Stuck on capacitance

[bdunham7]

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.

Hmmm...  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.

[David Hess]

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.

[m k]

I've no intention of leaving a power supply running above 50°C

Is there other reasons than personal discomfort?

[WattsThat]

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.

[davelectronic]

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.

[m k]

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?

[davelectronic]

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.

[bdunham7]

Why you are not using those primaries in parallel?

The 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.

[m k]

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.

Quora
https://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-secondary
has a nice curve of what can be.

[m k]

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.

[davelectronic]

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.

[m k]

If I've understood you correctly, that would mean splitting the core.

Sorry, 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/

[davelectronic]

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.

[m k]

You are also adding iron, it must mean something.

It's then like isolation transformer with some extra parts.

[davelectronic]

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.

[m k]

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.

[m k]

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.

[m k]

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?

[davelectronic]

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.

[davelectronic]

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.

[m k]

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.

[davelectronic]

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.

[m k]

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.

[davelectronic]

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.

[m k]

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.