12V to 6V resistor ? for elec gurus

[john walker's workshop]

is there any voltage reducer that can be rigged up, or purchased, for automotive use, that actually delivers a true 6V to a radio or gauge? if you measure the voltage at the supposed 6V end of a ceramic resistor, it's always a lot higher than 6V, like 8 or 9 sometimes, which would kill a 6V radio fairly soon. the more amperage the item draws, the more the resistor drops the voltage. a gauge or radio draws very little. so what can be done about this, if anything?

[david914]

I assume you're talking about a "vintage" radio since it's 6 volts. It can be done using a voltage regulator circuit (not like the one used for your alternator, but the same principle). You may want to check with those people that specialize in fitting older cars with more modern radios (Custom Autosound, I believe?). I think they have a pre-packaged solution that does what you want. A resistor, as you have found out, is not the best solution.

[thabaer]

Resistors will block current and you can make a small network called a voltage divider with two resistors, but you already described a drawback of that when you mentioned current draw from the radio changing the voltage.

What you really want is a regulator, like an LM317. You can go to a number of mfg. sites to get notes on how to configure simple circuits that will give the desired voltage. Then hit Digi-key or Newark to order a few parts.

[Matt77Euro27]

Try this.

It is a voltage reducer found at Napa. Echlin VT 6187. ~$9.

[john walker's workshop]

any more info on a regulator? what sites?

[thabaer]

www.digi-key.com
www.newark.com

NS usually provides lots of design notes:
http://www.national.com/pf/LM/LM317.html

I kept trying to remember to mention not forgetting to check current requirements, and of course I forgot

A good heatsink and some airflow around it, too...hanging on the back of the radio is usually a good place.

[john walker's workshop]

being that i'm an electronics dummy, i wouldn't know what to put together from those electronics sites. there's a lot to choose from. a tube radio would draw more than a transistor model, so what would i need to power each one?

[targa80]

Do you know what the current draw is for the 6 volt radio?

[rick-l]

SMPS buck regulator with synchronous rectifier? Just kidding...

[thabaer]

If you have a 15A or so meter, dig up a 6V source and bench test the radio. Lot's of tube radios were portable battery-powered units before transistor radios appeared on the scene, although they frequently used headphones to dodge the speaker level amp bullet. The heater filaments and plate voltage might not be a huge load for a unit targeted to a small car interior.

This approach may violate your space/weight/complexity/cost budget, but did you consider mounting an aux 6V generator+battery system ?

[thabaer]

Hey! Rick's on to something, or go scavenge an old diesel backup from a decommissioned computer room.

[fintstone]

http://www.eaelec.com/12to6.htm

for under $10, this would probably be a good option if your device does not draw too much current.

[thabaer]

$10 would buy a good-sized handful of VRs, but is also probably less than the time has cost you investigating this solution.

It'd be the way to go if the schematics in this datasheet don't give you enough to go on:
http://cache.national.com/ds/LM/LM117.pdf

[john walker's workshop]

on my 49 cad convert, i was thinking of the auxillary 6V gen/reg system, but for old porsche and vw 12V conversions, i need something compact. those 6V regulator boards from eaelec.com look like the ticket. they must adjust automatically to keep a constant 6V going. i assume they come with different ratings depending how much stuff you hook up to them. i'm thinking an old blaupunkt transistor radio on the porsche/vw conversions, and a tube radio and a couple of gauges on the old caddy.

[power]

I would try to find out how much current draw you need to supply, before choosing a regulator. The regulator will shut down (thermal protection) if it's overloaded. Power (Watts) = Volts * Amperes. If you know the Power your radio requires say e.g. 15Watts you can find out how much current you require to size your regulator. e.g. Amperes=15Watts/6Volts. You would need a regulator capable of delivering Minimun 2.5Amperes continuous.

Hopefully some familiar with the power requirements of the radio can chime in?

[Lorenfb]

Yes, the key is to determine the current requirement of the radio when operating
at 6 volts. So find a variable power supply or a 6 battery and determine the worst
case current demand (max volume). This value determines max load power of
your new voltage source.

Resistors make for good current sources and bad voltage sources. The best approach
is to use a series voltage regulator and not a parallel voltage regulator like a zener
diode because of the power needed by the radio. Some three terminal solid state
regulators (LM317, LM7812) will provide 1.5 to 2 amps max. If the radio requires
more current than this, they're not a good choice. You'll need a series pass transistor
to boost the current capabilities of the three terminal regulators.

As an example, if the radio requires 1 amp, then the regulator will pass 1 amp
and dissipate 8 watts (14 volts (battery) - 6 volts (radio) x 1 amp). As the current
increases above 2 amps, you'll have to use a switching regulator or your car will
have a nice built-in electric heater.

Bottomline: It's not easy to make a 6 volt converter especially if the radio requires
a lot of current.

[fintstone]

John
For your purpose, I would probaly use these:
http://www.jcwhitney.com/webapp/wcs/stores/servlet/ProductDisplay?productId=1569&catalogId=10101&langId=-1&storeId=10101
They have a fairly high current rating.

[power]

Thats the ticket! Probably DC to DC converter circuitry inside, I would think it perfect for John's use.

[dvkk]

I guess no one here has heard of Ohm's law. This is an extremely easy problem to solve, and it can be done with a simple resistor.

All you have to do is apply 6V to the accessory you want to run on 12V, and measure the amperage. Once you have that, you can calculate the resistance, R=V/I. Then get a power resistor of that same value and wire it up in series. The wattage of this resistor has to be greater than the product of VxI.

[power]

Unfortunately, Ohm's law is the problem with your solution. I believe the current of the radio is varying therefore in your solution, as the current rises the voltage drop across the resistor will also rise (V=I*R).

As Loren stated above, good current source but bad to use as Voltage source.

I think you need regulation, in this case a DC to DC converter of some kind would be the best soltion. A DC to DC converter circuit will maintain a constant voltage oputput, even with a variable voltage input and a varying load current. (within designed parameters). I think that's what you will find inside the JC Whitney Product above.