Stuck on capacitance - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 14, 2020, 02:49:37 pm
content: I've come so far with a power supply build, but my rectified filtered voltage isn't as high as I would like. I know to a certain extent filter capacitance can keep the input to regulator in the green, but I'm not sure on how much to use. The regulator is an LM2940-12 an LDR positive voltage regulator. My RMS voltage from two series wired transformers is 14.00 Volts AC before the bridge rectifier, and 13.00 Volts DC after its been rectified. I'm looking to get a final voltage of 12.00 Volts DC and 15 to 20 Amps of current from it.My use will have an estimated 50% duty cycle, I can see the input to the LM2940-12 linear voltage regulator is 13.60 Volts DC. I have capacitors totalling 282000uf large can electrolytic capacitors, this is 6 X 47000uf capacitors. I'm not sure how much capacitance to use to keep that voltage up under a 15 to 20 Amp load. I know there is a point beyond where no more adding capacitance will keep the voltage up, and if increasing load is added the regulator will fail to function below its drop out voltage. Two images below showing the two transformers arrangement with a 50 Amp 1000 Volt bridge rectifier tucked under the forward end of the aluminium plates. I plan to use 4 X TIP36C transistors for the power carrying ability in this power supply on a 20mm 100 X 200 solid aluminium plate. The case is two separate cases bolted together, with the transformers in the lower case. And the power transistors and control board in the top section. Two cooling fans and two analogue panel meters, and suitable 4mm terminal post to finish it off.Any help on capacitance for the input to the regulator would be really helpful.Thanks for reading.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 14, 2020, 03:02:06 pm
content: To give a better idea of the capacitors I have, and where the case is at the moment in image below.Could really use some help on the total capacitance to use, or some help working it out. Thank you.

author: ocw
author_rank: Regular Contributor
timestamp: October 14, 2020, 03:14:26 pm
content: You should already have an adequate amount of capacitance. I suspect that your problem comes from the voltage drop in your bridge rectifier. Your 1,000 PIV 50A diodes may actually have two or more diodes in series and have more voltage drop than you are expecting at that comparatively low voltage. Try measuring the peak voltage with a scope at the rectifier's DC output and see what voltage is going to those capacitors.

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 14, 2020, 04:14:06 pm
content: With 14 vac, I think you’re already in trouble, that’s simply not enough headroom to get 12 volts out. You’ve reached the point of quickly diminishing returns when adding capacitance. What you don’t discuss and is the missing key piece of information, is what is the dropout voltage of your regulator scheme?Assuming a full wave bridge, 20 amp load and 50Hz, your 6 x 47,000uf cap bank will result in approximately 700mv of ripple. (It/c). Couple that with the rectifier voltage drop and the input voltage to the regulator is probably too low to maintain regulation for a 12v output.IMO, you need a higher AC input voltage.

author: m k
author_rank: Super Contributor
timestamp: October 14, 2020, 04:46:18 pm
content: 14Vrms?

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 14, 2020, 05:14:05 pm
content: Quote from: m k on October 14, 2020, 04:46:18 pmGreat point, I had a brain fart when reading the original post. If the the OP has 14 vac rms, he should have some 19 volts dc output, ignoring the rectifier voltage drop. So, my comment about 14vac being too low is just wrong.OP: now I’m confused about your numbers. Is that measurement of 13 vdc without the capacitors in the circuit? I’m thinking that has to be the case. The caps are going to raise the dc voltage by roughly 1.4x the ac input.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 14, 2020, 10:09:54 pm
content: Thanks for your replys, 14.00 Volts AC is what comes from the series wired secondary windings. After the bridge rectifier it's approximately 13.00 Volts DC but not filtered at this stage. I did a test with 90000uf of capacitance from 9 X 10000uf capacitor bank. My filtered voltage after the capacitors was indeed about 18.70 Volts DC. With out any regulator I loaded it with 200 watts of halogen lamps, that dropped the filtered voltage to 13.90 Volts. I've yet to use the new capacitors in the image I posted. I wanted to maximize my chances of hopefully get 12.00 Volts from the two series wired transformers by using a high capacitance, I was thinking that a final output filter could reduce some of the ripple. I do have an old hameg ocillscope I could measure the output waveform with. Just thought a multimeter would be fine for measuring the output with.

author: srb1954
author_rank: Super Contributor
timestamp: October 14, 2020, 10:26:40 pm
content: Quote from: WattsThat on October 14, 2020, 05:14:05 pmThe 13V DC is about right for the average value of the rectified waveform without any smoothing capacitors. With smoothing capacitors the 19V DC output will only be obtained when there is no load applied.However, under full load the smoothed DC voltage is going to drop considerably to maybe only 13-14V DC. This drop is dependent on factors such as transformer regulation, diode drops and smoothing capacitance. Adding a huge amount of smoothing capacitance helps a little but I think the main problem here is transformer regulation. Basically, he needs a beefier transformer to do the job.

author: srb1954
author_rank: Super Contributor
timestamp: October 14, 2020, 11:41:26 pm
content: Quote from: davelectronic on October 14, 2020, 10:09:54 pmThe multimeter only measures the average DC voltage so does not give you the full picture of the performance of the rectifier/smoothing filter. You need to know the minimum voltage at the trough of the ripple to determine whether there is sufficient input voltage to the regulator.A scope is the most appropriate way to measure this although, if you only have an old analog scope, you may not be able to measure this minimum voltage accurately enough. Alternatively, you can use your scope in AC coupled mode and measure the peak-to-peak ripple voltage. Subtract half of this pp voltage from the DC reading of the multimeter to obtain the minimum trough voltage.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 15, 2020, 12:08:36 am
content: Thanks again for your replys, yes the transformers are basically the issue. They are two MOT transformers with rewound secondary windings with 12AWG silicone cable, I just fell short of enough turns. And pining my hope on an LDR 12 Volt positive voltage regulator, and enough capacitance to keep the voltage up under load. Alternatively I could have dropped to 14AWG cable and achieved more turns for a lot less current. I just thought I might get 15 Amps from these two series wired transformers. Each primary receiveing around 120 Volts AC. I can get 15 Amps and more from one single transformer, but it runs just to hot. Idle primary current is around 3 Amps at 240 Volts with one transformer. So I decided to use two transformers in series. But just can't get the wire turns count on each secondary winding. I've got 14.00 Volts AC from the pair. Had I had say 18 Volts from the combination of the two transformers that would have been great. I know these transformers are poor performers, it's exactly that the drives me on to achieve a working reasonable linear power supply. I think I could get 18.00 Volts with 14AWG, but doubt I would safely achieve 15 Amps at 12.00 Volts.Thank you for the ocillscope tips, I bought this 60Mhz Hameg a while ago now, but yet to use it for testing in a project. I will try and see if it's possible to squeeze 15 Amps at 12.00 Volts DC with the 12AWG cable. The transformers run reasonable in terms of temperature, just over 40°C with the two primary windings in series, but it just doesn't seem to have the power from the primary, or a powerful enough mutual inductance coupled magnetically to the secondary windings.

author: bdunham7
author_rank: Super Contributor
timestamp: October 15, 2020, 01:01:57 am
content: Quote from: davelectronic on October 14, 2020, 02:49:37 pmIs that measured with a DMM and no load?What would really help is to know what the AC voltage is when you have your 200W worth of halogen lights connected, which I assume are drawing 16-17A or so. Then you can calculate how much headroom you have for voltage regulator and rectifier drop and see what capacitance it would take to get the ripple low enough to maintain adequate headroom. If it is close, you might gain nearly a volt by using Schottky rectifiers in the bridge.

author: srb1954
author_rank: Super Contributor
timestamp: October 15, 2020, 01:03:10 am
content: Quote from: davelectronic on October 15, 2020, 12:08:36 amRather than using silicone insulated cable you should use enamelled copper wire, sometimes referred to as "magnet wire", for the windings. Enamelled copper wire has thin insulation relative to the copper size so there is less space in the transformer window wasted on insulation.The object in designing a transformer is to cram as much copper as possible into the winding window to get the maximum power out of the transformer. By using wire with thinner insulation you can get more turns in for a higher voltage or you can use a larger wire size for more current/lower losses.I see that there is a UK-based eBay seller that is offering enamelled copper wire in sizes up to 12AWG. Perhaps you should investigate.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 15, 2020, 02:49:42 am
content: The 14.00 Volts AC straight from the transformer with 200 watts of halogen lamps 4 x 50 watt lamps, drops the AC Voltage 10.50 Volts. but with 90000uf of capacitance drops from 18.70 Volts DC filtered down to 13.90 Volts. I've tried enameled copper wire before, its a nightmare unless i split the core. But the magnetic circuit is never as good as it was before splitting it. I'm using silicone sheathed cable with an added layer of heatshrink tube to be sure of good insulation. I'm just wanting to know if i can get 12.00 Volts at 15 Amps with this configuration. if i could bump up the primary series voltage, to say 150 Volts AC I'm fairly certain that would eliminate the voltage drop i have now. but i don't want to use a variac, and don't know of an AC circuit that could add another 30 volts AC to each primary winding.

author: bdunham7
author_rank: Super Contributor
timestamp: October 15, 2020, 03:15:31 am
content: Quote from: davelectronic on October 15, 2020, 02:49:42 am10.5VAC is pretty low for a 12VDC output, but with the huge caps you might be right at the edge. You might barely scrape by with Schottky diodes and some massively paralleled LDO regulator setup to operate at that current, but you usually want 1.5 to 2 volts Vin - Vout for best regulation. What is your load and how sensitive is it to ripple or a bit of voltage drop?QuoteI can see using fine-strand cable with a thin silicone insulation just so you can actually do the winding, but the heatshrink tube sounds insane. Why do you need extra insulation at 14VAC?

author: srb1954
author_rank: Super Contributor
timestamp: October 15, 2020, 04:26:36 am
content: Quote from: davelectronic on October 15, 2020, 02:49:42 amNo need for extra heatshrink on the wire - the silicone insulation is already an overkill for a low voltage winding like this.

author: bdunham7
author_rank: Super Contributor
timestamp: October 15, 2020, 05:14:22 am
content: Quote from: davelectronic on October 15, 2020, 12:08:36 ami didn't read carefully enough. OK, you are actually doing OK by reducing the primary voltage and taking the transformers well out of saturation. I just looked at your photos and I seems that it would be pretty easy to get plenty more windings without the heatshrink. Did you knock out the magnetic shunts between the primary and secondary? I'm not sure how much difference they would make at full load in your case, but if you are trying not to totally rewind the transformers, every little bit will get you towards your goal.

author: srb1954
author_rank: Super Contributor
timestamp: October 15, 2020, 07:04:01 am
content: Quote from: bdunham7 on October 15, 2020, 05:14:22 amThe magnetic shunts will likely increase the leakage inductance which will further degrade the transformer voltage regulation.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 15, 2020, 07:59:42 am
content: Yes I did take out the shunts in both transformers, the wire is silicone sheathed. But it looked a bit thin in some cross section looking at it end on. I thought the heat shrink tube just added that guarantee of insulation under load where heat increases. But now on reflection, I can see I've lost at least three extra turns by the space the extra thickness is taking up. I have rewound them three times so far. First time was wire just not suitable, and to thick, second was also to thick, close to 10AWG. Third time is this silicone 12AWG wire, it's very easy to use as it's so flexible, and the silicone sheath is rated for 200°C I think. I just wander now, is it worth a final rewind with the same 12AWG, but no heat shrink tube. Or drop to 14AWG cable, but I don't think 15 Amps at 12.00 Volts quite as safe with 14AWG cable.

author: m k
author_rank: Super Contributor
timestamp: October 15, 2020, 08:35:06 am
content: Quote from: davelectronic on October 15, 2020, 12:08:36 amHow hot is too hot, coloring paper hot?

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 15, 2020, 02:50:13 pm
content: Yes I missed an earlier question, that is 14.00 Volts AC secondary with no load. To hot is 98°C measuring with a meter thermal couple slid in a small space down the side of the primary winding. That temperature is for a single transformer, the pair in series don't get much above 43°C or close to it. I'm torn between trying this as it is, or rewinding with 12AWG with no heat shrink added. I'm sure I could possibly get up to 16.00 Volts with the few extra turns on the secondaries in series still. And just an observation, the no load temperature of a single transformers primary is hotter idling than when it's drawing heavy current. That's for a single powered transformer. With 14AWG I could easily achieve enough turns, but I think that's a bit close to 14AWG limits on power drawn. I was asked application for this in a previous post (just looking back) It was intended to power a HF linear amplifier. So a mix of inductance and resistive load I think. And about a 50% duty cycle. Just a mention on the pair of transformers series wired idle AC power, it's a little over 1.25 Amps.

author: TheMG
author_rank: Frequent Contributor
timestamp: October 15, 2020, 03:48:51 pm
content: Wait... are the transformer primaries also in series? If so, that would be the reason the output falls off so much under load, there simply isn't enough magnetic flux in the transformers with each primary running at half of the designed/rated voltage.As for the transformers drawing lots of current and getting quite warm with no load, that is a characteristic of microwave oven transformers. In order to get the size, weight, and cost down to a minimum, they operate with the core into saturation. Not a problem in a microwave oven because the transformer is always under full load when operating, but this does make these transformers unsuitable for general power supply applications where the transformer might spend long periods of time with only a small load.I expect the "sweet spot" is going to be somewhat higher than 1/2 rated voltage. Low enough to take the core out of saturation, but not too low as to have poor regulation. Ideally, you'd have to add turns to the primary to obtain just the right flux.

author: m k
author_rank: Super Contributor
timestamp: October 15, 2020, 06:05:47 pm
content: Quote from: davelectronic on October 15, 2020, 02:50:13 pmHow hot it is with 10Amps, or 7,5?

author: bdunham7
author_rank: Super Contributor
timestamp: October 15, 2020, 07:32:32 pm
content: Quote from: davelectronic on October 15, 2020, 02:50:13 pmHow many turns do you have per transformer right now?QuoteYou'd have to check with a wattmeter, but hopefully that is mostly recirculating current or reactive power.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 15, 2020, 07:37:57 pm
content: I loaded the single transformer with 200 watts in 50 watt lamps X 4. The temperature got up to 98°C with this single transformer driven from the full 240 AC mains. The insulation class for the primary seems to be up to 220°C maximum, I've no intention of leaving a power supply running above 50°C let alone over 200°C. Yes the primary windings are wired in series, and I know the inductance fall's short of ideal. Each primary winding is getting 120 Volts AC, it's probably not enough. There's no room to add further primary windings, so that's not an option. If I could get each primary up to 150 Volts each primary, that might pick up the mutual inductance. But I don't know of a way to do that.Laying around I forgot I had an LM317 with 2 X MJ2955 transistors, so put the two transformers in to that circuit, as it's good for 8 to 10 Amps. Adjusted to 12.80 Volts DC from this circuit it dropped to 11.90 Volts DC with a 100 Watt lamp load. So it's not going to do 15 Amps at 12.00 Volts regardless of any high value of capacitance. I have removed the 12AWG windings now. I'm considering 14AWG with a lot more turns, I had 10 meters of 14AWG I completely forgot I had it. It's still an unknown until I rewind it and see how it behaves powered up, idle, then loaded up. Looking at the 14AWG cable it looks like it could handle 15 Amps at 12.00 Volts, especially as it's only going to see a 50% duty cycle. But again I know there is going to be a substantial voltage drop with the transformers primary windings wired in series, and getting half there rated voltage. A voltage drop is fine, as long as I can achieve 12.00 Volts with a maximum of 15 Amps.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 15, 2020, 07:42:11 pm
content: I think it was around 10 turns per each transformer secondary windings. I'm going to wind as much 14AWG cable on each transformers secondary. Hopefully I can achieve 12.00 Volts DC at a maximum of 15 Amps.

author: bdunham7
author_rank: Super Contributor
timestamp: October 15, 2020, 08:15:59 pm
content: Quote from: davelectronic on October 15, 2020, 07:42:11 pmHmmm... If you can get 14 or 15 turns of 12awg in there it would be better. And you could consider removing some primary windings if that is doable and that would increase your voltage and improve your regulation. The 14awg will increase your OC voltage, but make regulation even worse.

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2020, 09:59:21 pm
content: It is not clear what relationship your LM2940 has to a 20 amp output so I will just discuss the LM2940.The LM2940 has a maximum dropout voltage of 1 volt so I usually figure double that for the minimum input voltage, or 2 volts, and that much ripple, so 2 volts of ripple above the 2 minimum volts. The rule of thumb for input capacitors is 8200 microfarad-volts/amp, which means 4100 microfarads should be suitable for an LM2940. If you somehow had a 20 amp regulator with the same dropout as the LM2940, then it would be 20 times higher or 82,000 microfarads.If you are just missing your dropout voltage specification, then switching to big schottky diodes is more effective than massive amounts of capacitance.

author: m k
author_rank: Super Contributor
timestamp: October 15, 2020, 10:18:12 pm
content: Quote from: davelectronic on October 15, 2020, 07:37:57 pmIs there other reasons than personal discomfort?

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 15, 2020, 10:21:54 pm
content: Not to be pedantic but the OP is in 50Hz land so the rule of thumb would be 5000uF for 2 volts of ripple per amp with a full wave bridge. Or, as we yanks would write 100,000uF for the 20 amps.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 15, 2020, 10:32:19 pm
content: Walking away from a transformer at 98°C is not an option. I'd be ok with midd to high 60,s but not much more than that. I ever thought about removing primary turns to increase power as the transformers are in series. But how many turns ? These are 700 watt microwave transformers from my last two oven failures. Capacitance, I've made accommodation for 4 X 47000uf 40 Volt capacitors. But the primary turns removal sounds like a great idea.

author: m k
author_rank: Super Contributor
timestamp: October 16, 2020, 12:39:39 pm
content: It seems that I'm still nagging.Half of above ambient temp is already high 60 and not much more.Why you are not using those primaries in parallel?

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 16, 2020, 02:31:38 pm
content: If I use the transformers primary windings in parallel, I will get the secondary voltage I need easily. And the secondary current by the bucket load. But these transformers run very close to 100°C and probably higher if I monitored them powered up in parallel for longer. 40 minutes no load each in parallel get them up to silly temperatures, and I had 140mm pc fan above them. Looking at the original microwave oven front panel, the appliance doesn't have longer cooking times beyond 1 hour in timer settings. Ok so you could restart the over timer and continue cooking, but I believe at 1 hours cooking is right at the limits of the ovens use. I've cooked in these for up to an hour (varied cooking powers) But I'm certain I can detect a faint odour of transformer windings insulation smell. I doubt it's dangerous, but it's running flat out.The no load primary power drawn from two transformers wired in parallel is around 6 Amps at 240 Volts, that's just crazy for long term use. I already had one transformer from a magnetron failure some years earlier, then a second oven failed in much the same way. So I decided to try running these transformers in series to see what they can output safely. The pair of transformers run fine with half the input voltage, so that's great for the primary windings longevity. But any secondary windings suffer from lack of power probably as already mentioned down to the poorer mutual inductance with half the input voltage per transformer. I'm trying to get enough voltage AC secondary to power an LM2940-12 and 4 X TIP36C pass transistors to achieve 12.00 Volts and a maximum current of 15 Amps at approximately 50 % duty cycle to drive a HF linear amplifier. I don't know if it's possible to get this voltage and power from these transformers. But I haven't exusted all options just yet. I'm only a hobbyist, and some of the crazy ideas I come up with have taken weeks of thinking to find a way forward. It might be that I have to abandon this idea eventually, but that won't be before I've tried all possible safe options. I'm sure I was a mad scientist in a previous life. I have even considered boosting the voltage with a boost converter then taking that voltage and putting it back through a linear regulator circuit. It's probably not even doable, but I wouldn't disregard any idea until I've gone over that idea at least once. I appreciate all the help and suggestions, but I'm not ready to quit yet.

author: bdunham7
author_rank: Super Contributor
timestamp: October 16, 2020, 03:14:37 pm
content: Quote from: m k on October 16, 2020, 12:39:39 pmThe transformers he has are designed specifically for use only at full output (and a specific type of load at that), intermittently and in an application that doesn't have a particularly long life. They are way into saturation at full input voltage and no load, so they get hot and wasted power. To make a reasonably efficient transformer out of them, you have to add primary windings or run them at a lower voltage.

author: m k
author_rank: Super Contributor
timestamp: October 16, 2020, 05:59:03 pm
content: Ok, I've missed the donors.Have you tried to find out the max AC of not so hot situation?Like adding light bulbs and so.Quorahttps://www.quora.com/What-is-the-purpose-of-the-magnetic-shunt-that-is-in-most-microwave-power-transformers-Can-the-shunt-be-safely-removed-when-winding-a-single-replacement-secondaryhas a nice curve of what can be.

author: m k
author_rank: Super Contributor
timestamp: October 16, 2020, 08:14:14 pm
content: Have you thought of putting them in a row?Like primary in first, secondary in second and 1:1 or the like in between.I'm too lazy even trying to calculate what the double core means.If that is enough for the temp then you can also use those other original windings.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 16, 2020, 08:56:59 pm
content: If I've understood you correctly, that would mean splitting the core. I've done it once, cutting the shallow welds. And removing the primary is reasonably easy. But the transformer never really worked like it did before splitting it, but I didn't weld it up again. I used high temperature epoxy. I still think splitting the core makes the magnetic circuit even more inefficient. My next try is with 14AWG cable with the added heat shrink tube, it doesn't add much in terms of diameter. But heat shrinking 10 meters of cable with a 120 watt gas iron and hot air nozzle takes a while. Whilst I remember for any one rewinding one of these transformers, once you start running tight, talcum powder is your friend. So is a flat smooth implement to make way for extra turns. Bit like a marlin spike splicing cable, but go easy, you don't want to damage any turns.Anyway, if I can get the voltage closer to 18.00 Volts AC I could absorb the voltage drops and still have enough overhead to stay in the green, and get what I'm looking for. But it far from a sure bet, I think the 1000 watt MOT transformers would be far better contenders. The removal of primary turns really needs the whole winding removed to do it in a safe way.

author: m k
author_rank: Super Contributor
timestamp: October 17, 2020, 10:37:18 am
content: Quote from: davelectronic on October 16, 2020, 08:56:59 pmSorry, too zipped.Much easier, like a protection isolator, like mains in to Primary1 then Secondary1 and Secondary2 together and then mains out from Primary2.But it becomes more difficult since you like to keep those primaries.Anyway, if you now connect them like above how much they heat up unloaded?How experimental this is?Use sides of an E and get double space.No idea how that will do but in "theory" you can get a center tap.I read somwhere that serial secondaries are naturally balancing but how it is in this case I don't have a glue.Below link has a segmented toroid, so windings partially here or there should be reasonable.https://talema.com/segmented-core-cap-medical-transformers/

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 17, 2020, 01:18:50 pm
content: Errm, ok, still don't quite understand that. Mains isolated in to primary 1, secondary 1 wired in series to secondary 2 ? But unless there is power to primary 2, then secondary 2 is just added turns secondary to secondary 1. I'm confused. I'm trying to understand this.

author: m k
author_rank: Super Contributor
timestamp: October 17, 2020, 02:02:11 pm
content: You are also adding iron, it must mean something.It's then like isolation transformer with some extra parts.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 17, 2020, 03:11:14 pm
content: I will try it, but there is only small inductance in primary 2, but that's not of any use ? It's the added turns to secondary 1from secondary 2 turns in series with secondary 1. But no power on primary 2, I think. How can it increase secondary voltage when primary 1 has no power applied to it. Is it not just like adding ballast to secondary 2 ? I will give it a go, but can't see than increasing voltage out form either secondary, but secondary 1 in particular. Anything is worth a try.

author: m k
author_rank: Super Contributor
timestamp: October 17, 2020, 04:44:29 pm
content: It's a temperature test.Since your first problem is over heated idling this tries to figure out how far abowe the balance your MOTs are.Maybe you've already tried this at the beginning and with original windings.

author: m k
author_rank: Super Contributor
timestamp: October 17, 2020, 06:34:38 pm
content: Did you do it already, don't.It's a cascade anyway and current go up, no matter the temp.Adding primary rounds is a better approach.

author: m k
author_rank: Super Contributor
timestamp: October 17, 2020, 07:22:26 pm
content: The temp test still feels like a good idea but kidneys say it's a brain fart.Better stop dreaming and start calculating.I have few UPS transformers, machine type was ower 600W.They are quite balanced and opposite idling current is 300mA.Core seems to be EI and is 7,5x6,5x5cm and ratio is 25:1.What kind of cores your MOTs have?

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 17, 2020, 07:55:50 pm
content: I will pull out a Vernier and see how large the appetures are. It's not much though. If I could have found a couple of 1000 watt MOT's that would have been much better. If I had a milling machine, I'd have removed a little material to allow for a few more turns count. But not two much otherwise the magnetic circuit would be worse.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 17, 2020, 11:15:56 pm
content: A bit more persistence, I think I've got it. Rewound both transformers with the silicone sheathed 14AWG with an added heat shrink layer (I wouldn't be happy with out the extra insulation) Drawing it through over and over is demanding on the silicone alone. Starting out with 10 meters, I got as many turns on transformers 1 (right in the image) Then continued winding on transformer 2. There is a slight mismatch in turns, few less on transformer 2. But run out of cable, more on the way. But it's looking promising, I'm getting just over 19.00 Volts AC no load. And around 16.20 Volts AC with a 200 watt halogen lamp load. When the extra cable arrives, and wound on the same turns as transformer 1, I think it will come to more than 20.00 Volts AC. The transformers are still wired in series, primary and secondary. With a bit of luck I should get the 12.00 Volts DC at a maximum current of 15 Amps. This test is just AC voltage straight from the secondary of both transformers.

author: m k
author_rank: Super Contributor
timestamp: October 18, 2020, 05:49:25 pm
content: It's close.Were your earlier 40 something temp with or without extra cooling?If with then what it is without?All primary winding addendums are still unknown waters but then your secondary space is even smaller.Winding around a side is a normal thing, UI cores and the likes have no choise.Other thing is what kind of a hassle that would be but then you could use them primaries in parallel.If you're still taking that one secondary out you can try that primary adding a la Quora link and then you would at least know the left over space.Here UPS trafos are 1/2 + 1/2 + 1 + 1/2 + 1/2 inch wide.So air space is 1/2 inch a side wide and around 13/8 tall.Iron is then around 250 - 50 cm3.My "Mauser" is from USSR and not for very small places, others are plastic and not for even that accuracy.One wire is around 1mm wide and other is less than 2, maybe around 1,5.No idea how thick the insulation is.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 18, 2020, 08:40:30 pm
content: I have some more turns to go on transformer 2 run out of cable. Final AC voltage should be around 22 to 24 Volts with no load. With a heavy load it's going to drop 3 to 4 Volts, guestamat. The 46°C was no load with both primary windings in series. I did measure primary voltages, one was 122 Lots AC the other was closer to 125 Volts AC. Tea / dinner time in the UK we drop a bit of voltage. Normally 240 Volts, can drop to 220 Volts at peak times.I've cut two 80mm holes for fans, bottom for transformers, top fan cooling transistors. I didn't want an external heatsink, so going to use a plate of aluminium 100mm X 200mm X 20mm it's substantial. Hopefully what it lacks in surface area, it makes up for in mass. Well that's the plan. Should be able to start assembling it soon. I'm only going to do the MOT thing once, unless some 1000 to 1200 watt transformers drop out the sky my way. It's quite cool repurposing what would otherwise be junk. I will measure the transformers appeture, but I'm fairly sure all the 220 Volt 700 watt oven transformer are the same. I've never opened a UPS to see the transformers in them, Mauser as in arms company ? Not sure what you mean about that, other than accuracy.

author: m k
author_rank: Super Contributor
timestamp: October 19, 2020, 09:53:33 am
content: It's Mauser Vernier, we've historically being in the other side of the no mans land.Is your power going to be a round the clock type?It could do a bit more relaxed work without that tea time mains drop.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 19, 2020, 08:10:26 pm
content: Yes I was planning to use it as a PSU for HF Radio, powering a transceiver and a linear power amplifier. It's only during TX that the power demand ramps up, at RX or standby it will draw not much more than a few hundred milliamps. It is a bit wasteful when you look at the numbers when the transformer is fairly idle. Around 1.25 Amps, that could be from 275 watts to 300 watts or so. I cleverer guy than me might be able to devise a circuit that switched over to a smaller power friendly transformer for RX purposes. But when power is demanded switch over to the MOT transformers. Interesting idea. But a switch over needs to be fairly quick initiating, but once no more power is demanded, switches back to low power transformer.

author: m k
author_rank: Super Contributor
timestamp: October 20, 2020, 09:38:12 am
content: If the thing finally fails and you don't like those earlier alternatives there are still some.Make an electomagnet that is in serial with a single MOT primary.But since you have two MOTs you can also use one for transmitting and two for listening.For transmit use a zero point switch and disconnect listening state, so that for listening state you have two MOTs in serial and then a bit before transmitting you change it to one MOT only in use.With equal secondaries the regulator can be the same.Secondary switch may have some timing issues but if it is guarateed later than primary the capacitor pack handles the situation easily.Then dimmer.You can also use regular AC dimmer for listening state.Capacitor pack guarantees your listening ripple is not more even with heavily limited primary.You still need a connector between the states but dimmer it self can be it.You can also change dimmers mechanical limit so that then you can just turn it to the south-west for listening.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 21, 2020, 01:46:31 am
content: Interesting ideas, i thought dimmers where not usable for inductive loads. Not the resistive types anyway. if i could have changed the wave form in the primarys (with out over heating the windings) I would have used a dimmer on the single primary winding. I have finished the pair of transformers in series, both primary and secondarys. I'm getting 20.70 Volts AC with no load, and 17.60 Volts with 200 watts of halogen lamp load, 4 x 50 watt lamps. The cable i used is advertised as being capable of currents up to 30 Amps at 12.00 Volts. I don't know it could handle that much current, but at 200 watts, i'm almost certain it can handle a bit more current. i'd say 18 Amps to 20 Amps with a 50% duty cycle wouldn't be unreasonable, although i'd expect to drop a bit more voltage form the secondary input due to the lower series primary input. But its looking good so far. I didn't do the stat's for a 200 x 100 x 20 mm piece of aluminium plate as a heatsink for 4 x TIP36C transistors. And the rectifier, i wasn't happy with the aluminium it was bolted to with the transformer chassis assembly. Hopefully with air flow it will keep things cool.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 21, 2020, 02:29:17 pm
content: Thank you for all the help with this issue, hopefully I've accomplished a usable power source in the two MOT transformers. Just sharing a tip or two, I found silicone PTFE spray really helped max out the windings count. Also made a purpose tool (old screwdriver) rounded off and polished the tip (now bull nose for want of a better discription) pushed in the other side allowing the cable to follow this tool until I could pull it through, and creating more turns count. Another quick tip on checking voltage per turns count, a dress makers pin pushed through the insulation to check the voltage. The tiny hole closes up when the pin is withdrawn. You could just take a reading on voltage from the end your winding, but I felt volts drop over a length of unwound cable might not give an accurate result.Thanks again for the help.