What are the different ways to convert mains to low voltage? - Page 1
Forum section: electronics / beginners

author: lscw
author_rank: Contributor
timestamp: December 28, 2021, 02:51:53 am
content: Parameters:Input 110-240V ACA fixed output voltage (AC or DC) anywhere around 5-25V,Output current of <1AI know of:capacitive dropper,series resistor and zener diode,SWPStransformer.Are there any other ways?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2021, 05:10:40 am
content: Motor-generator.On a similar angle, I suppose a "piezo transformer" is a new type, though it manifests as just another circuit component (as its name suggests).Optoisolator, namely LEDs into PV (photovoltaic panels/stacks). Awful efficiency, but high isolation (can be delivered via fiber).Various RF methods, though those are arguably straddling the line between "transformer" (near field, and resonant if the coupling factor is low) or optics (far field; arguably a PV panel is a rectenna of sorts) so might already be included in the above.Tim

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 28, 2021, 05:56:02 am
content: any energy transformation... e.g. even just a resistor coupled with any device able to convert a thermal gradient in an electric potential difference... even hydraulic energy... gas pressure... etc... if you do not mind about efficiency

author: Terry Bites
author_rank: Super Contributor
timestamp: December 28, 2021, 11:19:04 am
content: Thyristor and Triac power control.https://learnabout-electronics.org/Semiconductors/thyristors_62.php

author: Ian.M
author_rank: Super Contributor
timestamp: December 28, 2021, 11:53:26 am
content: The better part of a century ago, when DC mains supplies were still in common use, it was not unusual to charge a Lead Acid Accumulator (battery) direct from the mains supply utilizing a light bulb as a dropper resistor to limit the current. Several accumulators could be wired in series for charging. Of course this was horribly dangerous by modern standards, as the batteries were mains 'live' during the charging process, with the added hazard of a wet acidic conductive film on all nearby surfaces from them 'gassing' vigorously, resulting in an acid mist, as they reached full charge.

author: CaptDon
author_rank: Super Contributor
timestamp: December 28, 2021, 02:16:44 pm
content: Use convicted guilty serial killer as a 'series dropping power resistor'. Recommended input voltage 1200vac, output will vary.

author: themadhippy
author_rank: Super Contributor
timestamp: December 28, 2021, 02:31:54 pm
content: Diodes ,around 300-350 1N4007's in series would be a good starting point

author: Microdoser
author_rank: Frequent Contributor
timestamp: December 28, 2021, 02:36:43 pm
content: Pretty much every way involves turning the AC into DC and regulating the DC to the voltage your project requires.There are many ways of doing this. Some more efficient than others.You could just put a single 400V diode across the mains, add a 400V cap, then use a hefty resistor of the right value to drop the voltage to the one you want. Bingo, you have low voltage and could easily make that supply 1A.Of course, that would be really inefficient, really high ripple, and not as safe as you should aim for.TI has a power supply designer, you input your required parameters, and it spits out a design. Uses TI chips, of course, but can show you some concepts and lets you pay with the components to see how changing them changes the effifciency and ripple etc.Here is an example design that fits your stated needs, 240V50Hz -> 12V, 2A, 91% efficient.https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=AB1500BCB90C0220

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2021, 06:25:04 pm
content: Quote from: Ian.M on December 28, 2021, 11:53:26 amThink you're missing a step here! They used something slightly fancier than light bulbs:http://www.r-type.org/pdfs/tungar.pdfThermionic gas-discharge tube, having a fairly low voltage drop considering (~8V, with max. 14V turn-on being typical, by the looks of it), and ratings up to a few tens of amperes. The full output ripple sucks, but lead-acid doesn't mind too much all the same.A transformer affords somewhat better regulation than direct line operation (which could indeed be limited by a series light bulb, if you don't mind the horrible terrible efficiency?), but still not good enough to avoid overcharging (and yes, lots of hydrogen and acid mist!).Tim

author: Ian.M
author_rank: Super Contributor
timestamp: December 28, 2021, 06:37:56 pm
content: I did sayDCmains!Transformers require AC mains, so if you were on DC mains and needed a low voltage supply there weren't many options other than a rotary converter, or as I said, charging a series string of batteries that would later be used individually or in parallel as a low voltage supply.DC mains in the UK 'hung on' in certain areas into the '70sSeehttps://www.vintage-radio.net/forum/showthread.php?t=41380

author: radiolistener
author_rank: Super Contributor
timestamp: December 28, 2021, 06:38:42 pm
content: Quote from: T3sl4co1l on December 28, 2021, 06:25:04 pmGood choice for charging batteries used in telephone, telegraph, clock and alarm systems.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2021, 06:41:20 pm
content: Ah, right. DC went away very rapidly by the turn of the last century, AFAIK, here at least. (Skimming the thread, it doesn't look like much of a statistical sample -- a number of isolated cases, hardly exhaustive, so, no idea how much it actually was..?)Tim

author: tpowell1830
author_rank: Frequent Contributor
timestamp: December 28, 2021, 08:40:13 pm
content: For less than 1A, one could find or purchase a small inductive AC motor and mechanically couple to a small DC motor/generator. This would fit your requirements if the generator output 5-25 V.Other methods would be to use a running stream and a paddle wheel to drive the generator, or even a windmill. Solar panels? This would not qualify, since it is not using mains, sorry just ignore.EDIT: For fun one could use nichrome heating elements to make a boiler and a turbine to drive the generator instead of an AC inductive motor.

author: CaptDon
author_rank: Super Contributor
timestamp: December 28, 2021, 09:36:18 pm
content: Those Tungar bulbs were not used with D.C. mains because they had a low voltage somewhat high current filament to vaporize the mercury. Sort of like an 866 rectifier. I had a Tungar charger and indeed it ran on A.C. and included a filament and low voltage transformer. I think the transformer has some kind of current limiting magnetic shunt for the low voltage winding used as the charge winding because the unit would charge 6 volt or 12 volt batteries and could withstand short circuits for short periods without damage to the Tungar bulb. Interesting to note, it had no power switch or range switch and the Tungar bulb when being used properly needed some warm-up time for the mercury.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2021, 12:51:05 am
content: If you had the mercury variety, anyway; they made them in argon too, and I suppose other things would work too (xenon and mercury were common in thyratrons, to a similar end but with grid control; not sure about plain old B+ rectifiers, other than mercury).For DC mains, I'm not sure what OTHER than a dropping lamp they would've used! Could only be an MG set, or perhaps an early vibrator supply (which got more common into the vacuum tube days, e.g. for automotive radios)?Tim

author: CaptDon
author_rank: Super Contributor
timestamp: December 29, 2021, 03:34:06 am
content: Vibrator power supply, great memories. A friend of mine had a 55 Chevy and if you turned on the radio and after about 30 seconds it didn't play you would kick the floorboard over by the glovebox to help the vibrator start. Then the radio would instantly play!!! In between stations you could hear the sparking noise of the vibrator coupled into the RF/IF. A distinctive harsh thrashy buzzing interference!!Cheers!!

author: lscw
author_rank: Contributor
timestamp: December 31, 2021, 04:28:29 am
content: Quote from: Terry Bites on December 28, 2021, 11:19:04 amGood answerQuote from: Microdoser on December 28, 2021, 02:36:43 pmOf course it needs an account!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2021, 07:55:48 am
content: 6 or 12V.Tim

author: Circlotron
author_rank: Super Contributor
timestamp: December 31, 2021, 11:10:57 am
content: Quote from: CaptDon on December 29, 2021, 03:34:06 amQuite a bit of engineering went into a good vibrator power supply. Generally they had a high voltage cap across the transformer secondary calculated to resonate with the tranny inductance to achieve zero voltage switching on the primary side contacts! Pretty cool stuff for those days.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2021, 11:19:46 am
content: The motor generator way is not just for low power. It was also used for higher power ( e.g. kW range) with a multi phase generator one can get stable DC without huge filter caps.

author: lscw
author_rank: Contributor
timestamp: December 31, 2021, 01:26:35 pm
content: Quote from: T3sl4co1l on December 31, 2021, 07:55:48 amSo why the need for a vibrator power supply?

author: Ian.M
author_rank: Super Contributor
timestamp: December 31, 2021, 01:52:09 pm
content: Although low power RF, IF and AF tube amplifier stages could be (and later were*) designed to operate with a 'HT' as low as 12V, it was impossible to get enough audio power out of a tube output stage without significantly higher HT. Once you had it, it made more sense to also power the rest of the set's plate circuits from the vibrator's boosted HT.*https://www.angelfire.com/electronic/funwithtubes/12v_tubes.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2021, 04:07:57 pm
content: Quote from: lscw on December 31, 2021, 01:26:35 pmOh, you mean internal voltages? Usual tube stuff -- 300V or the like. Not sure how much battery-operated (as in low voltage, portable or hearing aid) types made it into automotive radios, some of those ran on 45 or 90V batteries for example. Low power output though. Or, as mentioned above -- some later oddball types designed for 12V-only operation... and still quite low power output, heh.Tim

author: CaptDon
author_rank: Super Contributor
timestamp: December 31, 2021, 07:59:54 pm
content: My 1948 truck was 6vdc with positive ground. It had a 6vdc radio with a vibrator power supply. It used the very common 0Z4 gas full wave rectifier tube. It was an absolute physical monster of a radio. My friends 55 Chevy had a 2 piece radio. The audio amplifier and vibrator power supply and speaker were near the glove box. The RF section was centered in the dash. I am not sure what tubes were used in the '55, I just remember the 'kick start vibrator' which was made by CDE and still available new into the late 70's. Do you remember those old radios having two Civil Defense emblems on the tuning dial. one at the low end and one near the high end for CONELRAD reception? When I worked in radio broadcast our old A.M. RCA BTA-5H was equipped with the CONELRAD oscillator and frequency changeover gear. Our 4-tower array could not make the frequency shift so CONELRAD forced us to go 'single tower' mode with separate relay switched tower tuning in the dog house.