Electronics > Projects, Designs, and Technical Stuff

Weekend project: 9V 2A linear DC power supply

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richlooker:
Did some scoping;

At 1.5A load, AC measurement:



And DC:



2.0A, AC:



2.0A, DC:



It seems the effective dropout voltage of the regukatir is about 2.5V. At 2.0A it breaks down. I'll try the 2x12V transformer instead, redo the measurements and check the temperature of the heat sink.

Cheers, Richard

tooki:
Or just replace the 7809 with a modern LDO regulator??

bdunham7:

--- Quote from: Ian.M on July 15, 2020, 02:48:19 am ---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. 

--- End quote ---

If you look at the performance chart, a 'typical' unit would only have a 3V dropout at 1A if it were at -75C.  At a more reasonable 25C, it's 2V.   And is warms up in operation it will go down more.
 I'm guessing he'll skate by.  Perhaps his isolation of the regulator will save the day by running it warmer!  :-DD

richlooker:
It took me a little while longer than a weekend, but I think I have nailed it now.

Did the overwind trick; 18 turns of 0.75mm²/19AWG wire:



My test load; three 10W resistors sqeezed between two heat sinks, each drawing 0.5A at 9V, plus a halogen bulb, drawing 0.6A:



Measurements at increasing load currents:



At 2.1A load, the regulator input voltage ripple troughs now dip down to ~11.9V and there is no breakdown to be seen. The transformer and the rectifier both get warm, but not toot hot to touch, and the regulator heatsink is even a bit cooler, while dissipating ~7.5W.

Attached AC and DC scope traces of regulator input (channel A / blue) and output (channel B / red) for the different loads.

Cheers, Richard

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