Roady and Ohm's Law Help

[jpalamaro]

A buddy of mine wishes to power his Roady XT thru the maplight, however he tells me that the Roady requires 5 VDC and apparently the cig. lighter adapter does the voltage dropping.

I don't have all of the data but can one of you engineers help with this question:

Ohm's Law: E = I R
Operating Voltage at Maplight: Nominal 13 VDC
Roady XT Amperage (guess): 500 ma
Dropping resistor on 'hot leg' of maplight, for roady. 26 ohms

Dropping resistor type/wattage: Ceramic, 5 watts, or higher

Does this make sense, has anyone done it?

Thanks

[Big_E]

An easy & cheap way is to buy a female lighter end pigtail at any parts store or WalMart, plug it in & wire the pigtail to the map light or any switched source, hide it all under the tupperware.

[jpalamaro]

Thought of that already, but sorry and don't mean to insult, a bit too clunky/low-tech for him/me. But thanks . . .

[dshealey]

A buddy of mine wishes to power his Roady XT thru the maplight, however he tells me that the Roady requires 5 VDC and apparently the cig. lighter adapter does the voltage dropping.

I don't have all of the data but can one of you engineers help with this question:

Ohm's Law: E = I R
Operating Voltage at Maplight: Nominal 13 VDC
Roady XT Amperage (guess): 500 ma
Dropping resistor on 'hot leg' of maplight, for roady. 26 ohms

Dropping resistor type/wattage: Ceramic, 5 watts, or higher

Does this make sense, has anyone done it?

Thanks
For a simple resistive load, adding a resistor is certainly the easiest way to do it. I would not use that method for something electronic though, as the voltage will change if the load changes any, and will also change as the dropping resistor changes temperature. Also, the dropping resistor is dissapating heat and energy. Of course this is pretty small, so not a real worry.

A far better approach is to use a three terminal voltage regulator, pretty readily available. These actually REGULATE the voltage so it is always the same, no matter the current or temperature changes. See Radio Shack #7805, $1.59.
Web site shows out of stock, check your local store.

[bmwmick]

John,
What I've done on a couple of installs is remove the 6V regulator circuit board from the Roady cigarette lighter adapter. Solder a lead to the + and - connection points and then put the whole regulator (1" X 2") inside a piece of large shrink tubing. Shrink the tubing and then cable tie this neat little package somewhere out of the way. Hook up the + and - leads to a switched 12V source and you have a nice neat install. :)
Dont try the resistor trick, it won't work well.

Mick

[DaveDragon]

Mick's solution is much easier and will work perfectly.

[jpalamaro]

Thanks Dave. BTW, I have tried attaching Howard's file on three websites that use vBulletin and cannot do it. I am going to photoshop is and save as . . . just nuts. Would you like the file e-mailed to you?

[DaveDragon]

Sure DaveDragon AT Hot Mail :)