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 Post subject: 12 volt to 6 volt dropping resistor
PostPosted: Apr Sun 14, 2013 9:58 pm 
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Joined: Jan Thu 03, 2013 5:17 pm
Posts: 111
Hello
I've bought a Philco 6 volt under dash mount radio for my hot rod which is 12 volt. I know you can buy dropping resistors for about 20 bucks but does anybody have any idea of what ohms they are and what wattage they should be? If I knew that I could probably cobble something together for a few bucks.
Thanx


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Sun 14, 2013 10:41 pm 
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Location: Livermore, CA
Hi

Need to know current drawn by your radio? If the radio draws 6 amps and you need to drop 6 volts resistance needs to be 1 ohm @ greater than 36 watts.

Voltage dropped divided by current gives resistance. Voltage times current gives wattage.

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Sun 14, 2013 11:32 pm 
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Might be easier to find a regulator that can handle the current. Unless the radio draws full current all the time a dropping resistor will drop less voltage as the current drops and depending on the radio may damage it.


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Sun 14, 2013 11:51 pm 
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Yeah, I agree with the regulator idea.

What is the maximum current drain for the radio?

An alternative would be to buy a second identical radio and wire them in series. Then you could have stereo.
[Boooo, hiss. Back back, foul cur. Under the stairs.] :shock:

- Leigh

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Last edited by Leigh on Apr Mon 15, 2013 12:04 am, edited 2 times in total.

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Sun 14, 2013 11:57 pm 
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The slackest method & a very effective method that I have found is to place some appropriately rated rectifier diodes in series.

The voltage drop across these is constant and independant of current. There is also nothing wrong with regulator idea but you need to watch out for vibrator spikes if it has one.

Marc


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 12:08 am 
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Rectifiers in series may be the cleanest option.
Just be sure they have a proper heatsink. They will dissipate the same heat as a resistor.

- Leigh

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 12:35 am 
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Two LM338k for $4 for the pair , a heat sink , and 5 resistors , a couple of by pass caps and your all set , wire the regulators in parallel and put a .33 ohm limiting resistor in series with each one

http://datasheetcatalog.com/datasheets_ ... M338.shtml

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 12:46 am 
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Running two LM338's in parallel to increase the regulated current is not a good idea.

Doing it properly requires additional circuitry and complexity, as shown in the datasheet applications.

You can do it with a single LM338 and a pair of pass transistors, but load balancing is an issue.

I use the LM338 in one of my products, and did some testing on this issue during the design phase.

- Leigh

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 12:58 am 
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Leigh wrote:
Running two LM338's in parallel to increase the regulated current is not a good idea.

Doing it properly requires additional circuitry and complexity, as shown in the datasheet applications.

You can do it with a single LM338 and a pair of pass transistors, but load balancing is an issue.


Exactly, what it has a tendency to do is shift all the load to chip with the lower regulation point (just from part-to-part variations). Which usually burns that one up, followed shortly by the other one burning itself up.

Brett


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 1:00 am 
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How about wiring the tubes in series pairs by current so they use 12 volts, and use 12 volt pilot lamps, then you just need to drop the voltage for the vibrator circuit.
Don

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 1:13 am 
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Location: Santa Rosa, CA
Using diodes in series is slightly better than using a resistor, however diodes do not have a constant voltage drop. Vf varies with applied current and with temperature. Here is a typical husky diode you might use, the 1N1184:
Image

To do the drop with diodes, you will need:

1. At least 6 diodes in series

2. If you use isolating washers (mica) under each diode, you could put them on one heatsink, other wise you need separate sinks for each diode.

3. As the radio draws more current, the diodes will heat up. The Vf drops and the current supplied to the radio increases, which may not be desirable.

The voltage regulator approach is best, though use only one regulator. A regulator chip with an external series transistor, like a 2N3771, is a useful approach. The transistor still has to dissipate about 30-some watts, so it needs a big heatsink. Paralleled regulators can be a disaster. They aren't resistors.

Rich


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 2:37 am 
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BikenSwim wrote:
How about wiring the tubes in series pairs by current so they use 12 volts, and use 12 volt pilot lamps, then you just need to drop the voltage for the vibrator circuit.
Don



This is the approach a lot of people have used, also changing them all to their 12 volt equivalents is even simpler and requires no rewiring of the filament circuit. Then you can more easily come up with a way to drop voltage to the vibrator, since most of the current is used by the tube filaments. It shouldn't be a problem to find a 12 volt vibrator transformer and 12 volt vibrator, then you would actually end up with a 12 volt radio.

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 3:13 am 
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Location: Lexington, KY USA
The resistor solution is not "good", but it works.

Plenty of radios were "converted" back in the 50s using big ole resistors.

Car radios are already designed to work over a range of input voltages, as the source goes from a soggy battery to the finishing voltage of a battery that is fully charged with the generator spinning.

Using a dropping resistor does make this worse. If you get it wrong, you may damage the radio, or melt a few wires. But it's cheap.

It is best if you choose a resistor value to match your radio's actual current draw. This may or may not be the value listed on any documentation. Better to measure it.

It might be a good thing to add a big capacitor across the radio's 6V input terminals. But it will work without one.

If you want to spend money, they make nice little boxes with regulated 6V outputs.

And paralleled voltage regulator ICs can work very well. Or very badly. Depends on the circuit and the application. Regulator manufacturer's app notes do cover this use.

So if you want to go the cheap route, get your current drain measured, apply ohm's law, and find a resistor. You might cheat a bit and make the resistor an adjustable one.

You can report back here with the current drain info; someone will find a suitable resistor.

Ted


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 1:38 pm 
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Location: Poplar Bluff, MO USA
I used to buy "Globar" 12 to 6 volt dropping resistors at the local jobber. The better of them
had a heat shield around the actual resistor. If you go the resistor route mount it somewhere
under the hood away from anything because they run really hot.

Here's one on "the bay" looks well made
http://www.ebay.com/itm/1-Ohm-Voltage-R ... 19&vxp=mtr

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 3:01 pm 
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Isn't there a device made to drop 12 volts to 6 volts specifically for this type of application?

Thinking it was so someone could install a modern alternator or engine in their 6 volt vehicle and still run the 6 volt accessories such as gauges, radio ETC...


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 3:11 pm 
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Tube Radio wrote:
Isn't there a device made to drop 12 volts to 6 volts specifically for this type of application?

Thinking it was so someone could install a modern alternator or engine in their 6 volt vehicle and still run the 6 volt accessories such as gauges, radio ETC...


Most people were happy to be able to use 12 volt stereo transistor radio-tape-cd players etc. The previous post showed resistors used for the blower. There are adapters for the gauges, and the lamps and blinker are all replaced.
Don

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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 3:37 pm 
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Location: Shelton, WA
If you do not want to fuss and bother. Check this out:

http://www.ebay.com/itm/121094863369?ss ... 1555.l2649

billn


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 3:47 pm 
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Just found this

http://www.studebaker-info.org/tech/6-12V/6-12-6.html Read farther down the page. Shows a circuit with a regulator and pass transistor wirth the thing good for at least 10 amps.

Should work fine for your radio.


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 4:35 pm 
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If you can locate an RCA tube manual you can look up the filament currents; that will enable you to wire tubes with like filament currents in series. If there are an odd number of tubes use a resistor to drop six volts for the odd tube (R=6/If). Measure the vibrator current when on six volts and put a dropping resistor in series (R=6/Iv). Now you have a 12 volt radio.


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 Post subject: Re: 12 volt to 6 volt dropping resistor
PostPosted: Apr Mon 15, 2013 4:41 pm 
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Location: Santa Rosa, CA
Quote:
If you do not want to fuss and bother. Check this out:

http://www.ebay.com/itm/121094863369?ss ... 1555.l2649

billn


This looks like a "boost" converter. 6 volts IN / 12 volts OUT. Not what was originally discussed.

Rich

PS: The most efficient route for 12V to 6V conversion would be a switch-mode "buck" converter. But this has some challenges:

1. If you have never built one, it can be a challenge to design and stabilize.

2. It will be "chopping" the DC at 20 KHz or more, so there is the possibility of RFI or audio noise.

http://www.ti.com/product/lmz12008


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