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DC wallwart selection for a σ25 with σ78

sigma25, sigma26, sigma24

DC wallwart selection for a σ25 with σ78

Postby johan851 » January 2nd, 2018, 4:56 pm

I have a γ2 that's powered by a σ25. When I built it a couple of years ago I used a 7805 regulator along with this 9VDC wallwart (Mouser PN 412-109051) to save space in a small enclosure.

I want to upgrade the 7805 to a σ78. The sonic improvement might not be tangible, but the warm fuzzies are.

Since this is an LDO regulator I should reduce the input voltage as well. Will a 7.5VDC wallwart like this (Mouser PN 709-GS06U-11P1J) provide enough headroom? It seems like it should - I didn't populate the bridge rectifier on the σ25 - but I wanted to make sure I'm not missing something.

There's a 6VDC version as well, but the data sheet for these states output voltage of +/-5% (5.7V on the load side) and that seemed to cut it too close to the σ78's 5.5V dropout voltage.
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Re: DC wallwart selection for a σ25 with σ78

Postby amb » January 2nd, 2018, 8:14 pm

It's your choice whether to reduce the input voltage. The σ78 won't running hotter than the 7805, but you could drop the input voltage to reduce the heat dissipation further. The Mean Well unit you linked is a switch mode wallwart. If your original one is linear then I would keep it. Otherwise the σ78 should be able to clean up the noice anyway and give you very clean DC output. I recommend at least 1V of dropout voltage (despite the regulator chip being able to operate below that).
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Re: DC wallwart selection for a σ25 with σ78

Postby johan851 » January 3rd, 2018, 9:53 am

The case is small with almost no ventilation, and it gets a little warm, so reducing the voltage would be nice. If there's a straightforward way to thermally couple the σ25 board to the case that would work too - maybe one of those sticky thermal foam pads?

From the datasheet on the current wallwart, it looks like it might just be a transformer + rectifier + cap? http://www.mouser.com/ds/2/449/XC-600192-1212536.pdf

I see that many of the wallwarts say "switching", the rest don't specify - is there a good way to tell if a supply is linear?
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Re: DC wallwart selection for a σ25 with σ78

Postby amb » January 3rd, 2018, 12:49 pm

The Xicon wallwart you linked does seem to be linear without any voltage regulation.

There are a few telltale signs that a wallwart is switching:
- Small size and weight vs. output current rating
- Wide input voltage range (e.g., 90-240VAC, 115-230VAC, etc.)
- Regulated output DC voltage
- RoHS certification

Due to RoHS, linear regulated wallwarts are becoming very rare.
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Re: DC wallwart selection for a σ25 with σ78

Postby johan851 » January 3rd, 2018, 1:54 pm

Thanks, I'll stick to the wallwart I have then. I'll get some thermal pad to mate the bottom of the σ25 PCB to the aluminum case. Or maybe I'll just drill a few ventilation holes.
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