Strictly Linear PSU's

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Re: Strictly Linear PSU's

Post by sureshot » 22 Apr 2018, 23:05

Got side tracked with the graphics card heatsink boards...
Wasn't going to post this, but might interest someone.
Needing a variable testing power supply, it's something in progress. I don't have milling machines or cnc etc.
Have to make do. Will be TO220 lm317, or similar, there MJ2955 and MJ11015 transistors.
Yes the holes for the screws are blind, so isolation seems to be fine that side of the screw. Other holes for regulator, and large centre holes for transistor wires to loop back through. A mm or so off on one hole, it's only feeding wire back through so not critical.
Regulator heatsinks are one side cut off to fit the board. Makes a small package 100 watt or there abouts unit.
Hopefully get an enclosure and meters this time, I'll only end up using one of those in the picture, undecided which yet.
Drilling cutting tapping does take time, precision is not easy with out machine shop tools.
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Re: Strictly Linear PSU's

Post by radio pete » 23 Apr 2018, 10:06

Nice work neat job

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Re: Strictly Linear PSU's

Post by sureshot » 23 Apr 2018, 18:21

radio pete wrote:
23 Apr 2018, 10:06
Nice work neat job
Cheers Pete :)
Anyone else doing anything interesting with linear power ? Post it up if you have. :)
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Re: Strictly Linear PSU's

Post by sureshot » 27 Apr 2018, 23:02

Tiny bit more progress, can't believe it would take so long to mount 4x transistors, solder and heatshrink.
A few years ago I stumbled on a Watson power supply, sadly it had been driven to death. Anyway the two output transistors had been factory mounted with nylon machine screws.
One had partially melted and lost contact with the heatsink.
I have been thinking of giving nylon a go for a while, but added some red loctite to each fastner for a bit more insurance on heat stability.
As long as the heatsink is under 80°C it should hold up. I'm curious to see if nylon holds up.
Through the centre of the heatsinks are the base and emitter leads. Collector off the cases.
Can see the wrap through from behind the heatsinks. Fans to refit.
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Re: Strictly Linear PSU's

Post by stanogs68 » 27 Apr 2018, 23:35

thats top work ,brilliant
computers mess with my brain !

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Re: Strictly Linear PSU's

Post by sureshot » 28 Apr 2018, 10:26

stanogs68 wrote:
27 Apr 2018, 23:35
thats top work ,brilliant
Thanks pal, they don't look much at this stage. The unique attraction is the small foot print for power size. The boards are 100x150mm For a power rating of 100 watts or close to it, that's not a bad footprint at all. It can be scalled up, more of the same cpu heatsinks for greater power handling capabilities.
Its a slow process unfortunately for me, but i will try and see it through to an end usable unit.
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Re: Strictly Linear PSU's

Post by sureshot » 17 Jun 2018, 23:07

It's been a while since I sat down with an iron..
Some more progress, although I'm not sure if it's the circuit that will end up in my purpose build bench test psu. On a plus there was no trouble shooting to do, as often there can be.
At the low end I was expecting 1.25 Volts, it's an LM317 so not to 0 Volts. But below 1.5 Volts is not usable for me, although it would be cool to achieve 0 Volts. In the images the low end shows just over 1.3 Volts, I know the data sheet says 1.25 Volts typical and 1.29 Volts maximum. I think it's that old meter, it's years old now. On the 20 Volt range that's as far as I took it. And switched ranges to get the maximum of 26 Volts.
The transformer is a toroidal type with two primary, and two secondary windings. The primary is in series for 240 Volt operation. The secondary is in parallel for maximum current, the transformer is 80VA at 18.0 - 18.0 Volts. That comes out at just over 4.4 Amps maximum.
Testing was with a 50 watt halogen lamp, just over 4 Amps was very steady at the high end of 12 Volts. The heat dissipation was reasonable, when it's in an enclosure there will be a cooling fan.
For testing circuits I think that's fine for a starter adjustable bench power supply. Still to put in some circuit protection both primary and secondary on a separate board. I've got a 10 turn precision potentiometer on it's way hopefully. Case and hardware are under the cuboard. That just leaves the meters, analouge or digital I'm not sure. I would like to look into adjustable current limiting, but yet to find something usable for this circuit.
Anyway it's a bit more.
Note ! If you play about with electronics, stay safe. Mains voltage is a serious thing, and great care should be taken. Use of an RCD is recommended.
If your at all unsure, please go out a buy what ever power supply you might need. :)
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Re: Strictly Linear PSU's

Post by sureshot » 18 Jun 2018, 02:24

In case anyone was interested in the circuit above..
Look at the 6 x transistor circuit on page 1 of this thread, knock off 4 x transistors. Use an LM317 or LM338 voltage regulator instead. Resistor values can be the same as the 6 x circuit on page 1.
Below is the circuit diagram, add two, or more series pass transistors for greater current. :)
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Re: Strictly Linear PSU's

Post by sureshot » 20 Jun 2018, 22:49

Been looking about to try and drop that 1.34 Volts to 0 Volts. Many over engineered ideas I've come across. Opamps and second regulator to drop below 0 Volts. It's a bit wasteful, but two silicon rectifier diodes drop 0.7 Volts each. You can't get below, or true 0 Volts, but that looks the simplest way to do it. I've yet to give it a go.
Over the years I've never owned a lab supply, just put circuits together and power them up with what ever was to hand, not ideal I know. Whilst the LM317 and multiple pass transistors isn't going to make a super class lab supply. It's a big step up from grabbing what evers to hand.
Got a case and parts, just looking at a different hardware circuit configuration before putting it together.
It will be handy for learning them radio or amp repairs that screwdriver experts have messed up. Well the power source for testing anyway.
More to follow soon...
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