Solenoid circuit - help me not set things on fire

Discussion in 'The Projects Forum' started by a4avant, Dec 15, 2010.

  1. a4avant

    Thread Starter New Member

    Dec 15, 2010
    Hey Folks,

    Pretty new to the whole circuit world (besides a required college class that had one of the worst professors I've EVER had), so I just want to make sure my assumptions are correct.

    I'm using an Arduino Uno to control a number of solenoid valves (pneumatic). I'm fine on the Arduino side, just want to confirm I'm not going to fry anything on the solenoid side - these valves ain't cheap!

    here's the schematic I'm following (1 per solenoid):

    I'm using the TIP102 transistor for switching and N4004 Diode for back EMF protection, as shown.

    Solenoid has the following specs: DC24V, 3.0W (by I=P/V, I get .125 amps)
    My Power supply is DC 24V 0.75A (18W).

    I shouldn't be driving more than 2 solenoids at a time. Most of the time I'll be driving one solenoid. There will be a total of 5-6 solenoid valves, each on separate transitors but with shared power supply.

    Arduino power is coming from USB for now (and a 9V DC transformer later). I'm connecting the 24V DC side ground to the Arduino's ground pins.

    I'm correct in saying that the solenoid will only draw .125A (if needed) and the transformer will only supply the needed .125A, making everything safe and sound, right? There's no danger in using the .75A transformer?

    Anything else wrong with my schematic or plan?

  2. #12


    Nov 30, 2010
    True. Loads only use the current they need. The power supply does not force the load to take all the current it can supply, just as the electric power company does not force your home to take all the current it can supply.
  3. SgtWookie


    Jul 17, 2007
    Welcome to AAC!

    The TIP102 is maximum overkill for the current you'll be sinking from the solenoid. Even a 2N2222 with a 330 Ohm base resistor would be more than adequate, would take up a lot less board space, and is available from your neighborhood Radio Shack store.

    Measure your supply's output voltage when there is no load on it. What do you get?
  4. a4avant

    Thread Starter New Member

    Dec 15, 2010
    Thanks folks.

    Reason for the TIP102 is I might end up using different solenoid valves later, possibly 1-3A. I'm using one I got for free (thanks Mead!) for a prototype for now. I might go beefier for the actual project.

    Board space isn't a huge concern at all for me - I'm only building at most 2 of these, and they'll be semi-permanent installations. The control panel box will probably be oversized as is to allow for easier control.

    The wall-wart power supply is delivering 24.3VDC unloaded. The valve has a listed range of 21.6-26.4VDC, so no problems there.