Toroidal transformer secondary voltage. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 10, 2023, 11:49:02 am
content: Not sure about this as I've never tried it yet. Is it possible to get a regulated 12 volts power supply from a 12 volt toroidal transformer. Using a fairly large capacitor bank, a low dropout voltage regulator, and a shottky bridge rectifier. Oh and with the voltage drop of a series pass power transistor for extra current and power. Thanks for reading thoughts appreciated.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: August 10, 2023, 12:07:01 pm
content: HiMax DC from 12VAC is 16.98VDC, then start subtracting voltage drops. You did not say current required, but why two voltage regs?G Edmonds

author: wraper
author_rank: Unknown
timestamp: August 10, 2023, 12:15:49 pm
content: Depends on particular transformer (how much it sags under the load), how much you load it and how much mains voltage sag it must accept. Note that transformers with large voltage sag often have quite a bit more voltage than nominal under no load as voltage normally is specified under load.QuoteAvoid that if you need to minimize voltage drop. It would be better to just make an opamp based regulator rather than beefing up an LDO.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2023, 12:27:37 pm
content: Not really, if you need to include low-line condition and modest output ripple. (You can use excessively large capacitors to get slightly higher unregulated output with less ripple, but the power factor goes down as well; and it's already about 0.5 at typical values.) In the average case, sure.Preferably you wouldn't have a boost transistor in there (presumably you mean a PNP in the usual connection?). Fortunately there do exist LDO controllers, somewhat of a boutique item, but they are out there. See this for example:https://ww1.microchip.com/downloads/en/DeviceDoc/mic5156.pdfNotice you will need a higher supply voltage for the reg, since it's driving an N-ch (follower) pass device. This is easily implemented on a transformer supply by making a half-wave doubler from one winding to "on top of" the +16V supply. Use a small-ish series capacitor so the supply has a modestly high impedance, and use a zener for one of the diodes to clamp peak voltage -- a C-dropper supply without being direct mains fed. (Or just let it full double the voltage, if it's within ratings. At 16V (32V when doubled) out of a 36V rating, I think I'd rather toss a zener in, simple enough.)Tim

author: themadhippy
author_rank: Super Contributor
timestamp: August 10, 2023, 12:28:35 pm
content: Quoterubbish,from 12v AC i can give you almost any DC voltage you would care forQuotehave a search for voltage doubler rectifier

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 10, 2023, 12:47:00 pm
content: I'm understanding most of your descriptions, but the voltage doubler would take to high for the current I'm looking for. That's approximately 15 Amps at 12 volts. I know 12 volts RMS ends up at 16.968 Volts after the capacitor bank. The 12 volts regulator is a LM2940 CT and I was going to find some high power shottky diodes for the bridge rectifier. Yes there PNP series pass transistors to boost the current to 15 Amps, 5 Amp max per transistor. The transformer is a vigatronix toroidal transformer, 2 X 115 primary and 2 X 12 volts secondary. They should be a bit over 12 volts RMS when powered on. I've got quite a few 10000uf 25 volts 105 °C electrolytic capacitors for the capacitor bank. So if I've got this right, there's 0.5 Volts drop for the voltage regulator, about 1 volt for the bridge rectifier, and 0.7 volts drop for the 3 X parallel series pass transistors. So a total voltage drop of around 2.2 volts. This should just about be enough over head voltage for the circuit to deliver the 12 volts regulated output....

author: themadhippy
author_rank: Super Contributor
timestamp: August 10, 2023, 01:12:09 pm
content: Quoteeven though its supposedly 230v if your in the uk its gonna be more like 240+v so that should gain you an extra 1/2 volt or so

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2023, 02:12:22 pm
content: LM2940CT is one of those archaic LDOs, the early ones with "low" as in "not two diode drops" (though it does happen to have a proper Vce(sat) as the limit, not a diode drop like LM1117), and all the stability and current limit issues (but, at least it doesn't suffer from excessive GND current in dropout). Pay close attention to the required output capacitor ESR. And, I have no idea what boosting will do to stability; probably safe to assume it'll make it worse. Notice they don't suggest such a connection in the datasheet; while that's not an active prohibition against it, it's not promising.Hm, I wonder if that's any different, as far as stability in boosted configuration, with modern (low-ESR compatible) LDOs.Tim

author: tunk
author_rank: Super Contributor
timestamp: August 10, 2023, 02:51:51 pm
content: I think a rule of thumb is to use ~2000uF/A and that this willgive you 4-4.5V ripple: 2000*15*4.5/(16.9-14.2)= 50000uF(Don't know how power factor will influence this.)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 10, 2023, 03:27:12 pm
content: There are lots of factors, it's not so straight forward.Nominal transformer output voltages are (usually?) specified under full rated load current, and at a low current output, the voltage will be higher. This can be a few percent (for a big transformer) to 20% or more (for a small transformer). And of course it also fluctuates with mains voltage changes. Just now I measured 232Vac on a mains socket (so quite close to nominal 230Vac here in the Netherlands) but it can be up to something like 253Vac, or dip to something like 210Vac. Such limits vary by country.Most basic rule for a capacitor is:I = C *dV/dtMains voltage here is 50Hz, so a half cycle is 10msSo if you pull a current of 1A out of a 2200uF capacitor, it's discharge over a half cycle is:dV = I*dt/C = 1*1e-2/(2200e-6) = 4.54VSo that is quite close to tunk's "rule of thumb".

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 10, 2023, 03:39:31 pm
content: Quote from: davelectronic on August 10, 2023, 12:47:00 pmThat seems quite optimistic.A bridge rectifier has two diodes in series with the load, and even for Skottky's 500mV drop is quite low at 15Arms current (and maybe 45A peak current). You can add a shitload of bulk capacitors. this will reduce ripple, but at the cost of ever growing peak currents, and you may saturate the transformer during those peak currents. Also, resistive (copper) loss is quadratic with current. so 45A peak will have a 9x higher copper loss, on top of saturation problems. But anyway, those peak currents will reduce the output voltage of your transformer further.---------------------------------It's not saturation, but overload problems I think, I'm not too well versed in transformer details.---------------------------------Also, another thing to consider:1. Use the voltage doubler (Only one diode in series so already less loss).2. Use a moderate capacitance.3. Add an SMPS circuit to get 12Vdc.It's higher efficiency both for the SMPS and the more even load for the transformer, and you avoid problems with headroom.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2023, 03:54:10 pm
content: Quote from: Doctorandus_P on August 10, 2023, 03:39:31 pmAp bap bap*--- saturation is a magnetization thing, not a load current thing. Load current doesn't affect primary voltage (at least, not in an unbalanced way, given the full wave rectifier). Indeed, flux densitydecreasesas load current increases, because of primary DCR. Which is a small effect, for the most part.*Um, hmm, I have no idea if there's a preferred transcription for that... it's a, not really a turn of phrase, more a sound. I imagine it's a simplification of something like "ah, but but--". ...Things I'd never have thought about, but for foreigners/ESLs being present..!Also regarding transformer impedance / regulation / efficiency, toroids are generally quite good at that, so even if it's a small transformer (and it sounds like it's not), I'd expect pretty good regulation, even while using large filter capacitors. Again, assuming you don't mind the low power factor.QuoteYup, just RMS current to worry about hereQuoteNote that doublers draw worse PF as well; at least for the same output ripple. Though if capacitance should be "moderate", that should work out favorably overall? For certain values of "moderate". I see your point. It's not like the full doubling is needed; a lot more ripple means higher PF and (somewhat) lower DC out. Which, being that DC would be "too high" here, that's advantageous.I think at that point, I'd be more worried about the capacitor(s) themselves -- electrolytics even at line frequency tend to be rather unhappy with more than say 10% ripple. Polymers might handle it fine? They're rather pricey though, for the kind of values needed here, nevermind film or ceramic...SMPS can also be a SEPIC type, allowing boost or buck output (freely and continuously above or below input) -- but any SMPS is probably a bit more advanced than OP is looking for here, unfortunately.There's also the boost/buck (transformer) strategy, where you take a low voltage transformer in series with the main winding to make a smaller change to it. Well, in this situation, good luck finding say a 2Vrms transformer, right? -- but, being it's a toroidal, there might be some room to add windings around the thing by hand. Hacky, but it's probably in the couple-turns-per-volt range, so, I'd guess less than a dozen turns would do, and, just use whatever hookup wire is rated for the (RMS) current required. Does make mounting it a bit annoying -- avoid crushing the added wires.Tim

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 10, 2023, 04:02:16 pm
content: Ok, I see it's not an easy straight forward answer that I was hopping for. The transformer is 300 VA toroidal type transformer. I've never made a voltage doubler, so would have to look this up on how it's done. I'm not aware of other LDO voltage regulators, so would have to research some suitable types to use. Here in the UK I've never measured less than 240 Volts AC, but it might dip lower at peak times of day. My maths isn't the best in the world, I can work out most basic formula with a bit of help from the internet. I'm unsure about different scenario power factors of different completed circuits. I know I can build this type of circuit with an 18 volt RMS secondary transformer, because I've done it several times, with good results. But I've never tried to get the same regulated voltage as the Volts RMS of the intended transformer I'd plan to use. Thanks for all your replies.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 10, 2023, 04:45:37 pm
content: Quote from: Doctorandus_P on August 10, 2023, 03:39:31 pmThat statement about "quadratic" and "9x higher" is misleading in the context ofvoltageloss. Power loss is quadratic with current but voltage drop from copper loss is close to linear with current until the current is high enough that the heat generated increases the copper resistance.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 10, 2023, 05:27:58 pm
content: Quote from: wasedadoc on August 10, 2023, 04:45:37 pmYes, you have to keep the distinction between voltage and power loss clearly separated to avoid confusion.The whole SMPS idea was probably not so bright either. I was thinking along the lines of buying some pre-made module, but those shiny plated rectangular SMPS bricks with holes are at such a price point that it is probably a better option.About the voltage doubler... There are quite a lot of ways to combine a transformer, diodes and capacitors to make a DC voltage. The attached circuit (found in 10 search) is one of the simplest.The capacitor alternately charges C1 or C2 to the peak output voltage of the transformer.

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2023, 05:31:58 pm
content: Quote from: davelectronic on August 10, 2023, 04:02:16 pmDon't bother with the voltage doubler, it's a very good way of squandering more than half of your transformer's secondary current rating and landing yourself with a lot of voltage overhead to shed. It will need some very chunky (more than double the planned value) and rather stressed electrolytics at that sort of current (basically you have two caps in series, which are 'pumped up' alternately at 50Hz, rather than 100Hz as you would with a bridge rectifier).Don't rely on the 240V mains these days either. Our supply step changed from 250V down to 230V (228V - 233V observed) without any warning few weeks back, when the DNO switched our local area to a lower output tap on the substation transformer. You really want a solution that works across the specified tolerance rainge (230V - 6% +10%) to avoid sudden disappointment.I think this is one of thosetoomarginal cases where you need to carefully re-evaluate all of your requirements and choices, maybe go SMPS for instance? I don't think you specified what sort of load this is for.EDIT: T3sl4co1l's suggestion of an overwind on the secondary is probably the most practical fit for what you're intending.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 10, 2023, 05:48:57 pm
content: Quote from: Doctorandus_P on August 10, 2023, 05:27:58 pmThe diodes and capacitors arrangement is the equivalent of the input circuit of a PC PSU with a 110/220 Volt switch set in the 110 position. See schematics in the third reply athttps://electronics.stackexchange.com/questions/479800/why-do-some-smps-power-supplies-require-an-input-voltage-select-switch

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 10, 2023, 06:39:34 pm
content: A very simple and effective technique is to just add some extra windings to your toroidal transformer. The only downsides are that it "looks ugly" and it costs some time. Just put the extra wire in series with the existing windings. (note: you can either add or substract, depending on polarity). (Oops, I see now that Gyro mentioned that T3sl4co1l mentioned this too.)It is also a good exercise to get some hands-on experience with transformers. But do make sure your wire is thick enough to handle your current.============And there are other demons lurking too.One of them is that when you turn the thing on, the empty capacitors form a near short for your transformer. This can lead to very high primary current peaks. They can be enough to blow the mains fuses in your house. For toroidal transformers over 500VA, a "switch on current limiting" circuit is almost mandatory, for smaller transformers it's often omitted. Your 300VA transformer is in the region where this may or may not be necessary. Such a circuit varies from a very simple NTC to a power resistor (10 to 30 Ohm or so) in series with the primary winding and that gets shorted by a relay after a few seconds. audio power amplifiers often use this circuit and schematics are easy to find.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2023, 07:45:41 pm
content: Quote from: Doctorandus_P on August 10, 2023, 06:39:34 pmThis, however, is also exacerbated by saturation! Toroids are notorious for inrush due to saturation, when the switch is closed near a zero crossing (which happens randomly). An inrush limiter (salvage an NTC resistor from some random power supply, if nothing else) is good cover for both effects.Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 10, 2023, 09:08:04 pm
content: Some 15 years ago I had a gadget that had a low power standby mode (which kept big elco's topped up) and a high power mode in which a 600VA 12V transformer had to be switched on quickly and reliably with a solid state relay.The solution I finally settled on was to measure zero crossings with a micrcocontroller, and then determine a (fixed) phase shift to turn on the solid state relay. When I got it completely wrong, the 16A fuses in the house blew. I had a 1A wirewound (slightly inductive?) shunt resistor to measure inrush current, and it just topped out at the mains voltage. So it was either 300A peak or something wrong with my measurement setup. When I got it a bit wrong the transformer hummed heavily for a few seconds before it settled.When I got it right (I think ideal phase delay was about 3ms.) I still had heavy humming sometimes. It took me a while to realize the humming was about half of the times the thing was turned on. Solution was to turn the transformer on during a rising mains flank, and turn it off on a falling flank, and after that it worked excellently. so it turned out that the remanent magnetic field in the transformer core was enough of an extra phase shift to change saturation effects on turn-on significantly.Transformers look simple on the outside, but it needs a surprising amount of work to get something like that just right. If I had known that in advance, I would have used a regular inrush current limiter.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 10, 2023, 09:19:35 pm
content: I won't bother with a voltage doubler, I don't know if to chance 300 VA 12 volts transformers, or parallel two 160 VA 15 volts transformers. I get that extra few volts at the expense of a little less space in the case. But two 160 VA 15 volt transformers would fit. A few posts back mentioned what it's for, it's for HF radio equipment, so needs to be reasonably electrically quite when in use. I can find some NTC inrush thermistors for current limited during turn on. My knowledge doesn't stretch to microcontrollers, but that seems overkill for a linear power supply.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 10, 2023, 09:21:33 pm
content: Quote from: davelectronic on August 10, 2023, 11:49:02 amMy own statistics says it is 50% possible. If you are ok to have a bit smaller regulated output voltage, e.g. 11.0-11.5 VDC then chances are higher. Or, possibly you have oversized low-impedance transformer, or you may use syncronous rectifier instead of Shottky diodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2023, 11:01:17 pm
content: Quote from: Doctorandus_P on August 10, 2023, 09:08:04 pmI even once had a situation where, ... well let me set it up. Low frequency inverter -- a couple kHz, I forget what range exactly (maybe 1-3kHz?); induction heating application. Transformer was output matching (and isolation) between inverter and resonant tank. At the low frequency, a stripwound iron core was chosen (thinnest gauge GOSS available). First test, assumed pulse width was balanced (H bridge inverter) and wired it directly to the inverter. Well, DCR was low enough, and mu_eff high enough, that didn't work of course. It mostly worked, but the peak current at turn-off was pretty significant, and only on one edge, because of saturation. It took some seconds to drift over into saturation, but would inevitably saturate.The weird thing is, after it was first magnetized in this way, it never recovered: it would always... hmm wait now what was it, it would still start randomly (depending on turn-off + ringdown phase; initial phase on the controller was always 1/4 cycle however) I think, but always drift back towards saturation to one side (less severe than before, at least), despite having installed a coupling capacitor after that first test. It wouldn't recover magnetization on sitting (IIRC?), but only while driven at AC.Whereas a "virgin" transformer not subject to saturation, was always well behaved, quiet running and balanced.While I hate the term "flux walking", which normally isn't any kind of strange dynamic process, just the literal DC balance in the circuit; this truly seemed to be some asymmetric, dynamic effect, for which "walking" might be an apt name.So, there's some weird magnetization history stuff, and remnant and permanent magnetization all going on here. Ferromagnetic materials are -- to be precise -- as a physicist -- weird.NiZn ferrites also typically carry a warning not to saturate them, or cause mechanical stress (force, shock, etc.). Ferrites in general have some stress response, where mu can go up or down depending on relative stress and field orientation, there's weird stuff like that too. (And not just because of magnetostriction; I mean change in mu_i.)Tim

author: IanB
author_rank: Super Contributor
timestamp: August 10, 2023, 11:12:52 pm
content: Quote from: davelectronic on August 10, 2023, 12:47:00 pmSeriously? Why do we keep seeing absurdly high currents and linear voltage regulators in the same paragraph?You don't just bung a few capacitors and and LDO after a transformer to get a regulated 15 A supply. Linear power supplies that can provide 15 A at 12 V have the size and weight of a dozen bricks and have cooling fans.If you were asking for 1 or 2 amps, this thread would make more sense.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 10, 2023, 11:32:34 pm
content: Quote from: davelectronic on August 10, 2023, 12:47:00 pmYou need to also include the ripple and the winding resistance, as well as some mains margin.Just wire something up on the bench, and load it up.As you increase the total C to lower ripple, you increase the peak current demanded from the transformer and diodes, and that means more droop.QuoteYou can also consider MOSFET ideal bridges, as your overhead here is tight...https://www.analog.com/en/parametricsearch/11518#/Another helper, for tight designs, is to generate a 20V rail from a simple charge pump, that is used as a regulator bias rail.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 11, 2023, 11:45:09 am
content: I have seen these ideal bridge circuits, but they have a higher components count and use the IC plus 4 X MOSFETs. And as for why 15 Amps... The amplifier I'm powering is asking for at least 10 Amps DC 11.40 - 14.40 Volts. It's a heavy power supply, there is real weight in the toroidal transformer, and heatsinks. But it's manageable. All high current linear power supplys are heavy. I would like to build the MOSFETs ideal bridge, but I would need to allow heatsink space to cool those extra components.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 11, 2023, 12:11:32 pm
content: Quote from: davelectronic on August 10, 2023, 09:19:35 pmSearched though it, did not see it.But more important. I've heared a few times they want to run their equipment at 13.8V, as more voltage is apparently more betterer (output power?) and that is apparently the highes safe working voltage for that equipment (suspiciosly close to a fully charged car battery)Quote from: davelectronic on August 10, 2023, 09:19:35 pmAren't those HAM guys supposed to have knowledge of building / designing good filters?And I guess plenty of SMPS circuits are used there already. Maybe some are designed to be relatively quiet, others may be heavily filtered.I guess that an easy to reproduce and good filter for SMPS power supplies would be a quite attractive thing for those guys. It probably already exists.

author: Sighound36
author_rank: Frequent Contributor
timestamp: August 11, 2023, 01:36:15 pm
content: Maybe I'm being over simplistic, consider using bigger casework?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 12, 2023, 01:05:30 am
content: Quote from: davelectronic on August 11, 2023, 11:45:09 amThe MOSFETS have much lower power than the diodes, so the cooling needed is much less. Instead of ~10W for STKY bridge, you can be looking at 400mW for 2mOhm FETsThis is sounding marginal, so I'd suggest you load up the transformer you hope to use, and measure the droop and ripple at the target load current.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 12, 2023, 11:21:40 am
content: SMPS aren't very desirable for running HF or any RF equipment. I have a retail 30 Amp 13.80 Volt PSU powering a radio and amplifier, it has a noise offset knob, but I use this PSU reluctantly. I might look into the MOSFETs bridge rectifier if the shottky diode bridge fails. I'm hoping the transformer puts out 13 volts or a little more. I'm going to power some halogen lamps off the AC just to see what kind of sag it's likely to have.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2023, 12:15:17 pm
content: Could've told... y'know about everyone..? All my test equipment use SMPS inside. Even the 40m tube radio I made some years ago, I ran with a converter for 6.3 and 120V, just boxed up (copper clad, soldered at the seams) with one extra layer of LC filters on the (input and output) terminals. No interference whatsoever.It's not hard to filter things to arbitrary degree; but it is tricky to figure out at first, as there are many possible permutations, only a few of which will work successfully.Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 12, 2023, 01:10:50 pm
content: This:Quote from: T3sl4co1l on August 12, 2023, 12:15:17 pmIs info that needs to be discussed and explained far more in those niche HAM groups. It also serves a wider community for general EMC knowledge. I also do not know much myself about details for such circuits, but I simply do not believe:Quote from: davelectronicI guess they call that attitude cargo cult engineering.Some time (years?) ago EEVblog Dave did a teardown of some Keithly (or other top notch) instrument with 24 digit (or was it bits, you never know with Keithley) measurement accuracy and there were quite big SMPS circuits all over in the thing. They did go through extreme measures to get it that good. I think they even used coaxial wires for one of the transformer coils. I think it was a high voltage and high current SMU, but I'm not sure.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 12, 2023, 10:14:26 pm
content: I've bought some toroidal transformers 300 VA 12 Volts secondary now. So going to give it a go with shottky diodes 30 Amp 50 Volts axial diodes. They drop 0.154 per diode. My use of SMPS on radio gear was ex server PSU's converted for the job, some where better than others, and some where hourendously noisy. Gave up using those, but they had bucket loads of current. I'm not a Ham at the moment, but wouldn't mind doing the foundation course here in the UK, then I would be a good guy and use the requisite RF power.... And stop freebanding with hundreds of watts.

author: IanB
author_rank: Super Contributor
timestamp: August 12, 2023, 10:37:41 pm
content: Quote from: davelectronic on August 12, 2023, 10:14:26 pmBut why would you buy 12 V transformers? You need some headroom to allow for rectification, smoothing and regulation. At least 15 V would be good.Also, you should probably de-rate your 300 VA transformers to about 100 VA for DC use, depending on length of use and how hot you can tolerate them getting. So two of them in parallel should get you to 30 A.Another point is that car electrical supplies are typically at about 13.8 V. So 12 V is already on the low end.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 13, 2023, 11:50:14 am
content: Car electrical supply ? A car battery... Not much good for deep discharge, and battery chargers are no good either. I'm looking for 15 Amps from a single toroidal transformer 300 VA 115 X 2 primary, 2 X 12 Volts secondary. I could put them in parallel, as I have three exactly the same specifications. But a single toroidal transformer should be able to supply 15 Amps continuous, although what it will power won't be much over 10 Amps. So it should be fine. I do have 4 X 300 VA 115 X 2, and 2 X 18 Volt secondarys. But won't be using those for this. I want to see if I can get 12 Volts DC from a 12 Volt toroidal transformer.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 13, 2023, 01:25:52 pm
content: Quote from: davelectronic on August 13, 2023, 11:50:14 amYes, you probably can get the amps.Recified output DC voltage (without considering diode loss) is 12*sqrt(2) = 16.97V300VA / 17V = 17.6A so you have some headroom left (and you need that, because this calculation is not accurate)Quote from: davelectronic on August 13, 2023, 11:50:14 amHe did not suggest to actually use a car battery, but that 12V ham equipment is often rated to be used at 13.8V and running it on 12V is already lowish.Quote from: davelectronic on August 13, 2023, 11:50:14 amWell, if a part of it is just curioucity, then wire it up.But as you know by now, it's quite possible you won't get the 12V out. So I suggest to not spend a lot of time to make it neat and tidy but start with a quick and dirty wire up on your bench. Mains voltage clips such as Wago 221 are quite excellent for short time wire ups on your bench. It is a bit annoying that the standard ferrules are too short to be used with these Wago's. but you can buy longer ferrules. You can also strip a few silicone isolated wires and tin their ends sparingly, so you still see the strands through the solder. I find this much more convenient for temporary connections then the big 4mm banana plugs. Also note that the Wago's are not designed for this, but you can use them a few hundred times before the springs start fatiguing, en then you notice it from the smaller force to open them. If you notice this, just trow it in the bin and use a new one.

author: IanB
author_rank: Super Contributor
timestamp: August 13, 2023, 03:16:13 pm
content: Quote from: davelectronic on August 13, 2023, 11:50:14 amNot the point. As Doctorandus_P said, ham radio equipment is often rated for 13.8 V, so supplying it with 12 V may be inefficient.QuoteA 12 V, 300 VA transformer would provide 25 A AC. If you rectify it to provide DC then you have to de-rate it to stop the transformer getting too hot. Typical de-rating factors range from 2 to 4. So you might be looking at between 6 A DC to 13 A DC for a 300 VA transformer before it overheats. You will have to test this with your particular application to find out what kind of current you can actually achieve.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 14, 2023, 10:41:19 am
content: Quote from: IanB on August 12, 2023, 10:37:41 pmI guess a reason is that powerful 12 V transformers are widely available and so are relatively cheap too. I would consider it too. Of cause, proper fine filtering and regulation tends to have 15 VAC transformer (or even 13-14 VAC made on order).

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 14, 2023, 12:07:56 pm
content: I won't do a mock up first, although I will measure the secondary AC voltage, and probably load it up with some halogen 12 Volt 50 watt lamps. Just to see how it drops under load. The cases I have are fully ventilated, and a hole cut out in the back of the case for an 80mm axial fan, that I will put on a thermal switch. I'm moderately optimistic it will work out fine. All RF equipment I have ever used is happy from 11.40 volts to 14.40 volts. But yes there is some power loss at the lower end of those voltages. As for derating I can't see 25 Amps AC dropping to as low as 6 Amps, these toroidal transformers are usually very efficient.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 14, 2023, 06:37:53 pm
content: I have measured one of the 115 X 2, 12.0 X 2 volts. The unloaded AC measured 14.17 volts, then I loaded it up with 200 watts, 4 X 50 watt halogen lamps. Then it measures 13.57 volts AC So it's dropping 0.6 Approximately with a 200 watt load on the secondary windings output. So 14.17 X 1.414 is 20.03, so I need to remove the voltage drop components from the rectified and smoothed voltage, and the volts drop under load. Does it seem like this toroidal transformer is doable ? What do you think...

author: IanB
author_rank: Super Contributor
timestamp: August 14, 2023, 08:44:05 pm
content: Quote from: davelectronic on August 14, 2023, 06:37:53 pmA transformer is a ratio between primary and secondary, so it missing a detail to give the secondary voltage unless you also give the primary voltage. It was probably about 240 V, but depending on time and location it could have been as high as 250 V or lower than 230 V. So 14.17 V by itself is incomplete information.Also, multiplying by 1.414 to give 20 V does not help you very much. That will be 20 V open circuit with zero amps flowing. As soon as you try to have a DC current flowing then the voltage will be lower. The higher the current the lower the voltage. Then the transformer will start to heat up due to the unusual current waveform from the rectifier and you will have to allow for that too.A good starting point might be to add a bridge rectifier, add some big smoothing capacitors, then add the same 200 W load, and measure the DC voltage at the load. This will be especially useful if you have a meter that can measure AC+DC volts simultaneously and show both AC and DC on the display.

author: Zero999
author_rank: Super Contributor
timestamp: August 14, 2023, 09:05:29 pm
content: What are you powering?I hate to be boring, but I would just by a 12V switched mode power supply. It's more efficient and invariably works out cheaper too. I've built enough massive linear power supplies to know they're no longer worth the bother for most applications in modern electronics.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 14, 2023, 09:38:32 pm
content: Yes I understand, it will be a few days before I have the filter capacitor board and the bridge rectifier installed. But I test then would be better I think, I only wanted to see what the AC voltage would drop to with a heavy load. I've already got switch mode power supplys. One is powering a HF radio and linear amplifier right now. I just enjoy trying to get a compact PSU unit in a well ventilated enclosure with a hole in the back of the enclosure for an 80mm fan. There are a few power supplys lurking around in my flat. I try to improve each one that I build, layout, heatsinks air flow path, and generally a tidy layout in a finished product. Really I've always wanted to manufacture a PSU for the market ( don't panic, it's never going to happen) but if when I'm dead and moved on... It would be great to see someone looking over all the power supply projects laying around that are finished. I'm just trying to make the next PSU better than the previous one I built. I have used a few now with no problems. My Son says " you built that one already" he can't see the layout and circuit is different in each one. It's a hobby really that's all. They have powered high current RF equipment mainly, but I have torture tested a few, and to my surprise they actually held up rather well. This time it's just to try if I can get a 12 volt high current power supply from a 12 volt rated toroidal transformer. As the lower the overhead voltage ( not to low) the more current will be available, with in the specification of the toroidal transformer.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 14, 2023, 11:52:39 pm
content: Quote from: davelectronic on August 14, 2023, 06:37:53 pmYou should also record the Mains In voltage.That's one reference point, but it's a skewed test as the diode conduction angle is ignored here.The voltage youreallywant to measure, is the bottom of the ripple on the filter caps, under load.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 15, 2023, 11:43:05 am
content: Yes I know that now, once I've put together the shottky diode bridge rectifier, and the filter capacitors, I will test it again. Also taking a note of the AC mains into the transformer. I'm optimistic hoping it might just give me the 12 Volts 15 Amps I'm looking for. But if it doesn't, I will know why.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 26, 2023, 07:33:29 pm
content: It's been a while, I have now made a shottky diode bridge rectifier, 30 Amp shottky axial diodes. And filter capacitors, 3 X 10000uf 35 Volt 105°C electrolytic capacitors. The voltage with no load is 19.40 volts DC. If I add the same 200 watt load as I did with AC voltage. I get 14.90 volts under load. So with that voltage load drop, do you think the LM2940 CT will function ok with that voltage drop. Thoughts appreciated.

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2023, 07:49:04 pm
content: You could also leave the 200 W load connected continuously and monitor the temperature of the transformer, see how warm it gets over time. That might be one of your other limiting factors.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2023, 08:13:09 pm
content: Quote from: davelectronic on August 26, 2023, 07:33:29 pmHow are you measuring the voltage?Is 14.9V the RMS, average or minium trough in the ripple?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 26, 2023, 08:24:15 pm
content: Quote from: davelectronic on August 26, 2023, 07:33:29 pmYou need to check theminimumvoltage on the sawtooth ripple, to find the margin.And also check the mains voltage, as all of this is ratioed to whatever the mains is at the time.Another trick to sneak a bit more headroom is to add two more stky diodes and another large cap.That rail has less ripple and less diode drop, and can be used as a bias rail.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 26, 2023, 09:18:22 pm
content: I don't have a usable scope, so that is a bit difficult. The two extra shottky diodes, are they on the negative rail, and up the voltage by 0.7 Volts per diode. But I've no way of looking at the waveform.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2023, 10:29:06 pm
content: Quote from: davelectronic on August 26, 2023, 09:18:22 pmHow about making your own trough detector, using a comparator, a voltage reference and a few resistors? You'll then be able to find out what the valley voltage is.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 27, 2023, 01:14:44 am
content: Quote from: davelectronic on August 26, 2023, 09:18:22 pmYou could use an AC meter, and check with maths I = C dV/dT so 10A 8ms and 30mF gives 2.67V p-p, meaning your 14.9 dips to about 13.56Vgoogle finds thisTriangle- and sawtooth-shaped waveforms have RMS values of 0.577 (the reciprocal of square root of 3) and form factors of 1.15 (0.577/0.5).

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2023, 08:51:49 am
content: Quote from: PCB.Wiz on August 27, 2023, 01:14:44 amGood idea. It That will only work if the meter measures true RMS though.Do you know of a way, which will work with a non-RMS meter?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 27, 2023, 10:00:04 am
content: Quote from: Zero999 on August 27, 2023, 08:51:49 amYou could try a sound-card as an oscilloscope ? You need to calibrate the gain.You could also try SKTY diodes to detect the peak + and - into two caps, and measure the difference.That will be a couple of hundred mV too low.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 27, 2023, 11:16:56 am
content: I know what the peak to peak means in a wave form, but I don't fully understand the formula you've laid out in your post, my maths isn't great, just about get by with maths. Wouldn't know where to start with building a device to measure the troughs of the waveform. The sound card idea sounds interesting, although probably beyond my level to make something like that work. I have two true RMS multimeters, I could try to see if they could read the ripple on the AC setting. I really need to get an ocillascope as soon as possible.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 27, 2023, 12:37:11 pm
content: Suppose I will have to taste it and see what happens, I'm a bit optimistic that it will be ok and I've just got enough voltage. I need 8 Amps continuous, 10 Amps occasionally, and 15 Amps peak. But for the most part 8 Amps will be the load, with a 50% duty cycle. There are 3 X TIP2955 to supply the current. To give an idea of what I've got, it's in the image below. I just have to make up the regulator board, and wire it up.

author: IanB
author_rank: Super Contributor
timestamp: August 27, 2023, 03:36:32 pm
content: Quote from: davelectronic on August 27, 2023, 11:16:56 amWhat multimeters do you have? Some advanced multimeters have a crest mode that will will record the minimum and maximum voltages in a cycle.Or if you have a meter that can take AC + DC readings, you could get some idea of the AC ripple sitting on top of the DC.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 27, 2023, 04:32:52 pm
content: I've got the auto ranging true RMS multimeters. I will look at the model numbers on them, sad I can't remember off hand. I'm certain one of them has a min / max for voltage readings. Although I don't know if it's feature you mentioned. Just haven't used them in a while, I've also got two uni T AC DC clamp meters, they go up to 20 Amps. I know these meters aren't the best there is, but I'm just a hobbyist.

author: IanB
author_rank: Super Contributor
timestamp: August 27, 2023, 04:40:24 pm
content: Unfortunately, basic min/max won't do it. You would need a feature described something like "1 millisecond crest detection". It would scan the voltage 1000x per second and record the highest and lowest voltage that it sees.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 27, 2023, 05:40:32 pm
content: No they don't have that function, all I've got is what's below.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 27, 2023, 05:41:22 pm
content: And these.