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| evildragon |
Posted: November 03, 2009 09:12 pm
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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? -------------------- Brandon
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| Jimthecopierwrench |
Posted: November 03, 2009 09:29 pm
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![]() Moderator Group: Moderators Posts: 7,767 Member No.: 561 Joined: January 10, 2004 |
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. -------------------- Stranded in time. Surrounded by evil. Low on gas...
"On a long enough time line - the survival rate for everyone drops to zero." When the going gets wierd, the wierd turn pro. Hunter S. Thompson. |
| VenomBallistics |
Posted: November 03, 2009 09:51 pm
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works great, voltage dividers have done this since man discovered that he could electrocute himself
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. |
| Colt45 |
Posted: November 03, 2009 09:53 pm
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gotta be piss low current, or it's uber inefficient.
-------------------- >]:::|-
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| Roccivic |
Posted: November 03, 2009 10:02 pm
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Moderator Group: Moderators Posts: 7,258 Member No.: 7,882 Joined: January 19, 2007 |
Stepping 120V to 12V would mean 10% efficiency
No isolation either... A faster way to die if you are playing with mains. |
| Jimthecopierwrench |
Posted: November 03, 2009 10:09 pm
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![]() Moderator Group: Moderators Posts: 7,767 Member No.: 561 Joined: January 10, 2004 |
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. -------------------- Stranded in time. Surrounded by evil. Low on gas...
"On a long enough time line - the survival rate for everyone drops to zero." When the going gets wierd, the wierd turn pro. Hunter S. Thompson. |
| tekwiz |
Posted: November 03, 2009 10:26 pm
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Huh? Say what? -------------------- Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring. To him, mere toys of the moment, to be overturned at the flick of a finger. Fortuna favet fortibus. |
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| Jimthecopierwrench |
Posted: November 03, 2009 10:38 pm
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![]() Moderator Group: Moderators Posts: 7,767 Member No.: 561 Joined: January 10, 2004 |
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.
-------------------- Stranded in time. Surrounded by evil. Low on gas...
"On a long enough time line - the survival rate for everyone drops to zero." When the going gets wierd, the wierd turn pro. Hunter S. Thompson. |
| CWB |
Posted: November 04, 2009 12:15 am
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![]() Forum Addict ++ ![]() ![]() ![]() ![]() ![]() ![]() ![]() Group: Spamminator Taskforce Posts: 12,937 Member No.: 15,154 Joined: May 15, 2008 |
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 ? |
| Sch3mat1c |
Posted: November 04, 2009 12:16 am
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![]() Forum Addict ++ Group: Moderators Posts: 14,967 Member No.: 73 Joined: July 24, 2002 |
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 -------------------- Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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| Nothing40 |
Posted: November 04, 2009 01:02 am
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-------------------- "we need an e-kick-in-the-nuts button" -Colt45
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