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BS2e to power 12v @ 0.16A — Parallax Forums

BS2e to power 12v @ 0.16A

nick412nick412 Posts: 16
edited 2007-03-10 14:30 in BASIC Stamp
I am looking for the simplest (reasonably safe) way to power an electronic door strike (which the documentation says it is rated at 12v, 0.16A) from a Basic Stamp 2e, along with a 12v, 100mA buzzer controlled via a separate output pin.· Possibly via an opto coupler?

I read the Nuts and Volts article "Silicon Steroids for the Stamp Help Your Projects Heft Big Loads – Using Switching Transistors" http://www.parallax.com/dl/docs/cols/nv/vol1/col/nv6.pdf and tried a 2N2222A (I could not find a 2N2222) transistor however could not seem to get it to work.· Not sure if I broke the transistor somehow, but from what I could see it was wired correctly, and I tried it twice with 2 new transistors.

Comments

  • FranklinFranklin Posts: 4,747
    edited 2007-03-04 21:11
    According to the article the load current of a 2N222 is 100ma and t=you are trying to switch 160ma you need to find a different transistor (there are two other options listed in the article) or try a mosfet which will switch several amps.

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-03-05 05:44
    Franklin,

    The 2N2222A should be rated for 800mA. I wonder if this is more a problem with lack of protection for back EMF…I didn’t see any mention of a protection diode on the device.

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    Parallax Tech Support
  • nick412nick412 Posts: 16
    edited 2007-03-07 22:44
    Thanks for the info.

    When testing, I was using the buzzer which I was guessing did not need a protection diode (for the EMF?) like relays etc do?

    The mosfet's appear to be even more complicated to use than transistors (at least the ones I found), therefore I think I will give it another go with the 2N2222A.
  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-07 23:08
    You do need a protection diode with a buzzer, really anything with a coil/solenoid. MOSFETs are actually easier to use than junction transistors (like the 2N2222A since they're controlled by the voltage on their gate rather than by base current.
  • nick412nick412 Posts: 16
    edited 2007-03-08 08:51
    I just had another look at the MOSFETs - initially I thought they were only available in the type that plugs into a socket, however after looking into it further I see they are also available in the same upright style as the 2N2222A.

    What do I need in terms of V, A, and W? (I am looking at what Maplin have available: http://www.maplin.co.uk/module.aspx?ITAG=SPEC&ModuleNo=33843&doy=8m3#spec

    From what I can see the case style I need is TO220.

    Would the IRF540A work, datasheet: http://www.chipcatalog.com/Datasheet/849B84DF180C0CE4634E41A874D1AB66.htm

    Post Edited (nick412) : 3/8/2007 8:55:29 AM GMT
  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-08 14:42
    The IRF540A should work very nicely. It even has a built-in protective diode. As you can see from the specs, it's rated at 100V, 28A. With the on-state resistance quoted, the amount of power dissipated by the device is pretty small, even at currents way more than what you need (1A @ .05 ohm = 50mw). It's not even going to warm up.
  • nick412nick412 Posts: 16
    edited 2007-03-10 14:30
    I have now purchased the "IRF540A", and am in the process of connecting it up.· Am I correct in thinking that since MOSFETs are "controlled by the voltage on their gate" I do not need a resistor between the Basic Stamp and the "B" pin of the MOSFET?
    I.e. I should connect it as in the diagram (without any resistors / diodes at all)?
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