Strictly Linear PSU's

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Buick Mackane
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Re: Strictly Linear PSU's

Post by Buick Mackane » 07 May 2017, 19:08

Forget about microwave transformers and other such things, If you want a proper meaty mans power supply check this badboy out for size ;)
Enough power to crank over a hillman imp on a frosty morning, :D
http://www.ebay.co.uk/itm/13-8v-3A-CB-H ... SwN6JY-6hg

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Re: Strictly Linear PSU's

Post by Admiral » 07 May 2017, 19:21

Even Buick Mackanes Brother in law Big Clive who seems pretty clued up on this stuff had a MOT vid pulled by youtube to stop the tards from killing themselves.
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Re: Strictly Linear PSU's

Post by sureshot » 07 May 2017, 19:40

Buick Mackane wrote:Forget about microwave transformers and other such things, If you want a proper meaty mans power supply check this badboy out for size ;)
Enough power to crank over a hillman imp on a frosty morning, :D
http://www.ebay.co.uk/itm/13-8v-3A-CB-H ... SwN6JY-6hg
It's OK for the case transformer and hardware, but I'd build a new circuit for an old box like that.

MOT or microwave oven transformers often get modded into spot welders, they can pass massive amounts of current at low voltages. As the transformer for a radio psu, I'd only use it for powering a serious amplifier for a DX session. The power is there in bucket loads, but the quiescent current drain on the primary is high. With a previous MOT I measured a no load primary current of 3 Amps, that creates a lot of heat to dissipate. But for a DX session it won't chew up to much of your electricity.

You need to constant cool the transformer with a decent axial fan. In the unit I've planned the cooling fan will spin on start up at about the speed of a pc case fan. A 12 Volts pc axial fan is ideal, a 47R resistor will keep it spinning at a modest rate. I'm adding a thermal switch to short out the fan series resistor, then than fan will be at full tilt until the temperature comes down under the value temperature of the thermal switch. The main constraints become pcb traces and wire sizes, everything has to be big to carry the large currents. I see a MOT as a means to an end, it will deliver the current for sure, but it's not energy saving in anyway. The main reason I'd only power it on for a DX session to power that 300 watts plus amplifier, during long RX periods I'd rely on a conventional modest power linear or smps power supply. It's just a bit of fun really, for me proof of concept, be it a power munching psu. :D
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Re: Strictly Linear PSU's

Post by sureshot » 07 May 2017, 19:45

Admiral wrote:Even Buick Mackanes Brother in law Big Clive who seems pretty clued up on this stuff had a MOT vid pulled by youtube to stop the tards from killing themselves.
Check out Awsomematt on you tube, completely safe if used for low voltage applications. The biggest users projects are spot welders. Awsomematt frequents a US electronics forum, his very experienced at all sorts of great projects, MOT'S being one of them. But no mains project is for the inexperienced, Big Clive is great by the way, I watch all his stuff.
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Re: Strictly Linear PSU's

Post by sureshot » 07 May 2017, 19:51

Have a look !
https://youtu.be/-NLy-LL_TGQ

PS. This one's more refined, although he's needs are different. But he makes a good job of it.

https://youtu.be/m-y0Cjx7KeE
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Re: Strictly Linear PSU's

Post by sureshot » 07 May 2017, 20:54

As I've said before ! If you are contemplating a MOT project, have a good look around the Internet, YouTube has many great MOT projects, if undertaken with the same caution as any mains voltage project it's safe. Any psu project can be considered dangerous if you don't follow safe practice. A MOT with a low turns count secondary can be termed the same as any other linear transformer, be it a little wasteful when idle. The only difference is a retail purchased transformer will have greater efficiency overall. Building a power supply around a rewound MOT is no more dangerous that modding server power supply, the same rules on safety and electricity still apply. It's possible to produce a really useful project and it be perfectly safe. The rule with MOT's is that it's never powered up with its high voltage secondary winding. This winding is removed, and a low turns count high current secondary is rewound in its place. As I've said check out MOT projects on YouTube, there are loads. I don't say there all perfect, but YouTube would have pulled them if deemed unsafe, being the project is undertaken with safety in mind. A plug in RCD (residual current device adaptor) will ensure and limit the risk of any earth fault during testing. If you've got this far, you know the do's and don't's for final safety on a finished project. I'm doing this as a proof of concept project, and will power a modest amplifier from the finished project. As long as electrical safety practices are followed, there shouldn't be a problem. And once again, modding stuff is not for everyone ! If you need a high current low voltage supply, and modding is not for you ! Purchase of a retail psu is the way to go. Might be a while before I've anymore progress, but will post up any progress as I go. If I thought for one minute it posed a risk, I wouldn't be doing it.
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Re: Strictly Linear PSU's

Post by Admiral » 07 May 2017, 22:32

I wasn't trying to wee on your bonfire sureshot, far from it, I just wanted to add an element of caution surrounding MOTs to the wider audience as they are a different bread to regular linear transformers, their science is very different regarding primary and secondary outputs, they are crude, cheap, and inefficient, they also have a disproportionate amount of amps in an unmodified state.
Unless you are technically capable of rewiring primary and secondary coils, leave MOTs alone as they are lethal.
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Re: Strictly Linear PSU's

Post by sureshot » 07 May 2017, 23:27

Admiral wrote:I wasn't trying to wee on your bonfire sureshot, far from it, I just wanted to add an element of caution surrounding MOTs to the wider audience as they are a different bread to regular linear transformers, their science is very different regarding primary and secondary outputs, they are crude, cheap, and inefficient, they also have a disproportionate amount of amps in an unmodified state.
Unless you are technically capable of rewiring primary and secondary coils, leave MOTs alone as they are lethal.
No problem "A" Agree totally.
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Re: Strictly Linear PSU's

Post by sureshot » 21 May 2017, 20:05

Well it's been a while..
Little more progress, I've had some issues with such a small transformer core. As it's only a MOT from a 700 watt machine, there's little space on the core for windings that need heavy gauge wire. If anyone tries similar projects, look out for 900 or even 1000 watt MOT, the core would be much bigger.

I'm going with what I've got in this 700 watt unit, after a couple more attempts at trying to increase winding count, I've made little progress at all.
So I've got 14.70 Volts AC no load, on the up side it only drops 1 Volts with a 300 Watt halogen lamps load. Things are tight on the voltage side, ideally I'd have liked 18 Volts or a bit more for head room.

What I'm going to end up with is shottky diodes rectifier bridge, or maybe even a mosfet bridge. And a low drop out voltage regulator. The LM2940C-12 is what I've got in mind, but not 100% sure on that yet. I'm at the looking round for components suitability stage at the moment. To make the headroom under load I need a rather large capacitors filter board to maintain peak voltage under load. I'm still mulling it over, there are a few tricks out there for a control board for steady regulation, even with little drop out headroom.

I ended up with 32 Amp wire on the core, it's silicone covered high temperature wire, but I've added a layer of heat shrink for longevity. I know some might scream cooling, but it adds very little barrier to cooling, and much greater insulation resistance. An offer up for size in the picture below, just rough at the moment. More to follow, be it slow. There will be a second board above the filter capacitor board for control regulation, and probably 100000uf below it.
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Re: Strictly Linear PSU's

Post by sureshot » 28 May 2017, 21:41

Not much more, just a bit for the minute. Waiting on some longer machine screws for the heatsinks to stand off, allowing maximum air flow through the case. These cases are cheap of eBay £12.99 , old stock from a dissolved company I think. They have a few holes punched out the steel already, where practical I try to use these first. Gone with 6mm aluminium plate for heatsinks, it's cheap, heatsinks are really expensive. The rear ones for the rectifier bridge, and one each for the transistors. I only plan on 10 to 12 Amps use, it's more proof of concept using a MOT. Dam it's going to weight over 10kg or there abouts. The blue sockets are for direct AC secodary take of if it's needed. The 4mm binding post are from an old audio project I never got round to, clear heatshrink covers them, don't fancy any dodgy shorts. Just waiting on the capacitors and the machine screws for the heatsinks.

I've got a feeling I might need a fan on the rectifier heatsink, the first project using this same case has a 100 watt cpu heatsink for the rectifier, and I've had that over 85°C That square off aluminium on the back is only 100mm x 100 mm, the transistor heatsinks are 200mm x 100mm. The missile switch cover was just a bit of fun really. Not having a jigsaw, the top fan was cut out with a coping saw, quite trying that was. Be back with any progress in the near future. :)
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Re: Strictly Linear PSU's

Post by oh5nxo » 29 May 2017, 05:56


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Re: Strictly Linear PSU's

Post by sureshot » 29 May 2017, 18:09

oh5nxo wrote:Space left for one of these ?

https://www.digikey.com/en/product-high ... ter-series

Juha
There probably is space for an IEC mains chassis socket, but I'd have a right old fun time fitting it with out a jigsaw, a tool i don't have unfortunately. But neat solution for mains power inlet.
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Re: Strictly Linear PSU's

Post by sureshot » 19 Jun 2017, 21:10

Got a bit further, it's getting a bit weird and wonderful lol. Stand offs on the back fan are a bit long, I needed them to mount two pcb's inside the case. It was never going to be beautiful, more to see how a MOT would run an amplifier. It's coming on slowly, I got to go with what I've got now, no more wonga on it. Only be me that ever uses it anyway. I will post up the final result soon, the voltage regulator is an interesting mount option using a graphics care heatsink and mounted reverse side of the board (another picture to come) Any way it's getting there slowly. :)
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Re: Strictly Linear PSU's

Post by sureshot » 28 Jun 2017, 02:02

Coming on slowly, I shortened those long stand offs on the rear fan. Looks bit tidier like that. Had to use a lot of capacitance as had very little regulator headroom to work with, even with an LDR regulator. Bank of 9 capacitors 10000uf each. If I'd have had a 900 or 1000 watt MOT, I could have used less than half that capacitance. So if your looking for a MOT bigger is better, more secondary turns equals higher output voltage, and headroom for regulator, rectifier and any drop across transistors. Getting there slowly, secodary rectified and filtered is 21 Volts, should be fine for straight 12 Volts. The transformers drop, even under a 300 Watt AC load was less than 1 Volt. But not surprising given the amount of grunt these things have. Last post when I've finished it, and it's powered, unloaded, and with load. Be interesting to see any voltage drops, hopefully minimal.
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Re: Strictly Linear PSU's

Post by sureshot » 08 Jul 2017, 22:05

Getting there, control board nearly finished, just bullet joints to solder to leads. There for hook up, and quick disconnect for the input, output, and transistors. Strip board very rarely looks pretty, it's not quite the same as pcb art work. But it achieves the means to function, good continuity through out, and heavy solder traces where there needed. Just some grommets for the pcb holes, and bolt it in and wire up. Testing the board gives 12.60 Volts with a 21 Volts DC input. The MOT was 14.70 Volts AC secondary. Should be interesting to see how it behaves under load. Slight hard ware modification was needed, the MOT cooling fan Fouled the pcb studs, so fan on the outside. Not what I planned, but lesson learned, check and double check the lay out of hardware.
Last post when it's finished.
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