Shop OBEX P1 Docs P2 Docs Learn Events
Vdd current specs — Parallax Forums

Vdd current specs

Christina KellerChristina Keller Posts: 8
edited 2005-08-10 01:24 in BASIC Stamp
How much current is the Vdd +5v output on the BS2p specified to sink and source??

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-08 15:29
    Christina,

    ·· The Vdd pin will not be sinking/sourcing per se...That is a function of the I/O pins.· If you refer to the Vdd output pin, it can supply up to 110mA not including anything connected to the I/O pins.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-08-08 17:17
    Chris, I think the scenario he wants is this:

    He's running the BS2p off of Vin, driving it with say 9 volts.
    Now, he wants to know how much current he could get from the BS2p Vdd pin, to use driving additional circuits on his breadboard.

    I know this depends on the exact regulator used on the BS2p -- but what is the amount of available current not already used by the Bs2p?
  • Christina KellerChristina Keller Posts: 8
    edited 2005-08-08 17:33
    Yes, that is what I mean: I am driving the BS2p with 12v, and have several pullup resisitors to the Vdd pin for switches (that in turn go to the I/O pins). I noticed the regulator on the BS was getting very hot when all the switches were closed, and I want know much how current it is supposed to handle. I think I was only drawing like 55mA, but it still got untouchably hot!
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-08-08 17:42
    I always cringe when the onboard regulator gets used -- it's very small and has little ability to cool itself. The problem is exacerbated by applying 12 volts to Vin; that means the regulator has to dissipate 7 volts in the form of heat (note our manual suggests 7.5 volts on Vin). I couldn't find a spec anywhere, but I would have sworn that we've stated in print not to draw more than 50 mA from the Vdd pin.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Jon Williams
    Applications Engineer, Parallax
  • YanroyYanroy Posts: 96
    edited 2005-08-08 18:45
    Jon, it's been my experience with the stamp manual that it has nothing in the way of specs other than the most general stuff.· I'm constantly trying to look up the logic threshold voltage or the input impedance of an I/O pin and coming up empty.· I don't know if that's because I'm not searching the PDF correctly or if it simply isn't there.· I'd like to see the traditional datasheet's "electrical specifications" and/or "absolute maximum ratings" sections added to the manual.
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-08-08 19:09
    The logic threshold is 1.4 volts -- except on the BS2px when you set to CMOS mode, then it becomes 2.5. There are several references to the 1.4 volt threshold through the documentation, but I don't know that it's printed as a specification anywhere.

    For the IO pins, you can reference technical docs for the PIC16C57 (stock BS2) or the SX micro (other BS2 members) as the IO pins connect directly to the BASIC Stamp module pins.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Jon Williams
    Applications Engineer, Parallax
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-08 19:19
    Sorry,

    ·· I was gone for lunch, so I didn't get a chance to clarify until now.· Allan, the original answer I gave is correct based on data available for the regulator.· This information is available in our knowledge base.· However, Jon is also correct in that what's on paper doesn't always work in the real world.· The higher the input voltage, the hotter the regulator will run, therefore limiting it's ability to provide the rated current.

    ·· I would take Jon's advice and not go the on-board regulator route.· I·didn't read in the original message about the 12V thing, as that's at the high end of the BS2p's voltage input range on Vin.· The recommended route here would be to use an external regulator and feed it's output into Vdd.


    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-08-08 19:46
    Christina:

    Personally, I tend to use 10 Kohm resistors for pull-ups/pull-downs. With 5-volts across them, (V = IR, I = V/R, I = 5/10000, I==500 uAmps) each resistor pulls 500 uA, aka 0.5 mA. 16 of them would only pull 8 mA.

    I would suggest you raise your pull-ups to 10 Kohms each, as 55 mA seems pretty high. I believe Chris's 110 mA for the VDD pin is valid when you are feeding it 7.5 volts.

    Regulator math: Power = Current * Voltage. (12 volts - 5 volts) == 7 volts. 7 volts * 110 mA == 0.77 watts.

    Now, if it had a 7.5 volt supply: (7.5 - 5) == 2, 2 * 110 mA == 0.22 watts.

    So, currently you are making that tiny little regulator dissappate about 3.5 times the heat it was designed for. It's going to get hot, yes. I believe it can go into thermal shut-down if it gets too hot, so you probably haven't broken anything yet.

    If all you are doing is driving pull-up resistors, give the poor thing a break and increase the value of your pull-ups to 10K.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-08 20:38
    Also,

    ·· What are the output doing/driving?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • ShortShort Posts: 26
    edited 2005-08-10 01:24
    What I allways do is get at least a 1 amp 7805 reg, put a bridge in front of it
    put A 470 mf cap after the bridge that way you can plug in the board with an ac or dc adapter with any voltage up to 35vdc. use pin 23 of stamp for ground and 21 for
    inputing the 5vdc off of reg.
Sign In or Register to comment.