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Topic: Best way to get 12 V from 28 V or 5 V ? (Read 748 times) previous topic - next topic

magagna

Thanks for the info guys. Three more questions:

1 - The solenoid will have a duty cycle of 17% (on for 1 S, off for 5 S, repeat) so I think the LM7812 will be dissipating 400 mW instead of 2.4W correct?

2 - Do I wire the LM7812 up with just a .33uF cap on the input and a .1uF cap on the output, like in the "Typical Applications - DC Parameters" section of the spec sheet?

3 - Crossroads you said to use this chip because a resistor voltage divider's voltage out will vary with the current being drawn. Is that really important for a solenoid? My (limited) experience with electromechanical things is that they're pretty tolerant of voltage ranges. (I'm going to use your advice and use the LM7812, I'm just trying to figure out what's appropriate when).

Thanks again,

Chris
http://en.wiktionary.org/wiki/magagna <-- My last name.  Pretty apt.

magagna

dc42 - Thanks for the idea. I will look at that.
http://en.wiktionary.org/wiki/magagna <-- My last name.  Pretty apt.

CrossRoads

You can go whatever route meets your needs & budget.
I went with the switching regulator as creating 5V from a 12v supply with a LM7805, the unheatsunk 7805 for the main box got really hot! Didn't measure the current, but I didn't all that heat in the box.

For a solenoid, yeah, maybe not so critical. Check its specs, see what absolute max it can handle.

As to  what's appropriate when, hard to say.  Depends on budget, enclosure, end use, etc.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

winner10920

If you know it draws 150ma you can try just puttin a 180 ohm resistor in series and that should work

magagna

The specs are a little weird, they say "100mA or 350mA".

http://shihhsin.en.ec21.com/offer_detail/Solenoid_Valve_SH_V0829--329695.html?gubun=S

It was only $2.50 from All Electronics (http://www.allelectronics.com/make-a-store/item/SOL-132/SOLENOID-VALVE/1.html) and since they're only about 90 minutes from me I get overnight shipping for dirt cheap.

I think I'm going to try both the resistor and the LM7812 solutions; the LM7812 is only $0.70 at DigiKey and I'm sure if I don't use it here I will find something to do with it down the road. I've got a bunch of transistors but no Darlingtons; hopefully something I have will work.

In case you're interested it will be used to control air flow between a vacuum pump and a SMD pick and place head.

I hope.

Thanks to all for your help.
http://en.wiktionary.org/wiki/magagna <-- My last name.  Pretty apt.

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