Strictly Linear PSU's

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Re: Strictly Linear PSU's

Post by sureshot » 18 Apr 2017, 12:23

A little more progress, just the transistors and regulator to wire up now. I went with an LM7812K as opposed to an LM317K, i am only needing 12 Volts. Its a genuine motorola regulator, lucky to find these new old stock. My expectations won't exceed 150 watts total load, so should run warm but not burning hot. Future proofed with a thermal switch, just in case i add fans in the near future. Although i do prefer silent operation. I will post the final test and load metered pictures in the near future hopefully.
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Re: Strictly Linear PSU's

Post by Buick Mackane » 18 Apr 2017, 12:39

Fantastic work dave. Wish my projects were as neat as yours.
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Re: Strictly Linear PSU's

Post by sureshot » 18 Apr 2017, 18:45

Buick Mackane wrote:Fantastic work dave. Wish my projects were as neat as yours.
Cheers mate, it really is a simple circuit, it only looks more complicated because of the wires to terminal blocks. This makes it serviceable friendly with out having to unsolder a ton of wires. I know most people prefer the LM723 as the back bone for a linear psu, if I'm here long enough maybe I'll get to try a psu using the LM723. The LM317K and LM7812K and TO220 variants are over looked as not usable for high current use. And the PNP emitter follower is not worth the time with such monolithic regulators. I say the circuit at the start of this thread is an excellent start place for a first go at a high current linear psu. Performance is very good, but you do have to add some output protection for over voltage and short circuit. Current limiting is not really a problem, as this is what where after for driving power hungry Amplifiers. If you use the LM317, a custom output voltage is obtainable with just two fixed value resistors, or a potentiometer. Its not going to save money in the pocket, unless you have a transformer knocking about already. And a project case is a bonus if you've got that as well.

The transformer was the best part of £40 but the case was an ebay find for £12 bargain for the case i thought. Transistors are old new stock, genuine motorola MJ4502's but you can use MJ2955's or TIP36's etc any high current PNP should be fine. Its just a hobby really. I know its an odd one, but I've had a fascination for power supplies from a kid playing with scalextric and train sets lol. Everything , or most stuff we use needs power of some sort. Its just an interesting subject. I like the modding stuff just as much. The magnetron just went pop in the microwave a week ago.... You can guess what i gleaned from it lol.
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Re: Strictly Linear PSU's

Post by stanogs68 » 18 Apr 2017, 18:57

sureshot wrote:
Buick Mackane wrote:Fantastic work dave. Wish my projects were as neat as yours.
Cheers mate, it really is a simple circuit, it only looks more complicated because of the wires to terminal blocks. This makes it serviceable friendly with out having to unsolder a ton of wires. I know most people prefer the LM723 as the back bone for a linear psu, if I'm here long enough maybe I'll get to try a psu using the LM723. The LM317K and LM7812K and TO220 variants are over looked as not usable for high current use. And the PNP emitter follower is not worth the time with such monolithic regulators. I say the circuit at the start of this thread is an excellent start place for a first go at a high current linear psu. Performance is very good, but you do have to add some output protection for over voltage and short circuit. Current limiting is not really a problem, as this is what where after for driving power hungry Amplifiers. If you use the LM317, a custom output voltage is obtainable with just two fixed value resistors, or a potentiometer. Its not going to save money in the pocket, unless you have a transformer knocking about already. And a project case is a bonus if you've got that as well.

The transformer was the best part of £40 but the case was an ebay find for £12 bargain for the case i thought. Transistors are old new stock, genuine motorola MJ4502's but you can use MJ2955's or TIP36's etc any high current PNP should be fine. Its just a hobby really. I know its an odd one, but I've had a fascination for power supplies from a kid playing with scalextric and train sets lol. Everything , or most stuff we use needs power of some sort. Its just an interesting subject. I like the modding stuff just as much. The magnetron just went pop in the microwave a week ago.... You can guess what i gleaned from it lol.
fancy making me one sureshot :thumbup: :)
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Re: Strictly Linear PSU's

Post by sureshot » 18 Apr 2017, 19:59

Thanks for the inquire, but its just a hobby. i keep everything i make. thinking of it..... I've got psu's in boxes from 6 years or more ago. Its not that hard to do one yourself. But do some study first as mains voltages are involved. :)
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Re: Strictly Linear PSU's

Post by sureshot » 18 Apr 2017, 20:11

That salvaged MOT from are popped microwave oven, found some high temperature silicone wire to rewind the secondary for about 15 volts. If you do reclaim a MOT (microwave oven transformer) do your home work first. On the plus side there cheap and easy to come by, and on the down side the primary idle current is high, and they need air cooling as they run hot with out it. And my advise is never play with the transformer with its original high voltage winding. They can top 2000 Volts AC incredibly dangerous. But ok for a psu project where you want cheap high current.
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Re: Strictly Linear PSU's

Post by stanogs68 » 18 Apr 2017, 20:18

ive seen people on ebay using micro oven parts to fry radios there powerful arnt they
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Re: Strictly Linear PSU's

Post by sureshot » 18 Apr 2017, 20:37

stanogs68 wrote:ive seen people on ebay using micro oven parts to fry radios there powerful arnt they
They sure are, but there not very efficient when just idle. Primary current idle can be as high as 3 Amps, quite wasteful in electrical terms. We can add primary windings to improve this, but then you have to increase secondary turns for that same voltage you want. Check out AwsomeMatt MOT winding on youtube.
For use in powering Amps there ok i would say, but wouldn't use a MOT for continuous use project. For radio its only used for a session with an amp, then you can power it down. Otherwise continously you'd be chewing up your electricity. But its a means to an end, at a budget as well.
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Re: Strictly Linear PSU's

Post by Admiral » 19 Apr 2017, 06:47

Jesus, those things need a health warning,

' WARNING, this unit is guaranteed to kill most idiots and quite a few semi-intelligent people too'

You wouldn't catch me messing about with one of those, they're deadly, people should only think about it if they really know what they're doing.
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Re: Strictly Linear PSU's

Post by sureshot » 19 Apr 2017, 09:29

[quote="Admiral"]Jesus, those things need a health warning,

' WARNING, this unit is guaranteed to kill most idiots and quite a few semi-intelligent people too'

You wouldn't catch me messing about with one of those, they're deadly, people should only think about it if they really know what they're doing.[/quote



Agreed, its for people with a lot more experience with electrical projects etc. But the one above could give me currents as much as 30 Amps with the correct gauge secondary wire. Once its rewound its a normal 240 Volt mains transformer, be it a little wasteful when idle. A lot of people make spot welders and other stuff with them. I might use it for a future psu project, it still needs rectification and regulation to useful as a radio power supply.
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Re: Strictly Linear PSU's

Post by Admiral » 19 Apr 2017, 13:51

For the average Joe reading this then sureshot seems to know his stuff, for Gods sake don't be pulling these MOTs out of microwave ovens in skips as they are absolutely deadly and they will kill you, you don't even need to touch the thing as they are so amped up they can and will arc at you, you wouldn't catch me within 10 feet of one of these things. Volts don't kill you, but amps do.
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Re: Strictly Linear PSU's

Post by sureshot » 19 Apr 2017, 22:10

Admiral wrote:For the average Joe reading this then sureshot seems to know his stuff, for Gods sake don't be pulling these MOTs out of microwave ovens in skips as they are absolutely deadly and they will kill you, you don't even need to touch the thing as they are so amped up they can and will arc at you, you wouldn't catch me within 10 feet of one of these things. Volts don't kill you, but amps do.

Yes not for the average joe. You need experience to play with a MOT (microwave oven transformer ) If in any doubt with any project, then your better off buying a retail power supply ! Modding stuff is fun, but very dangerous if your inexperienced. Save the modding until you've got some low voltage battery circuits under your belt.

If your interested start out with books the internet etc trawl through books, and start with battery circuit projects. Electronics is a lot of fun, but there is a lot to learn.
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Re: Strictly Linear PSU's

Post by sureshot » 27 Apr 2017, 21:10

Finally finished this power supply, I think it's taken about 4 months or more. For now it's the last one I'm building. Not the best image, the transformer is rated at 20 Amps and is a toroidal type. My goal was 15 Amps maximum, with 10 Amps typical constant and 12 Amp 50% duty cycle.

At 12 Amps I'm dropping 0.4 Volts, this is from after the regulator. The voltage measured from the circuit board regulator input / output still measures 12.04 Volts. It's forward from this where the drop is, fairly typical of linear regulators. On reflection I could have used an LM317K and set the output voltage higher. My DC rectified voltage is 22 Volts, the transformer is a 15 Volts AC secondary. In reality it reads 17.2 Volts AC unloaded. I'm happy with it for what I'll use it for. A couple more pictures on my little cybershot, but I've not uploaded them to the pc yet, post them another day.

I really wanted to build a monster, but I had many constraints working against me. All in I'm happy with the result. If I get a break in the near future I might try something bigger. But will try to do some smps modding in and out. Check out the server power supply thread. :)

PS, No idea why it posted it side ways, the cybershot pictures should look clearer. Post another day.
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Re: Strictly Linear PSU's

Post by sureshot » 28 Apr 2017, 01:45

Just another couple. I might swap out the 7812K for an LM317K or even a 7815K and drop a Volt from the regulator. Undecided..
See how a shade under 12 Volts goes. Most gear will run happily between 11.40 Volts and 14.40 Volts.
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Re: Strictly Linear PSU's

Post by ch25 » 28 Apr 2017, 07:22

Does it have over voltage, current and shortcut protection?
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