Radio Question

[Civil_War_Buff]

Now that I have a few outings under my belt with the TM, I am ready to compile my winter upgrade list.  Top of the list is to finally install an AM/FM/CD Stereo.  I was wondering if there is an easy way to add a 9V battery that will hold the presets if the main batteries are disconnected?  I am thinking that I would need a relay so that the 9V wasn't used unless the main battery was out, so I was wondering if anyone else has done this mod?  If so, how did you do it.

Thanks.

Actually, they make a simple to use device that runs off of a 9v battery. All you do is plug it into a cigarette lighter socket, and you're good to go! Best place to find it probably would be Radio Shack. It's very small, not much bigger than the plug itself. Just plug it in before you close the TM and it should be good to go.


[glow=red,2,300]Happytrails........[/glow]

Does your battery actually disconnect when you fold up the trailer?

I don't think my 12V power turns off.

I've looked all over for some type of cut-off switch and haven't found one.  I also tried turning on the lights after it was folded up and the lights still work...  

Maybe the automatic cut-off is on later model trailers...  I have a 1996 model.

[Civil_War_Buff]

Does your battery actually disconnect when you fold up the trailer?

I don't think my 12V power turns off.

I've looked all over for some type of cut-off switch and haven't found one. I also tried turning on the lights after it was folded up and the lights still work...

Maybe the automatic cut-off is on later model trailers... I have a 1996 model.

The battery power (12V) doesn't actually disconnect when I close down, but I was speaking of the times when I remove the batteries for maintenance or pre-trip charging.  

Larry

[Civil_War_Buff]

Actually, they make a simple to use device that runs off of a 9v battery. All you do is plug it into a cigarette lighter socket, and you're good to go! Best place to find it probably would be Radio Shack. It's very small, not much bigger than the plug itself. Just plug it in before you close the TM and it should be good to go.


[glow=red,2,300]Happytrails........[/glow]

Would this go in the receptacle for the TV?  Or do I have to put a new 12V recept. in to accomodate the device?  I don't want any other draw on the 9V battery, as I don't want to run that one down too quickly.

Thanks,  Larry

[Bill]

Um, I hate to disagree, Bill, but I'm not sure that the 9-volt battery/cigarette lighter device is going to work out. The reason? Since it is plugged into the 12-volt distribution circuit, it will try to feed power to any 12-volt devices that are turned on. Yes, you can turn off the big devices - lights, refrig, etc - but there are a few small ones that you can't turn off. The propane leak detector is one. The water heater gas control module may be another. The furnace control module may be yet another. I might be wrong, but my guess is that the total "hidden load" is high enough to kill a 9-volt battery in a few hours. We're having a nor'easter here this morning, so I won't go outside and measure the current drain, but I'll try to do so in the next couple days and report back.

CWB, I think you can accomplish the function you want, and probably a little bit easier than you described. Try this. Most car radios have three power wires. One is ground, one is switched 12-volt power (in a car installation, it would come from the ignition switch), and one is always-on 12-volt power (in a car installation, this would come directly from the car battery). This latter line is the keep-alive function you describe, holding the presets.

You will need two silicon medium power diodes. They are very common, Radio Shack sells a three-pack called "1KV 2.5A Silicon Diodes", part number 276-1114, for a couple bucks.  If you don't have a R-S nearby, any diode rated a couple hundred volts (or more) and an amp (or more), will work.

Cut the keep-alive wire, and insert one of the diodes into the break, with the cathode (banded end) toward the radio. Connect the cathode of the second diode to the cathode of the first. Connect the anode (non-banded end) of the second diode to the + side of your 9-volt battery. Connect the - side of the 9-volt battery to ground. Solder or wire-nut all connections, make sure they are well insulated, and you're done.

How does it work? As you may know, a diode allows current to flow in only one direction, from anode to cathode. When you have a 12-volt battery in place, current flows forward through the first diode, and brings the keep-alive point to 12 volts. The second diode therefore has 12 volts on the cathode but only 9 volts (from the small battery) on the anode. Current would like to flow backward into the small battery, but the diode blocks it.

Now if you remove the big 12-volt battery from the TM, the situation reverses. The 9-volt battery, via the second diode, brings the keep-alive point to 9 volts. This brings the cathode of the first diode to 9 volts, but the anode is at zero volts because the big battery isn't there. Current cannot flow backward through the first diode, and therefore can't flow to the propane detector, etc. The result is that your 9-volt battery powers the radio's keep-alive function, and nothing else.

Hope this helps

Bill

[Bill]

I have to make a small correction to my previous post, in which I talked about the "hidden load" of the propane leak detector. In the later models, at least, the leak detector is disconnected when you fold down the bathroom wall. You know if your model does this - there is a small "chirp" sound as you drop the wall. I'm not sure when this cutoff feature was added - I am under the impression it was relatively recently, so owners of earlier models will want to beware.

However, there may be another hidden load which I didn't mention, and this is the lighted dial display of the radio itself. The display is powered by the "switched 12-volt" line, and in a car installation, this line has power only when the ignition switch is on. But since the TM doesn't have an ignition switch, this line is simply connected to the 12-volt feed, and is always on. The problem is that on my radio at least, the dial display is lighted even when the radio is turned off! Very annoying.

To get to the point, if you plug a 9-volt battery into the cigarette lighter outlet (and therefore into the 12-volt power distribution), it will try to power the radio dial light in addition to any other hidden loads. The dial light will kill the small battery in a hurry. The diode solution that I described prevents this problem also.

Bill

[Civil_War_Buff]

Bill,

What you describe is EXACTLY what I am looking to do.  One quick question though, with the diodes, the cathodes are hooked together, and then one anode goes to the + side of the 9V, does the other anode go to the +12V?  If so, then I understand your fix entirely, and this is what I am trying to accomplish.  Any idea on how long a 9V will hold the memory?

Thanks for the help though.

[Bill]

Larry -

Yes, you have the hookup exactly right.

The life of the 9-volt battery would depend on how much current your radio draws for this function. Your radio may be different, but the keep-alive line on my Jensen AM/FM/CD radio draws about 150 mA. This is considerably more than I expected, and unless your radio is different (you should measure it), that news is not good.

A common rectangular Duracell alkaline 9-volt battery has a capacity of about 350 ma-hours. That is, if you draw 1 mA of current, the battery will last about 350 hours. However, if you draw 150 mA, the battery will last only about 2 hours. My guess is that you had a longer time span in mind.

As I say, I don't know why the keep-alive line draws so much current - there is nothing inherently power-hungry about the application. Perhaps it is simply because no effort was made to minimize it. After all, 150 mA is negligible in the context of a 12-volt car battery. Perhaps your radio draws less, but until you measure it, that's the way it looks from here. If you seriously want this function, you may have to consider using a stack of 6 D-cells. This should last about 100 hours.

Bill

I have radio/cd in my TM and it seems to me that it runs hot. Would a cooling fan installed nearby to move fresh air in/out of the space above the refrigerator help?

Dave