How to understand dodgey toroidal transformer advertisements. - Page 1
Forum section: electronics / beginners

author: tifkat
author_rank: Regular Contributor
timestamp: July 26, 2020, 08:29:37 pm
content: Hi all,Many moons ago, I made a simple regulated DC power supply for my grade 12 physics assignment. It was a chunky EI transformer and LM350T setup. Etched my own PCB and everything.Now I'm looking to make something more up to date, because I have no idea where it is, and I want to do something bigger. Looking to make a 3-channel regulated supply with Master/Slave rails plus a fixed, multi-select rail all controlled by an MCU with an LCD display interface, rotary encoder and keypad input, stored settings etc.I'm considering the use of a toroidal transformer with multiple secondaries to make a multi-channel bench supply, and I'm cramming to get back what little knowledge I had back then. But I'm a bit confused by some of the advertised specs of the toroidal transformers I'm coming across.There is one I have found which is 2x36v/5a + 2x12v/1a secondaries. Not bad, but those 12v windings May be just under spec. This is listed as a 384VA transformer, and I get how that's calculated.I have found a 500VA transformer, but it doesn't list the current capacity of the secondaries. It's listed as having 2x3-wire 36-0-36v Secondaries and 2x12v 2-wire secondaries.I assume the 36-0-36 can be used by connecting to one of the 36 and the 0 to get 36v. Or from the 36 to the 2nd 36 to get 72v, or do the 36-0 and the 0-36 (somehow) to get effectively -36 to 36v (after rectification)The 2x 12v 2-wire secondaries would work as expected.If any of the is wrong please correct me.Now my problem. No current rating. No winding counts. How do you know what the current rating of any given secondary winding is?Finally, it seems to be a selling point that these are for hi-fi amps... like every toroidal transformer I can find seems to be listed for hi-fi amps. I'm confident that it's just for use as a transformer for any kind of power supply, but I'm not omniscient, so here I am. I also wonder if this kind of one-track-mind advertising results in ridiculously inflated pricing because the 'niche' market is too well funded and not sensible or informed enough to know that $450 for the above specced transformer is just taking the piss.If you can shed any light on my thirst for knowledge, I'dReally appreciate it.tifkat

author: Gyro
author_rank: Super Contributor
timestamp: July 26, 2020, 08:56:28 pm
content: Hi, welcome to the forum.I don't know the exchange rate, but $450 for a 500VA transformer sounds a lot in anyone's dollars.Regarding the windings, it's clearly designed to drive a stereo audio amplifier with independent split rail supplies for each channel...Each of the "36-0-36 volt" windings is a 72V centre tapped winding. Each would normally be bridge rectified [Edit: and smoothed] to provide positive and negative DC supply rails of +/-50V.The 12V windings will be lowish current (1A sounds reasonable) auxiliary windings for misc purposes, maybe a pre-amp stage, muting relay supplies or whatever.Most of the 500VA rating will be directed at the two main 36-0-36 secondaries. The 12V secondaries would only account for maybe 24VA max. so say, 240VA per main secondary [Edit: Approx 3.3A each].There's nothing 'special' about toroidal transformers used for audio amplifiers - other than the above secondary winding and voltage selections, which are a dead giveaway. Toroidals tend to be used in big amplifiers for lower weight and slimmer profile, which helps with the overall form factor. No audiophool magic involved.ChrisP.S. The reason that you are seeing lots of audio power amp transformers is probably that they are one of the few applications that tend to use big linear supplies these days. Most other stuff has gone SMPS.

author: Brumby
author_rank: Unknown
timestamp: July 27, 2020, 12:28:04 am
content: Quote from: Gyro on July 26, 2020, 08:56:28 pmDon't worry about the exchange rate - $450 for 500VA is NOT a value purchase. I don't doubt they will be out there, but for a generic use such as a power supply, you can easily do better.Being a little flexible on the specs, at Altronics you can get a 2 x 30V 500VA toroidal for $179 or a 2 x 30V 300VA for $99.95. They also have a 2 x 45V 300VA for $86.95.At those prices, you could gettwo300VA toroidals to give you 600VA and still have change out of $200 ... assuming you have space to fit them.Checking RS Components reveals they have a choice including a 2 x 35V 500VA toroidal for $146.97 (on back order until 31/7). Element14 also has a 2 x 35V 500VA toroidal for $161.72.Just FYI

author: tifkat
author_rank: Regular Contributor
timestamp: July 27, 2020, 12:39:28 am
content: Thanks for your replies guys. After reading Gyro's reply I realised that I can indeed ease my requirements to something like 25v, which should net me around 36v after rectification.I was hoping to get 2 big secondaries and 2 smaller secondaries in the one transformer. One of the smaller secondaries would be for the multi-choice fixed rail, and the 2nd to run the MCU.I'll change my search parameters and keep looking.

author: tifkat
author_rank: Regular Contributor
timestamp: July 27, 2020, 12:41:17 am
content: The question still remains tho, if an item doesn't show a current rating for the secondaries, should I just move on, or is there some way of getting a useful figure?

author: james_s
author_rank: Super Contributor
timestamp: July 27, 2020, 03:55:38 am
content: It's rare to find a transformer without current or VA ratings for the secondaries. One nice thing about toroidal transformers is that unless it's potted you can very easily add additional windings over the top which can be used independently or wired in series to buck or boost one of the existing windings.

author: Gyro
author_rank: Super Contributor
timestamp: July 27, 2020, 10:55:36 am
content: Quote from: tifkat on July 27, 2020, 12:41:17 amIt would be fine for a bargain priced 'surplus' transformer where the vendor didn't actually know the figures. As I showed in my previous post, it was fairly easy to work out ballpark ratings. It's not acceptable for a brand new one though.Aside from the price, that transformer would have made for a beefy PSU. If the windings were paralleled and full wave rectified it would have given you a +50V rail with reasonable current capability. If your thermals could handle it, that would give you maybe 0-45V @ 5A... or a twin PSU with two outputs of around 2.5A. The problem with starting with such high voltage windings though is that you have no opportunity to 'tap change' to lower secondary voltages to reduce the thermal dissipation of your series pass element(s) at low voltage, high current output... They would be dumping the whole lot as heat.

author: tifkat
author_rank: Regular Contributor
timestamp: July 28, 2020, 09:03:13 am
content: Chris, thanks for the input.I have found the same transformer for sale at around $65 AUD and it looks like shipping will be around the same.I've contacted the seller to ask for actual values. They have a real-time chat service and so far have gleaned that the stated voltages are RMS figures. I've asked about the current rating and am awaiting a reply.I'm curious to know, if I had 12vDC (ripple) at 2A supplying a voltage regulator (such as the LM338, my current target regulator), that's 24VA, can I expect that getting 5V / 3A out (15VA) is achievable? This is something I haven't learned yet. Is the conversion arbitrary, and VA in > VA out is the only real requirement (when used in a step down fashion), or does 3A out require 3A in, regardless of the voltage change?

author: Gyro
author_rank: Super Contributor
timestamp: July 28, 2020, 09:54:26 am
content: You're welcome, that sounds more like the price.Sorry, no. It's not just a matter of VA with a linear regulator. You can't pull more than the rated current of the winding. After rectifying and smoothing cap you will get about 15-16V raw DC (RMS * SQRT2). But in the process you loose current capability to about 0.62 of the winding rating (see attached), so it will yield about 1.24A for a 2A winding.You can probably push the 12V windings a little bit harder if you're not using the main secondaries, but ultimately it comes down to the wire gauge of the windings.

author: james_s
author_rank: Super Contributor
timestamp: July 29, 2020, 01:55:57 am
content: Another option is to do what I did several years back and wind my own. Toroidal cores are a bit of a pain to wind by hand but with a larger core you can get away with fewer turns. Going from memory it's a 600VA transformer, the 120V primary is about 200 turns and the secondaries are much fewer. It took a good bit of reading up on things but in the end I wound the thing, it came out beautifully and worked perfectly. It was a fun project and took a lot of the mystery out of transformers. Suddenly I realized "Hey I can actually make these things from scratch to exactly the specifications I want!"

author: tifkat
author_rank: Regular Contributor
timestamp: July 31, 2020, 11:16:59 am
content: Gyro,In that sheet you attached, is it correct to think of the V(peak)DC figures as being the voltage measured after rectification, when there is no load connected?

author: Gyro
author_rank: Super Contributor
timestamp: July 31, 2020, 08:20:22 pm
content: Yes, that's correct, that's the figure with the rectifier charging the output capacitor with no load. The loaded DC voltage and ripple depend on the load current and capacitor value as well as rectifier forward drop and transformer regulation. At minimum load, Vpk is the voltage that your capacitor and regulator must be able to cope with.The main usefulness of the sheet is to let you judge how much current you can safely pull in different transformer / rectifier configurations.

author: tifkat
author_rank: Regular Contributor
timestamp: August 01, 2020, 07:36:39 am
content: Sweet! So, I'll need to aim for 24v secondaries on my transformer.Ideally, in such a precision power supply, would you want to have each channel completely isolated from the others?My consideration is: a 48v winding with a center tap, or 2 discrete 24v windings? My thinking is that discrete windings would be safer, to avoid the possibility of one rail affecting the other. If this is true, then I'll also want a discrete winding for my MCU pcb, and one for my fixed voltage rail.

author: tifkat
author_rank: Regular Contributor
timestamp: August 01, 2020, 08:06:07 am
content: Another pertinent (I think) question would be, if a center tapped winding is rated for 5A, is that 'total', and each half of the winding gets half of the current?So a 48v winding with a center tap, rated at 5A, that would give 2x24v @ 2.5a, or a +24/-24v @ 5A. You're not going to get 2x 24v@5a are you?

author: mariush
author_rank: Super Contributor
timestamp: August 01, 2020, 08:25:01 am
content: The VA rating will tell you that.A 500VA transformer with 48v center tap (2 x 24v) will have 500VA / 48 = 10.41 A ... so you have 24v AC 10.41A secondary windings.When rectified with a bridge rectifier, you'll have a peak DC voltage equal to 1.414 (square root of 2) x 24v AC - 2 x voltage drop on individual diode on bridge rectifier (usually 0.8v-1.2v) ... so around 32v peak dc voltage.The rectified AC current will be around ~0.62 x 10.41 = 6.4AIf the cost is not critical you could also check out ideal diode controllers like LT4320, which - with the help of 4 mosfets - can replace bridge rectifiers resulting in less voltage drop produced and less heat generated.Seehttps://www.analog.com/media/en/technical-documentation/data-sheets/4320fb.pdfFor example, at 5A of current, a standard bridge rectifier will have a voltage drop of around 1.4v so there's gonna be around 1.4v x 5A = 7 watts of heat on each bridge rectifier, or 14w in total for both windings. This is not very important, because normally you'd have a big heatsink for the pass transistors anyway.But, not having that 1v+ voltage drop can help keep the costs down when it comes to the bulk capacitors.... not at 5A, this isn't a good example, but it would certainly make a difference at 15-20A of current.For example, if you want to make a 24v psu, you'll want at least 26v minimum .... and you know with the formula above that your peak will be around 32v... so you can estimate the capacitance needed to keep the 26v at the current you want :capacitance = current (in amps) / [ 2 x ac frequency x ( Vdc peak - Vdc min desired) ]So C = 5A / 2 x 60 x (32-26) = 0.006944 Farads or 7000uF , so you'll want to use a 8200 uF 50v rated capacitorWith an ideal bridge rectifier, the voltage drop will probably be less than around 0.25v, so let's say your peak dc voltage will be 33.5v :C = 5 / 2 x 60 x (33.5-26) = 0.0055 Farads or 5555 uF ... so you could use a 6800uF, or 2 x 3300uF in parallelIn your particular case it's not worth it from a cost point of view, the lt4320 chip itself is around 4-6$ and the four mosfets you'd need would probably cost you another 1-2$ ... you can instead just parallel 2 0.5$ 20-40A bridge rectifiers to get a lower voltage drop and be done with it.

author: tifkat
author_rank: Regular Contributor
timestamp: August 01, 2020, 11:22:39 am
content: Maybe I'm reading mariush's post wrong, but it seems to be describing a primary winding with a center tap. For clarification, I'll give a hypothetical example.Say I have a transformer with the following specs:Input: (P)rimary winding - single winding, 240v AC inOutputs: (S)econdary winding - single winding with center tap, 48v@5a out (or 24-0-24 from my understanding)I could use that (S) without the center tap as 48v@5a, but how do the 24v uses work? I know I could make a +/- 24v rail, but at what current rating? Is it 2.5a each? And I could theoretically make 2 individual 24v rails. Would these also be 2.5a each?I appreciate the proposal of running 2 regulators in parallel. I'll keep that in mind if I'm struggling to find a regulator to suit.tifkat

author: mariush
author_rank: Super Contributor
timestamp: August 01, 2020, 01:06:04 pm
content: No , you get same current .If it's 48v 5A (240va) , then you have 24v at 5A (120VA) + 24v at 5A (120VA) = 240VA in total.and i didn't mean regulators, I meant bridge rectifiers. You'll still need voltage regulator or pass transistors after the bridge rectifier(s) and capacitor for bulk storage.How much paralleling bridge rectifiers help depends on rectifier and what diodes are used inside (or what diodes you use to make a rectifier, because you could just use multiples of 4 diodes)Some bridge rectifiers use diodes that have high voltage drop no matter the current, for example even at 1A they'd still drop 0.7vOther bridge rectifiers may drop only 0.5v at low currents, and up to 1v or something like that at high currents (per diode, so double that for a bridge rectifier)

author: tifkat
author_rank: Regular Contributor
timestamp: August 01, 2020, 03:40:23 pm
content: Thanks mariush. I just came back to say I'd been mulling it over in my head and came to that conclusion. The VA in must equal VA out.