Shop OBEX P1 Docs P2 Docs Learn Events
Board of Ed power question — Parallax Forums

Board of Ed power question

Mr. RichardMr. Richard Posts: 51
edited 2007-01-11 17:57 in BASIC Stamp
I just wanted to confirm my understanding before I try this and release the magic smoke. I have a Board of education (rev B) and a BS2. I would like to use a transistor to control a solenoid. I have specked out the solenoid and transistor. I can trigger the transistor with 5 volts and it will handle the power I need.
··
Can I connect a 12 volt battery to the power jack, run my stamp and pull 12 volts from the Vdd pins located above the breadboard, sent the voltage through a transistor to a solenoid and return it to Vss pins above the breadboard?
·
Thanks for the help
Josh

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2007-01-10 15:41
    There are three voltage identifiers you should be aware of.

    1. Vin -- this is the 'raw' voltage input to the BOE board. This can be anything from 6 volts DC up to 20 volts DC.

    2. Vdd -- This is the output of the regulator on the BOE board. This will be 5 volts, always. If you attach a power source directly to this greater than 5.5 volts you will probably destroy the BS2.

    3. Vss -- This is also known as "ground". This is the point you should attach grounds to.

    So, to answer your question -- you can never pull "12 volts" from the Vdd pins, because the Vdd pins can never be at 12 volts. You COULD attach that 12 volts to the 'top' of the solenoid coil, attach the 'bottom' of the solenoid coil to the collector of the transistor, connect the base of the transistor through a 2K ohm resistor to a BS2 I/O pin, and connect the emitter of the transistor to Vss. Make sure you put a 'protection diode' across the solenoid coil if you do this.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-01-10 15:48
    Josh,

    ·· Some points of clarification…On the Rev B Homework Board your input voltage should never exceed 12V to VIN (The battery terminals). You should never draw more than 35mA from the VDD terminal, not including any current being drawn from the I/O pins. See the attached schematics for connecting a solenoid. Note that a relay is electrically equivalent in this case.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
    Driver_Relay_MOSFET.JPG
    320 x 240 - 6K
    Driver_Relay_NPN.JPG
    320 x 240 - 6K
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-01-10 16:29
    Umm -- he's talking about the BOE. I believe the 2840-5 on the BOE can supply up to 1000 mA to Vdd, no? That's assuming your wall-wart is rated for 1000 mA, of course.

    And note, if you ARE trying to use more than 500 mA or so from the BOE, lower wall-wart voltages (6 volts, 7.5 volts, 9 volts) are much better.
  • terahertzterahertz Posts: 55
    edited 2007-01-10 16:33
    Anyone mind a semi-hijack? cry.gif

    While we're on the subject...yesterday I was building the power supply network for my project. My plan was to build a 12V to 6V regulator circuit to power the 2 BOEs I'm using,·but I couldn't find a zener in my box-o-stuff. While looking for a zener I found a 5V regulator I didn't know I had. You know where this is going right?

    So I wired up the 5V regulator and the 2 BOEs seem to work fine (wired to the 9V battery connector). Will this cause a problem?

    On a side note, I was a little curious WHY this worked, I checked the voltages on the regulator on the BOE, and there was 5V on the input and output. I wonder if the onboard regulator has a 5V passthrough?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-01-10 16:50
    Allan,

    ·· You’re partially correct…I saw the Rev. B and somehow replace Board Of Education with Homework Board (since the current BOE is Rev C.). So while you can get roughly 750mA of output current (you won’t get 1A due to heat-sink constraints) the input voltage must still be adhered to since VIN directly feeds the VIN of the BASIC Stamp. So while·the BOE on-board regulator can handle higher input voltage, the BASIC Stamp on the board would not be as forgiving, which could be costly. I guess I tend to err toward the safer side of things.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
  • Mike GreenMike Green Posts: 23,101
    edited 2007-01-10 16:59
    Terahertz,
    The BOE (except for Rev-A I believe) use low drop-out regulators. They stop regulating somewhere between 5.5V and 5V depending on current load, but do pass the input voltage through with typically less than 0.3V drop from input to output. For hobby use, this will probably work fine. The Stamp (BS2) is tolerant of supply voltages below 5V, has a brown-out detector that triggers a reset at around 4.2V.
  • Mr. RichardMr. Richard Posts: 51
    edited 2007-01-11 17:57
    Chris and Allanlane5,

    Thank you for your help and advice.
Sign In or Register to comment.