12VDC to 6VDC converter

Hello all, I'm new to the forum but I love the site and have been inspired by it for a while now. So, I'm working on a project to do GMRS radio communications between my wife and I between our motorcycles. The radio and headset part are pretty well taken care of but the radios don't natively accept vehicle power - only batteries. This is a bit of a bummer if you're on the road for a couple of days or more so I want to tie them into the existing 12 volt power on the bike. The radios have no plug for this but I'm assuming if I can knock the bike power down to the 6 volts normally delivered by the 4-AA batteries, I should be able to tie that in to the terminals in the radio that normally take the battery power. So, what is my best option? Hack a cellphone charger? Build from scratch? Size is a factor as it will be stored in the tiny glovebox in the tail of the bike. Any thoughts or links to appropriate stuff would be helpful! Thanks very much. -Ryan

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Kiteman9 years ago
Check the resistance of the radio as a whole (use a multimeter). Add an equivalent resistance in series and that will split the voltage - half will be across the resistor, half across the radio.

It can't be that simple, can it? What current will be involved?
Nope, it's really that simple. What you're talking about is a potential divider, with two equal resistors in series. The point between these resistors will have a voltage equal to half of the input voltage.
I think building a small power supply with LM317 would do it. You'd need a couple filter capacitors and a couple resistors to set the voltage. Like this one... Or just use a 6V zener diode.

The other problem with a voltage divider is the current wasted by the resistors. To get a useful voltage from a divider, you need small resistance values, hence lots of heat loss.
CameronSS gmoon9 years ago
Is there anything you can't use an LM317T? They seem to be pretty ubiquitous.

Could you use a 7806 from the 78xx series?
gmoon CameronSS9 years ago
Coitainly. The 7806 might be slightly harder to find locally, if say, Rat Shack was your only source....
CameronSS gmoon9 years ago
However, you're more likely to find a 78xx on a dead circuit board than a LM317T.
compassrose (author)  gmoon9 years ago
Aha... very helpful... I looked all over the place but that was exactly the kind of link I needed and failed to find. Thanks, gmoon! That should get me going right quick.
Sure, no biggie. I like the regulator better than the zener approach (it's more stable, since the input voltage will fluctuate, and the zener needs a load resistor selected for the current draw..) There's another low-tech approach--many car cigarette-lighter plug-in power supplies use a standard diode, which has ~0.6 voltage drop. So they string together several diodes to reach the target voltage. But that's about 10 diodes for 12V. :P And unregulated, to boot. If a 'cycle is anything like a car, the DC varies from 14+V to below 9V, depending on the state of battery charge, and if the starter is cranking ...
compassrose (author)  gmoon9 years ago
Yeah, the regulation is definitely a big issue. There are lows when starting, spikes when shutting down, etc. Flip on the heated grips or vest and the voltage is certainly effected. My hope is to leave the rig in all the time if I'm on the road for a few days and not have to worry.. a regulator is certainly the way to go there. Thanks again!
just to clarify, if you used a zener diode, i dont think youd need any filter capacitors, seeing as any spikes would be grounded by the zener.
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