battery, and the accessory socket turned off when the vehicle turned off. Not everyone has the luxury of a rear accessory socket, but it worked great for me for many years.

As was pointed out earlier in this thread, if you are going to wire directly to the battery, be cautious of tying directly to the negative terminal of the battery especially on newer vehicles that use current sensors in the negative battery lead.

- Glenn DJ0IQ and W9IQ
« Last Edit: October 19, 2012, 06:30:37 AM by W9IQ » Logged

Posts: 47


« Reply #22 on: October 19, 2012, 08:04:10 AM »

I've never done it with a mobile rig, but I have used a relay connected to the switched power lead for the stereo to turn power straight from the battery on and off. That way,you still get power from the battery through adequate wiring, but the stereo power - switched on and off through the ignition's accessory position - turns that power on and off automatically. I chose the stereo wiring as the trigger because it's fairly easy to get to in most cases - especially for me, since I almost never keep the stock stereo in my cars, so I know the wiring well since I made it myself.

73 - Justin

Posts: 399

« Reply #23 on: October 19, 2012, 08:53:20 AM »

For all of the speculation about transients in this mobile forum, it is interesting to study the application note from Harris regarding surge suppression requirements in a vehicle (referenced earlier by AA4PB

The three most damaging transients are infrequent events in the life of a vehicle: load dump (disconnecting the battery while at high charging), losing the regulator, and getting a 24 volt jump start. The two next events with the greatest energy potentials are inductive load switching transients and alternator field decay. These later events have <1 J (1 watt second) of energy and happen often and at turn off, respectively.

My take on this is that while we might rely on internal transient protection on our radios, we would be well served to consider an additional level of protection with an appropriate varistor type device. It is a bit of belt and suspenders but may be worth consideration. It certainly would be an easy addition to most mobile radio installations. It could easily be added to the cut off circuit in the latest issue of QST that I mentioned earlier.

- Glenn DJ0IQ and W9IQ
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