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Author Topic: Wall power to three different voltages  (Read 670 times)
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Hi all,
I'm building a circuit that requires a 110VAC wall power to power a 5V Arduino, a 12V (1.2A max draw) solenoid, and a 10V sensor. I'd prefer to keep the number of plugs at 1, so I was thinking of buying a 12V 1A "wall wart" to power all three.

The circuit will be rather simple: the Arduino reads some value from the sensor and, if a certain state is reached, output a value on a LCD and fire the solenoid. I don't think there will be too much current draw and I'd imagine the 1A wall wart to be sufficient to power everything (please correct me if I'm wrong).

My question is: I hear wall warts are notoriously dirty and suffer from serious ripple. I hear the Arduino has its own regulator/ripple protection built in, so I'm not too concerned. The solenoid won't need anything special other than a darlignton transistor and doesn't require a clean supply (again, please correct me if I'm wrong... I'm very new to this!) I can probably just hook that straight up.

I'm most concerned about the sensor; it probably needs a good regulator and some filtering (Low pass? high pass?) and god knows how many other provisions... What would you recommend? Also, I've read that I should protect the Arduino against backflow in case the transistor going to the solenoid fails... How would I do this? A single diode?

Thanks all in advance! I'm looking forward to some great advice.
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Get a good wall wart, they are not all bad. Generally it is the heavy ones that are poorly regulated, so a light one will be a well regulated switching type.
So with 12V you need to regulate it down to 10V with a linear regulator, something like a UA78M10 and capacitors.

This can then be used to drive both your sensor and the power jack of the Arduino.
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Regulating 12 volts down to 10 volt may leave insufficient overhead for the 10 volt regulator. Also when the solenoid activates it will pull the voltage down possibly by a volt or 2 depending on the quality of the supply.

Potentially this could cause you spurious resetting of the Arduino. The power supply should be rated at a higher current than 1 amp to minimize this possibly a 2 or 3 amps unit with an output of 13.8 to 18 volts should be considered. The solenoid voltage is probably not that critical but if your supply is higher than 13.8 use a resistor in series with it or some 3 amp diodes to reduce the voltage a little.

If you have a 12 volt 1 amp supply try it and see if you get away with it but if not follow the above.
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Quote
it probably needs a good regulator and some filtering (Low pass? high pass?) and god knows how many other provisions...

It probably is a good idea to understand what the sensor expects from its power supply. Get a datasheet to read about it. Otherwise, relying on speculation by complete stranger is unlikely a winning solution.
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Standard laptop power supplies are 15V 5A. This is sufficient overhead (even with a voltage drop from current draw) for an LM7810 (or any other 10 volt linear regulator). I suggest running a 5V regulator off the 10 (7805 or 78M05) instead of relying on the small internal arduino one. If you do choose to supply power to the arduino, use the regulated 10 volts and not the full 15, that will overheat the regulator with any reasonable current draw.
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