I have a need for a 12 volt to 6 volt dc dropping resistor, so that I can operate a 6 volt auto radio from a 12 volt source, drawing 2 to 4 amps.
Need a source for this product; any ideas out there?
Allan

This is a risky way to do the job. At 2 amps, you need a 3 ohm resistor, probably 25 Watts. If you draw 4 amps, the resistor needs to be 1.5 ohms, probably 50 watts. If you use only the 1.5 ohm resistor, the voltage supplied to the radio will go up significantly when the load is lighter. A better approach is to use a voltage regulator IC, like an LM350 on a heatsink.
Rich

:I have a need for a 12 volt to 6 volt dc dropping resistor, so that I can operate a 6 volt auto radio from a 12 volt source, drawing 2 to 4 amps.
:Need a source for this product; any ideas out there?
:Allan
:

Yep, active regulation is the way to go. A dropping resistor wastes power and is a fire hazard, to boot.

Multiple decades ago, when there were still a lot of 6V auto radios floating around, it was common practice for skinflints to recycle old but good-operating 6V car radios for use in their newer, but radio-less, 12V vehicles by tapping the 12V car battery at the midpoint. This was easier to do than it sounds, since most batteries back then had exposed lead-metal cell connector bars on top of the battery. The enterprising cheapskate could drive a self-tapping screw into the middle con-bar and run a 14 gauge wire (with inline fuse, of course) back to the radio. The only drawback to this was that the hot wire didn't run through the ignition switch, so you could run your battery down if you forgot to turn off the radio after you shut off the engine. (This could have been viewed as a "feature" rather than a liability if one were on a date with a woman who was wise to the "we're out of gas" line.)

Most car batteries are now cased in molded thermoplastic, not hard rubber and tar, and the con-bars have been thoughtlessly covered up. I suspect it would be possible to access them if you know where to cut the hole; however doing so would surely void any warranty on the battery!

Another tack would be to convert the 6V set to 12V operation. This would take an entirely new set of 12V tubes, a new 12V vibrator and a fairly small dropping resistor to reduce the current into the B+ transformer (most of the current that the set draws is consumed by the heaters, so this resistor would be relatively low-power-dissipation).

If you are dead set on using the brute-force dropping resistor approach, operate the radio on a 6V battery (or 10 amp DC bench supply, NOT a 6V battery charger!) and take a current reading. This will tell you exactly what ohms/power rating your dropping resistor needs to be.

:Yep, active regulation is the way to go. A dropping resistor wastes power and is a fire hazard, to boot.
:
:Multiple decades ago, when there were still a lot of 6V auto radios floating around, it was common practice for skinflints to recycle old but good-operating 6V car radios for use in their newer, but radio-less, 12V vehicles by tapping the 12V car battery at the midpoint.

How about two 6 volt radios hooked up in series, for AM stereo? :>)

No, the regulator circuit is by far the easiest and best way.

I knew a guy that had a 6 volt accessory fan on his 12 volt '55 Chev wired that way, screw in the middle bar. He'd swap ends now and then to load the other side for a while.

Thanks to all of you who responded so quickly; I will go the regulator route with heat sink.
Thanks again, Allan

::Yep, active regulation is the way to go. A dropping resistor wastes power and is a fire hazard, to boot.
::
::Multiple decades ago, when there were still a lot of 6V auto radios floating around, it was common practice for skinflints to recycle old but good-operating 6V car radios for use in their newer, but radio-less, 12V vehicles by tapping the 12V car battery at the midpoint.
:
:How about two 6 volt radios hooked up in series, for AM stereo? :>)
:
:No, the regulator circuit is by far the easiest and best way.
:
:I knew a guy that had a 6 volt accessory fan on his 12 volt '55 Chev wired that way, screw in the middle bar. He'd swap ends now and then to load the other side for a while.
:
:

As Lt. Columbo is wont to say, "There's just one more thing..."

You might want to verify that your 6-volt radio is set up to run on a negative-ground vehicle. Virtually all vehicles nowadays are negative ground, but when the 6V battery was king, they were split about evenly between negative- and positive-ground arrangements. Some sets running off positive ground systems used the fact that the "hot" lead going into the set was at less DC potential than the chassis, so it could also be used directly to negatively bias the tube grids without having to play games with the B+ supply.

Not all sets were set up this way, and generally this design practice was abandoned after self-biasing (via cathode resistors) came into general use around 1940. But it's something to take into account if you can't get your radio to work on a negative-ground battery setup.

The negative/positive ground issue may also affect the vibrator type used, but my knowledge of exactly why that is so is deficient. I just recall that "back in the day", replacement vibrators were differentiated between the positive and negative ground radio types. May have had something to do with the way the vibrator switch contacts were configured, but I can't say for certain. If you are running a solid-state ersatz vibrator, it would definitely be a concern.

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Quote:
Yep, active regulation is the way to go. A dropping resistor wastes power and is a fire hazard, to boot.

Unfortunately, a regulator wastes just as much power and will generate just as much heat. That's why you will need a heatsink. You could use a switch-mode regulator, which is much more efficient, but much more complex and may generate a lot of RF noise in the radio. An "active" regulator is just a smart resistor.
RICH
*********************************

:As Lt. Columbo is wont to say, "There's just one more thing..."
:
:You might want to verify that your 6-volt radio is set up to run on a negative-ground vehicle. Virtually all vehicles nowadays are negative ground, but when the 6V battery was king, they were split about evenly between negative- and positive-ground arrangements. Some sets running off positive ground systems used the fact that the "hot" lead going into the set was at less DC potential than the chassis, so it could also be used directly to negatively bias the tube grids without having to play games with the B+ supply.
:
:Not all sets were set up this way, and generally this design practice was abandoned after self-biasing (via cathode resistors) came into general use around 1940. But it's something to take into account if you can't get your radio to work on a negative-ground battery setup.
:
:The negative/positive ground issue may also affect the vibrator type used, but my knowledge of exactly why that is so is deficient. I just recall that "back in the day", replacement vibrators were differentiated between the positive and negative ground radio types. May have had something to do with the way the vibrator switch contacts were configured, but I can't say for certain. If you are running a solid-state ersatz vibrator, it would definitely be a concern.
:

:********************************
Unfortunately, a regulator wastes just as much power and will generate just as much heat. That's why you will need a heatsink. You could use a switch-mode regulator, which is much more efficient, but much more complex and may generate a lot of RF noise in the radio. An "active" regulator is just a smart resistor.
==============================================
Techno-semantics. The inefficiency charge is true for a linear regulator, or a zener diode with a series current-limiting resistor. These regulators are inefficient (power-wasters). I consider these to be "passive regulators" since all they do is clamp the voltage and discard the excess power as heat. A switching regulator ("active regulator"), on the other hand, wastes far less power as it incrementally passes only the amount of power needed from the source to the load. These modern 3-terminal IC devices still require heat sinks, but they are much more efficient than the zener/resistor approach. Any RF artifacts generated by the switcher can be easily dealt with with bypass caps at the supply.

:********************************
:Quote:
:Yep, active regulation is the way to go. A dropping resistor wastes power and is a fire hazard, to boot.
:
:Unfortunately, a regulator wastes just as much power and will generate just as much heat. That's why you will need a heatsink. You could use a switch-mode regulator, which is much more efficient, but much more complex and may generate a lot of RF noise in the radio. An "active" regulator is just a smart resistor.
:RICH
:*********************************
:
:
:
:
::As Lt. Columbo is wont to say, "There's just one more thing..."
::
::You might want to verify that your 6-volt radio is set up to run on a negative-ground vehicle. Virtually all vehicles nowadays are negative ground, but when the 6V battery was king, they were split about evenly between negative- and positive-ground arrangements. Some sets running off positive ground systems used the fact that the "hot" lead going into the set was at less DC potential than the chassis, so it could also be used directly to negatively bias the tube grids without having to play games with the B+ supply.
::
::Not all sets were set up this way, and generally this design practice was abandoned after self-biasing (via cathode resistors) came into general use around 1940. But it's something to take into account if you can't get your radio to work on a negative-ground battery setup.
::
::The negative/positive ground issue may also affect the vibrator type used, but my knowledge of exactly why that is so is deficient. I just recall that "back in the day", replacement vibrators were differentiated between the positive and negative ground radio types. May have had something to do with the way the vibrator switch contacts were configured, but I can't say for certain. If you are running a solid-state ersatz vibrator, it would definitely be a concern.
::
:
:
You can also go the cheap way and use diodes, i.e. 1N5400, in series. They each drop 0.7V with very little to no heat and cost less than 10 cents each. I have been using this system for radios, wiper motors, fan motors, etc.

That's a good approach, too, for an application where the power supply voltage is reasonably stable (i.e, a car battery). Just be sure that the diodes can support the current needed. I think that 1H5400 diodes are good for 3 amps, but higher-current ones are probably not much more expensive. Nine silicon diodes in series would forward-drop 6.3 volts, which would be about right. Heat sinking will still be needed but since the power dissipation is equally spread over nine parts it should be fairly easy to accomplish.

****************
A diode string has to dissipate just as much power as a resistor. If you are dropping 6V @ 3 Amps, that's 18 watts, whether it heats up diodes or resistors.
A 1N5408 is rated at 3 amps, but you have to keep it on an ice cube to do that. Axial lead diodes are hard to heatsink. Better to use stud-mount diodes or sections of a molded rectifier bridge.

As to making a switch-mode supply with acceptable RFI emission, it usually takes more than a few caps. Good shielding; proper snubbing or waveshaping at the switching transistor; maybe an L-C filter on the input side. Not a good first project for someone who is starting out with dropping resistors.

Rich
******************************

:That's a good approach, too, for an application where the power supply voltage is reasonably stable (i.e, a car battery). Just be sure that the diodes can support the current needed. I think that 1H5400 diodes are good for 3 amps, but higher-current ones are probably not much more expensive. Nine silicon diodes in series would forward-drop 6.3 volts, which would be about right. Heat sinking will still be needed but since the power dissipation is equally spread over nine parts it should be fairly easy to accomplish.
:
:

Or perhaps one could experiment with a series power transistor(s)/zener diode regulator.

Clifton

:****************
:A diode string has to dissipate just as much power as a resistor. If you are dropping 6V @ 3 Amps, that's 18 watts, whether it heats up diodes or resistors.
:A 1N5408 is rated at 3 amps, but you have to keep it on an ice cube to do that. Axial lead diodes are hard to heatsink. Better to use stud-mount diodes or sections of a molded rectifier bridge.
:
:As to making a switch-mode supply with acceptable RFI emission, it usually takes more than a few caps. Good shielding; proper snubbing or waveshaping at the switching transistor; maybe an L-C filter on the input side. Not a good first project for someone who is starting out with dropping resistors.
:
:Rich
:******************************
:
::That's a good approach, too, for an application where the power supply voltage is reasonably stable (i.e, a car battery). Just be sure that the diodes can support the current needed. I think that 1H5400 diodes are good for 3 amps, but higher-current ones are probably not much more expensive. Nine silicon diodes in series would forward-drop 6.3 volts, which would be about right. Heat sinking will still be needed but since the power dissipation is equally spread over nine parts it should be fairly easy to accomplish.
::
::
:
:

A diode string with heat sinks

:Or perhaps one could experiment with a series power transistor(s)/zener diode regulator.
:
:Clifton

OR ...
Use a cheap inverter to convert the 12 VDC to 120 VAC. Then just use a tranformer and diodes to create 6 VDC.

The inverter introduces a lot of noise, though, so the 6 volt ppower supply will need some big filter caps.
I starting to think a 6 volt battery could be kept in the trunk. :>)

Now that we have thoroughly discouraged you, you might want to try a step-down regulator.

http://www.amazon.com/DROK-4-5-30V-0-8-30V-Converter-Regulator/dp/B00C4QVTNU/ref=sr_1_4?ie=UTF8&qid=1393518709&sr=8-4&keywords=buck+regulator

the investment is pretty small, so may be wort trying. You might have to put it in a metal box to prevent interference and "hash" on your radio, but it might work!

Rich

:I have a need for a 12 volt to 6 volt dc dropping resistor, so that I can operate a 6 volt auto radio from a 12 volt source, drawing 2 to 4 amps.
:Need a source for this product; any ideas out there?
:Allan
: