Reverse-feeding stepdown transformer as stepup (12VAC to regulated ± 30V, 60mA) - Page 1
Forum section: electronics / projects

author: Nick_Denison
author_rank: Contributor
timestamp: May 21, 2025, 01:55:43 am
content: I'm seeking some advice as to what rating a stepdown XFMR (VA, voltage) would require to be a candidate for stepping 12VAC up to 25-26VAC/CT (i.e. 50VAC series) when reverse-fed (i.e. primary/sec swapped). This is to create a bipolar supply (typical 317/337 regulation) of ± 27-30V, with 60mA draw on each rail.I have tried this with a 6VA 48V (24-CT-24) split bobbin XFMR (Triad Magnetics FS48-125-C2), and the results were abysmal. Flipping the XFMR and feeding the series secondary as a primary yielded 2 X 23.3VAC, or 32V rectified, (no load). These rails collapse to <14V with even a 1K load across them. Obviously this XFMR is woefully underrated for what I'm trying to do. The 12VAC supply was a 10A rated supply; the 12VAC supply did not sag, nor did it have any DC on it.I now understand that XFMRs are not inherently bidirectional, and have extra windings to account for regulation. So it seems one must up the VA rating to antitipate lossy operation when reverse-feeding, and plan for the loss of voltage due to regulation compensation, the question is by how much? Are split bobbins notoriously bad for this? I've read toroids might offer better performance in this regard (?)A copmpany engineer suggested a 7VA toroid would hold up to my demands when I enquired by email, but I'm not so sure.This is for a guitar effects pedal with discrete op amps that run at 25-30V. Connecting to mains isn't an option for me (and effects pedals typically have wall adapters anyway), and the emissions testing required for a SMPS is also prohibitive at this stage (I may make these units for sale at some point). The plan is to utilise wall wart 12VAC adapters. There are other effects pedals that flip prim/sec sides to step up voltages in this manner (e.g. SD Twin Tube Classic, for tube plate voltages).I'm going to have to buy a bunch of different XFMRs to try out I guess, but any advice on ballpark ratings (and what I need to consider generally) would help me greatly in saving on getting redundant parts.TL;DR: Seeking advice on mimum XFMR specs for reverse feeding as a stepup (12VAC into secondary, now acting as primary) to obtain bipolar supply of ± 27-30V, 60mA draw per rail.Thank you very much!

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2025, 03:28:33 am
content: Quote from: Nick_Denison on May 21, 2025, 01:55:43 amA custom wound one would likely manage that, but it is a rare requirement, so is not likely to be off the shelf.Quote from: Nick_Denison on May 21, 2025, 01:55:43 amUsing part this way, is going to have a much higher winding resistance thancouldhave been used, were it custom wound.You are feeding a little over 10% of the rated voltage, into a high resistance winding.Whatever you do is also going to be 'soft', so the load regulation / sag will be poor. Is the load somewhat constant?You may need to add a clamp / shunt regulation / minimum load, or use higher voltage parts in the circuit.You already have one data point, perhaps try 30VA and carefully measure the load line on that.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2025, 03:43:55 am
content: Quote from: Nick_Denison on May 21, 2025, 01:55:43 amAn alternative here would be to rectify that 12VC and use a pre built DC-DC module, that already has the emissions testing done.You can get standard DC-DC parts rated at eg: 9-18 VDC, 6 W, 2 outputs, ±15 VDC.You mention 'discrete op amps that run at 25-30V' so one of those may do ?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2025, 06:04:11 am
content: A transformer is in principle bedirectional. Only the very small ones (like < 2 VA) tend to have assymmetry with more volume for the primary to acount for the magnetizing current, that with the small one is significant compared to the load current.The point is that when using the transformer for lower than the nominal voltage the winding resistance gets much more significant. One would need a higher power transformer to compensate for this. 7 VA would be about the right size when wound for 12 V and some 30 V.For low winding resistance the toroid form has some advantage.Wall warts are usually also limited power - there source resistance also adds to the problem.An alternative to a transformer could be a charge pump / voltage multiplier with diodes and capacitors.The obvious choice would be to skip the discrete OP-amps and use chips that can run from +-12 V or so and get away with less power.

author: Nick_Denison
author_rank: Contributor
timestamp: May 22, 2025, 02:30:31 am
content: Thanks for the reply, I really appreicate it.QuoteYeah, all op amps are run single ended, class A, so 60mA at idle per rail, with little change with signal injected.QuoteYeah.... I've considered this, and gone down the rabbit hole of XFMR winding. Good for a personal one off, but major drudgery in numbers!QuoteI'm thinking of trying a 25VA toroid next (44V, 22-CT-22). This is easily available to order. 30VA and above are trickier for me to find (at sensible prices). Has to be better. Even if it's inefficient as heck, it beats winding customs by hand. But we'll see.QuoteThanks - I didn't consider pre-certified boost converters I guess. I've been trying to make the DUT in-house as much as possible (pride, ego perhaps). The project is sort of hinged on the contentious perspective of the gains from hotter supply voltages (headroom etc), so I'm really trying to get ±25-30V if at all possible.Thanks again for your input. I'll try the heft VA XFMR and see where that lands me - then attend to the other possibilities.The prototype has a SMPS - and it works fantastically off a 9VDC 1A supply. Easy. Emissions testing is the big sads.

author: Nick_Denison
author_rank: Contributor
timestamp: May 22, 2025, 02:44:12 am
content: Hi Kleinstein, thanks for replying.QuoteAlas, I only found this out when I hooked up my undersized XFMR. I've tested it in Ltspice, now that I've measured the DCR of the windings, and made a guess as to the inductances. The spice model performs as poorly as the real thing, and the 300 ohm per dual primary side resistance is awful for this.QuoteTo this effect, I'm going to try a 25VA toroid (44V, 22-CT-22). Pluggin in an educated guess of 75ohms per primary side, it looks possible in Spice, but I am guessing at inductance to get the rated performance to look right when operated in the correct orientation. Feeding the 12VAC in parallel to the secondaries gets enough voltage to deal with the sag - but at the price of getting very close to the 1A supply limit, and as you alluded to, the drawbacks of the wall adapter might compound those woes. So it might be diminishing returns. We'll see.QuoteI overlooked that, thanks. I believe anything that switches over 9Khz falls into emissions testing headaches, so I'm not sure how viable that is?QuoteI'm notorious for making my own life difficult. The project's premise is to be discrete op amp-based, and also of high-ish voltage (at least well beyond the typical operating area for the average guitar effects unit). Why? Foolhardiness probably.Thanks again.Nick

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2025, 04:45:33 am
content: For the charge pump idea I was thinking about mains frequency. This would need a bit larger capacitors of maybe a few 1000 µF, but it could still be a good alternative to a 30 VA transformer.The ready made DCDC converters can have quite some higher frequency noise, not much better than a normal SMPS. There are a few low noise ones, but this are likely not the cheap ones.

author: IanB
author_rank: Super Contributor
timestamp: May 22, 2025, 05:05:36 am
content: Quote from: Nick_Denison on May 21, 2025, 01:55:43 amJust jumping in here with a thought. 12 V RMS AC has a peak voltage of ±17 V near enough, or about 34 V peak-to-peak. A simple voltage doubling rectifier should be able to produce this output. I think it should be possible therefore, by arranging two parallel voltage doublers in a bipolar configuration, to obtain ±34 V without any transformers in the picture. That should be enough headroom to regulate down to a stable ±27 V DC. I have not done any design on this, so I am not sure what size capacitors you would need to provide the desired 60 mA, but I'm just throwing it out there as an idea to consider.(I think this is basically the charge pump idea put forward by Kleinstein.)

author: Nick_Denison
author_rank: Contributor
timestamp: May 22, 2025, 05:18:33 am
content: QuoteOh I see! I defaulted to thinking of the 555 timer-type that is somewhat common. I'm not familiar with the mains frequency topology at all. Do you have a link perhaps to where I can educate myself on it? A quick google turned up the usual higher frequency suspects.QuoteI was just having a look at this too. 24V bipolar supplies. They look ok, until you see 100mV output noise. My prototype's SMPS outputting ±28V from 9V in has < 5mV spike noise with 20MHZ bandwidth limiting on the scope, and no visible ripple at all.Thanks for the suggestion on the charge pump, I will defintely look into this, and apologies for jumping to conclusions in my assumption about switching freq. I ordered the 25VA toroid to try out, but it's nice to know I might have options beyond hand winding my own, and pushing my own SMPS through testing. Cheers!

author: Nick_Denison
author_rank: Contributor
timestamp: May 22, 2025, 07:14:26 am
content: QuoteThanks Ian, yeah, sounds similar to the charge pump idea. I'm familiar with the 555-timer type charge pump, but less certain about a charge pump of a direct AC source like my 12V, and less certain again about the parallel bipolar topology for it. I have the gist though...Then I wonder about the parallel doublers, vs pumping up to a single >+60V, rail splitting with the usual opamp/BJTs splitter for bipolar supply and ground, then regulating to clean up. What are the pros and cons of those two approaches?Thanks again. You gentleman, are recuing my sanity. More options is always fantastic. Cheers.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2025, 07:20:40 am
content: A virtual ground would add a bit more losses, if there is an assymmetric loading.From the effort 2 short chain are about the same effort as one longer chain. The only saving would be in having only 1 linear regulator.

author: Ian.M
author_rank: Super Contributor
timestamp: May 22, 2025, 08:40:54 am
content: The charge pump idea is certainly viable, but you need to be aware of the high ripple current in the pump capacitors. They'll need to be long life high temperature low ESR aluminium electrolytics with a RMS ripple current ratings of at least 1/3 A. Also the RMS supply current is surprisingly high (if you are unfamiliar with line frequency doubler rectifiers). I found anything under 330uF pump capacitors didn't leave enough headroom for regulation, so to allow for aging you probably need 470uF. The pump capacitors only 'see' half the output voltage so a 25V rating is adequate. LTspice sim attached.

author: Nick_Denison
author_rank: Contributor
timestamp: May 22, 2025, 09:14:33 am
content: Fantastic. Thank you both.I have just been playing on my own version, trying to get my head around it. I had a go with your sim too (and thank you for taking the time to put that together). Took me a second to realise our circuits are the same - just differently presented.indeed - I saw the lower cap values degraded the output voltage, and ripple considerably.You'll note I've plonked large 3300uF ones in my sim. I suppose, should I go forward with this solution, I'll size as large as I can with space and budget. Ignore the diode choice, it was simply convenience to get me going.The ground is ok on this topology? Real-life measurement showed the supply rails are within 15mA for load, so maybe 60, 75 for + and - repsectively. Those values are stable as that is idle current usage (all single ended class-A), and signal changes the draw imperceptably.I'll be interested to see how the toroid performs, but this is looking like a really good way to go. I can't think of what the step-up transformer offers over it, except a rock solid ground I guess.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2025, 02:51:39 pm
content: For the simulation one may want to include the transformer resistance. The impedance is usually a little more than 2 x the DC resistance measured for the secondary, as there is also a similar resistance from the primary and a little inductance from imperfect coupling.I would make C1,C3 a little smaller and keep the output capacitors large to keep the ripple small. The ground is OK, but one needs some care with the layout, so that the charge peaks don't cause extra ripple at ground trace resistance.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2025, 04:25:18 pm
content: Quote from: Nick_Denison on May 21, 2025, 01:55:43 amThe 120 volt 60 milliamp primary implies a 7 watt transformer, however I would have derated for the expected voltage drop in the primary from the winding resistance, meaning multiplied the power rating by 120/30 or 4 times making it a 28 watt transformer. This is not ideal because the transformer core is much larger than necessary, but it keeps the resistive losses in the winding reasonable. In practice it might need to be double even this, or 50 to 60 watts.Another way to think about it is that operating the transformer below its rated voltage, but at the same current, makes the losses from winding resistance proportionally higher.

author: Nick_Denison
author_rank: Contributor
timestamp: May 23, 2025, 03:27:41 am
content: QuoteFor the 1A wall wart 12VAC supply? I don't have one at this moment to test/measure, just a large 10A 12VAC supply. I tried 5ohms parasitic Rser for the voltage source, and added its stated 15% unloaded output voltage. It looks happy enough. 10 ohms Rser and the headroom takes a beating, but it can't turn out to be that bad?QuoteThanks, I played around, and Ian's 470uF seems the lower threshold. I'll try 680uF, and for the output, 2200uF.QuoteI'll be using a ground plane, so hopefully no issues there. Well, a star ground in intent, with separated 'planes' for power, signal, all meeting at the singular plane at the PWR supply. A Fat, plane-like star ground if you like.

author: Nick_Denison
author_rank: Contributor
timestamp: July 08, 2025, 04:02:11 am
content: Thought I'd follow up here with how everything went. The voltage doubling rectifier is very suited to my needs. On the breadboard, the ground was noisy as heck, and ripple. On perfboard things looked so much better, and finally nice thick traces on PCB everything is excellent. Layout needs consideration, as the ground in immediate vaccinity of the doubling caps and diodes is pretty noisy. With a wimpy (12VAC 1A rated) wall wart that gives 13VAC unloaded at full line voltage (240V here) I can get regulated 25V rails at around 80mA draw on each at low mains supply times (e.g. 223V).As for the 25VA toroidal I ordered to try backfeeding as a step up XFMR...... it fared much better than the 6VA split bobbin, and looked good for giving rails < 20V, maybe 15-18V with centre tap. Hopefully that's helpful to someone thinking of trying the same in the future.Thanks to all of you who helped out. I really appreciate it. Cheers.