Simple electronic transformer; increase/change 230vac-12vac, to 230vac-24vac? - Page 1
Forum section: electronics / beginners

author: geir
author_rank: Newbie
timestamp: November 23, 2014, 01:49:06 am
content: I bought a couple of cheap ass chinese 220vac-12vac, 160w electronic transformers.I believe they work by some sort of "self-oscillating half bridge" scheme, but I have no idea really.I actually want 24vac output for an soldering iron project, so cunningly I thought; "if I simply double the the windings of the output transformer secondary, from a 75:8 turn ratio, to a 75:16 ratio, I should get24vacoutput instead of the measly12vac"The toroid input X vac from the circuit, to the 12vac output, is done by sometoroid transformer, where the current from the circuit enters a fine gage copper wire that is wound 75 turns (the primary, if I've got it right).All unmodified, the output with no load is ~<2vac at a ~50khz freq.With a 12v load (a 12vac halogen bulb) I get about 12.4vac and about 90khz freq.So far, so good.On top of the primary wire, are wound 8 turns of a larger gaged wire in a heat shrink (it's actually multistranded wires inside the heat shrink).I assume these are the secondary windings.So I replace these (8 ) secondary wires with a similar thick (single stranded) 2.5mm copper wire, using 16 turns (instead of the original 8 ).I then, probably naively, expect to get double the out voltage; 24vac.However, the voltage drop I get when hooking up 2x12vac halogen bulbs in series are only ~15-16vac, not the expected ~24vac voltage drop.What gives? Am I thinking all wrong? Have I ignored some obvious law of nature, punching big holes in my brilliant idea?What is wrong?Any insights or suggestions to the clueless (me) would be very welcome!The pseudo-schematic of the cheapo electronic transformer is here:Thankyouverymuch!

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2014, 01:10:26 pm
content: If you're expecting the same current, at double the voltage then you're overloading it.

author: bobcat
author_rank: Regular Contributor
timestamp: November 23, 2014, 02:13:56 pm
content: With the frequencies that you are describing, it sounds like a switching regulator. Changing the windings on the transformer is not the way to go.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 23, 2014, 02:30:55 pm
content: Please provide a link to the gadget you bought. You probably can't post an image as a newbie, so your pseudo-schematic didn't work.As bobcat said, it sounds like you are describing a switch-mode power supply (SMPS). Where the mains power is rectified to a high-voltage DC, and then a high-frequency oscillator drives a high-frequency step-down transformer. This is done because high-frequency transformers are smaller, lighter, and cheaper than the equivalent "heavy iron" linear transformers which operate at mains frequency (50 or 60Hz)Re-wiring a SMPS transformer is an extraordinarily aggressive and speculative thing. Most of us with years of experience would NOT attempt to do such a thing. Because the primary has a rather high voltage on it, this is NOT recommended! Doing such modifications to mains-connected power supplies is NOT recommended unless you have specific experience designing and debugging SMPS circuits. Many of us with years or even decades of experience avoid SMPS circuits. They tend to be rather fiddly and rarely have documentation available. And they are rather a "house of cards" where if one thing fails, it catastrophically takes out many other parts of the circuit.What you are forgetting is that most components (especially transformers) have design limits. Transformers have a maximum power that the core of a specific size will handle. If you look at Ohm's Law, you will see that for equal power (Watts), if you double the voltage, you cut the current in half.

author: geir
author_rank: Newbie
timestamp: November 23, 2014, 05:45:42 pm
content: Hi, thank you for replies!Gadget at ebay:http://www.ebay.com/itm/350629903027Image of the "guts":https://www.digitalinferno.com/wiki/attach/GizmosAndGearAndStuff/220vac-12vac-transformer-top.pngI realize I would only get half the current at 24VAC (with a different turn ratio).However I only need at most 50-60W (~2-2.5A) at 24VAC, and the thing is rated 160W at 12VAC, so I thought I would be in the clear.(And I was under the impression that an "electronic transformer" was a different beast than a switching transformer.)Ok, if upping the number of turns on the transformer-toroid thing is a big no-no, would it be worth a try to series connect 2 of these 12VAC gizmos to get a total of 24VAC?Thanks again!Geir

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2014, 06:27:46 pm
content: Assuming it's a very simple electronic transformer, such as the one described in the application noted linked below. It should be fairly straightforward to modify it to give 24VAC, just by rewinding the secondary. The reason why this may not be behaving as expected, could be because the coupling isn't as tight, due to the way the new turns were added. Try using slightly thinner wire and a extra turns.http://www.st.com/web/en/resource/technical/document/application_note/CD00003902.pdf

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 23, 2014, 06:51:18 pm
content: The term "electronic transformer" is artificial marketing-speak made up by non-technical copy-writers who don't know a transformer from their left elbow.I can't open that link to your photo, but I would be quite certain that it is simply a switch-mode power supply.What is your obsession with using 12V power supplies for a 24V load?Why not simply use a 24V power supply? I don't get it.

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2014, 07:19:21 pm
content: Here's the corrected link:Yes it's a type of switched mode power supply but it shouldn't be confused with a typical SMPS. The output not regulated and rectified so it's gives high frequency AC rather than DC,

author: geir
author_rank: Newbie
timestamp: November 23, 2014, 07:50:33 pm
content: Ok, thanks!!As long as I know itmaywork, I'll try rewinding the bugger tighter, with a few more turns!Would anyone care to explain roughly how the circuit works (the app-note was a little short on the bare basics)?Or perhaps someone has a pointer to some resource for this "classic half bridge" design?As for my obsession with 12VAC; 24VAC transformers seems to be very unusal in scandinavia, at least I've been unable to find anything with a sane pricetag.And, heavy things = insane freight + add 25% VAT to all (including shipping), and suddenly light weight moddable 12VAC "electronic transformers" become very attractiveThanks again, I appreciate it!Geir

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 23, 2014, 08:16:58 pm
content: I found many 24V switch-mode power supplies on eBay.For example here is a seller offering sizes from 24W (2A) up to 360W (15A)http://www.ebay.com/itm/12V-DC-24W-60W-102W-120W-150W-180W-240W-360W-Regulated-Switching-Power-Supply-/271641035431?pt=US_Lighting_Parts_and_Accessories&var=&hash=item3f3f1142a7

author: geir
author_rank: Newbie
timestamp: November 23, 2014, 08:37:39 pm
content: Sorry, I don't understand. Those are DC power supplies.Spesifically, I'm trying to gather the required parts for the "Open soldering driver" project:http://dangerousprototypes.com/forum/viewtopic.php?f=19&t=3475They make a point of using 24V-AC, not DC.Am I misunderstanding something? Are DC and AC powersupplies interchangeable?Thanks!

author: johansen
author_rank: Super Contributor
timestamp: November 23, 2014, 08:49:34 pm
content: its "160 watt rating" is probably disingenuous, so by doubling the turns and running two bulbs in series, it can't turn the lamp on.. cold resistance and all that.what voltage do you get when its unloaded?cutting out half the turns, can it still produce 12vac at 90 khz and the usual power? or did something change.could you light both lamps on 12 volts? electrically it will be the same as double the turns and two lamps in series.

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2014, 08:55:51 pm
content: A soldering iron's heating element is just a resistor do it doesn't make any difference whether AC or DC is used.Here's more information on those electronic transformers.http://webstaff.kmutt.ac.th/~kamon.jir/Documents/ISCAS03_kamon.pdfhttp://electronix.org.ru/datasheet/Motorola/mos/ar609rev0.pdfhttp://www.irf.com/technical-info/appnotes/an-1069.pdfMy advice is don't bother: use a 24V power supply, whether it's DC or AC, to power the soldering iron.

author: LukeW
author_rank: Frequent Contributor
timestamp: November 23, 2014, 09:03:19 pm
content: I can understand why those little inverters intended for running 12V downlights are appealing to newbies as power supplies... they are produced on a large scale and are very cheap. But in practice they're not really that useful for any other application.

author: geir
author_rank: Newbie
timestamp: November 23, 2014, 10:34:10 pm
content: Quote from: johansen on November 23, 2014, 08:49:34 pmYes, you're probably right. For starters, the box says 160W, but the printing on the pcb actually reads "120 watt"But at $3-4 I wasn't really expecting a whole lot.QuoteWhen nothing is connected, the multimeter seems to flutter around ~2V, and ~50kHz.QuoteI don't know, didn't try that.QuoteWell, one by one they were fine. In series yes, kinda... I think. However, each bulb only barely lit up.As far as I can remember I measured ~6V across both bulbs, but I'm getting confused now. Perhaps it was 6V across each bulb... I didn't try to light 'em in parallel.After adding the extra 8 turns they lit up just fine (in series), but I measured only 15-16V (and there was some slight flicker).Quote from: Hero999 on November 23, 2014, 08:55:51 pmHrmph... I feel I'm regressingI get that the heating element can take either DC or AC, but if acircuitis designed for AC , I cannot just plug in DC... can I?And I assume converting a curcuit from AC to DC is not (neccessarily) trivial?Anyway, thanks for all the input and the links!Useful or not, I like the fact that these cheapo transformers has (relatively) few components.Perhaps it would be more useful to try figuring out how these suckers work, rather than actually using them for anything.Thanks again!Geir

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2014, 10:41:38 pm
content: Quote from: geir on November 23, 2014, 10:34:10 pmIf the circuit is designed to switch power line frequency AC, i.e. 50 to 60Hz, then it won't work with an electronic transformer which produces 50kHz.

author: geir
author_rank: Newbie
timestamp: November 23, 2014, 11:06:15 pm
content: Ah... yes, bugger!That makes sense, unfortunately.Thanks!

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 24, 2014, 12:17:57 am
content: Quote from: geir on November 23, 2014, 08:37:39 pmThere is no particular advantage to concealing the details of the project until halfway through the discussion.QuoteNo, It says....QuoteFurthermore, your "electronic transformer" probably puts out high-frequency (ultrasonic) AC which is NOT what the "AC" input of the circuit is expecting.