Resistors Over Transformer

I just thought of something.

Wouldn't a network of resistors take the job of a transformer and step voltage down?

Or am I misunderstanding ohms law?

Sure, place a pot across a given voltage and use the wiper to derive any voltage between the rails, but keep in mind how much power that pot has to dissipate whether the tap is floating or loaded. If you need anything above milliampres then the resistors have to be big and they're burning up a lot of the supply as waste heat. To get tens of amps at a tap at 1/2 the mains voltage you'll be burning up KiloWatts just to get there.

It should be noted that this method was (is) acceptable for biasing tube elements, such as focus and grids in CRTs where very little current was required and the total divider might run 50-100 MOhm between the 'ends'. But for supplying low voltage electronics in general? No way.

Some very old AC/DC radios did use dividers on the input. I guess efficiency wasn't on peoples minds then. Nor was safety. Running heaters (and sometimes lamps) in series string across the mains to avoid a transformer wasn't uncommon either.

works great, voltage dividers have done this since man discovered that he could electrocute himself :P

now the issue with it is that it is not a one size fits all solution.
if you need substantial current to go with that order of lower voltage, its a transfos job since most resistors as we know them will smoke above their rated .25 watts.

gotta be piss low current, or it's uber inefficient.

Stepping 120V to 12V would mean 10% efficiency :P

No isolation either... A faster way to die if you are playing with mains.

It's 100 percent inefficient, no? Whatever you want out of the divider has to burned up again across the supply.

Note that the example of series heaters on the other hand is 100 percent efficient (outside the tubes themselves) as all of the input is directly being converted to the job at hand.

I wonder if there were any layouts ever built with the doubler/B+ divider pulled directly from a rectified (indirect heater) series, with the cathodes pulled from the bottom end of a lamp, and grid from just above? Or if such a layout would have worked.

Huh? Say what? :huh:

Yeah, something that would need to be drawn. I can picture it in my head but it doesn't translate well to text. Might scribble it later.

i think i know what he is talking about ...
sounds like something that was used in "el cheapo" radios .
but you get right back to the isolation/shock hazard issues .

i got knocked on my butt when i was 6 by a transformerless AA5 .
the volume control ("D" shaft) knob had split out .
sooo ... i grabbed it with a pair of pliers . i happened to be leaning across the gas range when i did it .
let me tell ya , i almost peed myself a little .

polarized plug ? what polarized plug ?

I've heard of some amps that heated the front end's 12AX7s with the 6L6 output tube's cathode current.

AA5 radios often have the diode connected to a tap on the rectifier's heater, to reduce peak surge current. (The tap is usually provided for lighting a small 6.3V panel light.)

Tim

http://www.users.qwest.net/~ptaylor/Audio/Bell6060B1.gif