# Silly noob question: Driving a 12v solenoid valve with a 9v battery

Discussion in 'The Projects Forum' started by mcgski, May 6, 2009.

1. ### mcgski Thread Starter Member

May 6, 2009
13
0
OK folks, I'm just getting started with some ideas and have little or no experience with building a circuit board. I'd like to build a pneumatic device that is driven by a solenoid valve that gets opened when a signal comes over a wireless device (XBee Pro). I saw a video on adafruit.com on the XBee. In the video a battery box was supplying the power for the XBee and the relay. Got me thinking about the pneumatic valve and when I started looking around all I could find was a 12v DC that consumes 3-6.5w of power.

solenoid valve URL here.

Now like I said, I'm new to IC design and electrical theory so I may need a little encouragement and/or a push in the right direction. Even a "what are you crazy!?!" would let me know I'm barking up the wrong tree. I know enough about batteries that I could string a bunch of them together but I'm trying to keep the package small and light, hence the 9v concept.

My first question is can I use a battery pack for this application? The XBee I think is a 3.3v-5v device so I know a 9v will work. What about the 12v solenoid valve? I was thinking of having the relay "switch" the valve when it gets the signal from the XBee device. That way the valve is "dumb" and is controlled by the digital signal it's getting from the XBee which in turn gets it signal from some distance.

Any thoughts on a direction here? I'm more than willing to read up if someone can point me to the right beginners guide or some advice on how to get started.

Thanks in advance for the help.

2. ### gryskop Member

Mar 1, 2008
26
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Depending on what pressure you want to control (lets assume for this case max up to 60 PSI) the solenoid draws 3Watt. For a 9volt battery that would require (P=VI, thus I=P/V = 3/9 = 333.3mA). For instantaneous operation (1-3sec) the 9 Volt may work, but for long time operation 1m to x hours, that poor 9 Volt will die a cruel and unnatural death.

For your need, using the 9V may work for the XBee, (if it does have an internal voltage regulator, i.e. dropping the 9V down to 5V for the microcontroller inside). I dont know XBee, but if it does have a potential free relay contact, I would suggest getting a 12V 1A power supply for the solenoid, switched through the relay contacts.

Those solenoids you mentioned are typically used in process automation being controlled by PLC control systems, but a PLC Digital output card drives 24V @ about 200mA, hence they can easily drive those solenoids without the use of interposing relay.

3. ### mcgski Thread Starter Member

May 6, 2009
13
0
gryskop, thanks for the reply. The solenoid is in the "Normally Closed" state and will only need to be energized for a few seconds at most a few times during the day.

I'll have to check on the XBee's ability to drop a higher voltage to 5v. If it needs to be regulated, what's the component I'd used to do that? I like your idea of switching the solenoid through a 12v 1A power source. I'm thinking I could power the XBee and the solenoid through the the same 12v supply, right? I still need it to drive off battery so would 8 AA's in series work? I know the voltage adds up but most batteries I've seen are about 1100mA. Is that cumulative as well?

4. ### gryskop Member

Mar 1, 2008
26
0
Google for the datasheets on simple 1Amp voltage regulators 7805 (for 5V), 7806, or 08, or 09 or 12 (for 6, 8, 9 or 12V). There's also the LM317, that can be used via an adjustable/variable resistor (POT) to be setup for any of the abovementioned voltages ranges.

Remember to read the datasheet well, as it explains what the recommended voltage is for supply vs. output. If you supply the regulator IC with a very high voltage level, the IC will need to dissipate the access power as heat, hence the need for a heat sink.

LM317, 7805

Batteries in series does totalize the voltage, but the current is limited by the max current the battery can deliver. The circuit will only draw as much current as needed, determined by the way the circuit is built. The total wattage supplied by the 8x1.5 AA batteries will be (8x1.5V) x 1.1A = 13.2Watt. That should theoretically be enough to drive the solenoid.

5. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
The "1100mA" spec you mentioned is actually 1100mAH, which is milliamp hours. This means that an AA cell can deliver roughly 1.1A continuously for an hour, or 110mA for 10 hrs, etc. As mentioned by gryskop, if you put 8 in series, you will get 12V at 1100mAH. Be aware that the voltage at the end of life predicted by this spec may be too low for your application.

6. ### mcgski Thread Starter Member

May 6, 2009
13
0
gryskop/Ron H, thanks for your input. You've given me some fuel for thought and a lot of extra reading. I think I have a handle on the design and the constraints and need to chew on it for a bit. Thanks again.