Electronics > Projects, Designs, and Technical Stuff

Weekend project: 9V 2A linear DC power supply

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richlooker:
I have a bunch of micro hardware synthesizers and effect units powered by 9V DC. Frustrated by the chaotic mess of wall warts and the noise induced by these cheap switch mode supplies, I decided to build a PS to power them all - a 9V 2A linear supply with 10 output sockets. No need for PCBs for such a simple design; I ordered some parts, got them the next day, and assembled this during last weekend.

2 x 9V 30VA toroidal transformer, bridge rectifier, 2 x 10,000 µF filter capacitors. L78S09CV regulator, aluminium oxide insulator, 3°C/W heat sink. C8 AC socket, SPST toggle switch, 5x20mm fuse holder. Red LED, led holder, 10 x 5.5/2.1mm DC sockets.

Tested for an hour with a lamp that draws ~1A, 8.9V and runs cool. At 1.5A load, measured 0.5mV RMS AC at output.














Cheers, Richard

Pawelr98:
Why isolate the regulator ?
Without isolator the thermal resistance would be lower and the case could also work as a ground terminal, saving on wiring.

Seekonk:
That has to be ground loop heaven. Great if you don't know the words to the music. All my audio devices require 9V AC so they can get the +- for opamp.

richlooker:

--- Quote from: Pawelr98 on July 14, 2020, 11:36:31 pm ---Why isolate the regulator ?
Without isolator the thermal resistance would be lower and the case could also work as a ground terminal, saving on wiring.

--- End quote ---

I just assumed it's good practice. The thermal resistance of the insulator is 0.3°C/W, so maybe an added 0.5°C/W including thermal paste. I don't like the idea of using an aluminum case as a conductor, and I would not save on the number of wires anyway, but maybe the length - but I would need to have somewhere to connect those wires to the case.


--- Quote from: Seekonk on July 15, 2020, 12:04:49 am ---That has to be ground loop heaven. Great if you don't know the words to the music. All my audio devices require 9V AC so they can get the +- for opamp.

--- End quote ---

What is ground loop heaven, my wiring or what Pawelr98 proposed?

Cheers, Richard

Ian.M:
Put it on a Variac, turn down the mains supply to 216V (the legal limits for line voltage in the EU and most of the EEA is 230V RMS +10%, -6%) and scope the regulator input and output at full load current to check how much headroom its got left on the ripple troughs and if its got any signs of ripple breakthrough on the output.  N.B. the headroom required drops with increasing regulator temperature, so start with it cold and only apply the test load when actually taking measurements! |O


I reckon you are in trouble as the L78Sxx series regulators are specced to need a minimum input  to output differential (aka 'headroom') of 3V at 1A load current, and by my reckoning you'll be lucky to have 2V at the mains low line limit.  I *THINK* its also going to be out of spec at your nominal mains voltage.  :scared:

However if there's no ripple breakthrough at full load low line, you've got a 'golden' L78S09, and can box it up and call it good as a 'one-off'.   8)

You may be able to save the day by replacing the bridge rectifier with beefy Schottky diodes and significantly increasing the reservoir capacitance to lift the ripple troughs, but if you are unlucky, you'll have to find a true 2A LDO, with a high enough max input voltage to survive the no-load input voltage it will see during high line (254V), no load conditions.  :popcorn:

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