Car Batteries

I said exactly the same thing, did I not? Forward drop across a junction is NOT a regulator. Then again, nor is a series resistor, which has been used in many applications over the years to run 6 volt radios, instruments, etc on

12 volt cars. The resistors needed to be the right value for the job at hand or the voltage would be too high or too low.

I've done design work too. I designed and sold several lines of equipment for towing 12 volt trailers with 24 volt tow vehicles (in particular Toyota BJ Cruisers) including units to adapt 12 volt trailer brakes, using 12 volt Tekonsha brake controllers, on the 24 volt cruiser. Also built and drove an electric powered Fiat128, with a single contactor solid state series parallel switching mechanism of my own design.

I DO understand the issue at hand. Very well. I have also installed MANY 7805/LM350 replacement instrument regulators. I like the LM350 best - I used a small trim pot mounted directly on the terminals of the chip as a voltage devider to set the output voltage calibration and locked it in place with Hot Glue. The T03 case unit was mthe one I used most, fastened to a 5 square inch aluminum heat sink, which was screwed to the back of the instrument panel mounting brackets (steel) Never had one fail. These were installed to eliminate noise in radio equipment caused by the original equipment thermal/electric instrument regulators used in the seventies and eighties.

I still have the prototype of that unit floating around my office somewhere.

Would work just fine if you could find a source!

Who's worrying about efficiency? We are talking dissipation of something like 3 watts maximum using a resister, and something less than that with a linear IC. POWER efficiency might be better with a switcher, but by the time you adress the RF implications your total cost/benefit ratio would kinda outweigh the benefits.

To stay with the "era" of the car, he can go to NAPA and order a VT6188 or VT6187 Voltage reducer. It is a 1.5 ohm resistor for use on a 4 amp maximum load. The 6188 is a 6187 with a mounting bracket.

An 8 volt regulator would be more to the point, this would be pretty close to the charging voltage for a six volt system.

Later year instruments designed for 12 volts actually use a bimetallic strip as a regulator, it heats up, opens the juice to the gages, then cools off & re-establishes current flow, happens a few dozen times a second IIRC. This results in PW voltage control.

H.

You did say that. A zener has about the same fwd drop as any other silicon diode, but breaks down at zener voltage in the reverse direction. So, it can provide a fairly constant drop of Vz (could be

6 volts) if used that way. Power zeners are made, but not nearly as readily available as 400 mW and 1W units. The reason may be because it is very easy to "fake" a "power" zener using a small zener and a power NPN transistor. Zener goes from collector to base. The collector becomes the cathode (K), emitter becomes the anode.

Fwd junction drop in germaninum is less, not more, than silicon including Schottky.

Actually -- germanium diodes significantly *less* -- as is supported by the site which you point out below. Germanium diodes at about 200 mV forward drop, Silicon diodes at about 700 mV forward drop. (Actually, I had remembered germanium as being closer to 150 mV, but haven't checked one in years.

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The problem here is that a 12V automotive system goes through a wider voltage swing than a 6V one does, and if you are using a zener in series, (assuming that the load draws enough to get past the knee of the zener's curve) you will wind up with more voltage swing at the 6V load, by subtracting something approaching a constant from the 12-14V swing of the charging system and battery.

Enjoy, DoN.

Surely that will be a small effect compared with the noise from the ignition and the noise from the vibrator for the HT supply for the radio?

Not necessarily. The vibrator supply is well filtered and so is the ignition. And on the vehicles where I was using the regulators the radios were not vibrator units (they were solid state 12 volt units) and with proper snubber caps on the coil and good RF Suppression plug wires, the instrument regulators were the only thing causing problems. Except for the odd vehicle where we had to put suppression caps across blower moors and wiper motors.