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Board of Education - Maximum Input Voltage? — Parallax Forums

Board of Education - Maximum Input Voltage?

everesteverest Posts: 141
edited 2009-07-25 15:59 in BASIC Stamp
Hi All,

For my application, I really need to be able to use a 12v battery to power my BoE. . .I've looked at the specs for the on board voltage regulator and I can't see why it would have a problem managing 13-14v from a battery. The max stress rating on the voltage regulator is 26v and clearly I'll never be anywhere near that. So why does the board indicate that it needs 6-9v? I actually have been running it on 12v for a while now before I even noticed the rating.

Any thoughts? Am I okay to continue operating on 13.8v?

-Jeff

Comments

  • dev/nulldev/null Posts: 381
    edited 2009-07-25 10:11
    Good question Jeff, I've been wondering the same thing. I've been using diodes to reduce to 9V until now. I have a Javelin board that accepts 24V with the same LM2940 regulator.

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    Don't worry. Be happy
  • Mike GreenMike Green Posts: 23,101
    edited 2009-07-25 14:10
    It's all about heat. Linear regulators (like those on the BOE) dissipate the extra voltage as heat and the amount of power involved depends on the voltage drop and the amount of current drawn (Power = Voltage x Current). If you're running the board at 13.8V, you're dissipating 8.8V x <current> as heat. If the current drawn is low, it won't matter. If you're powering a couple of servo motors and some LEDs and a couple of other things and drawing an Ampere or more, the regulator is dissipating 8-10 Watts and will get VERY HOT! It will probably shut itself down long before that since it has protective circuitry built-in for over-temperature and over-current.

    The 6-9V rating is based on near maximum current and the amount of heatsinking provided on the BOE (and typical ambient temperature).
  • everesteverest Posts: 141
    edited 2009-07-25 15:26
    Hi Mike,

    I'm guessing that a resistor based voltage divider is essentially the same yes? What abour using a zener diode? All of the simple circuits I see are one of those two, aren't both going to dissipate voltage as heat and have the same issues?

    I'm driving two ULN2803A transistor chips, a Ping)) sensor, a Sensiron Temp/Hum, an IR sensor, and reading a bunch of active high switches. . .seems like that might overwhelm the power regulator eventually. . .?

    -Jeff
  • Mike GreenMike Green Posts: 23,101
    edited 2009-07-25 15:39
    It's very common to use a 2nd voltage regulator when powering a BOE from an automotive power source. The 1st is an LM7808 which drops the 12-14V supply to 8V, then you run the BOE off the 8V supply. You'd typically have a 1N4001 diode in series with the input to the LM7808 which would drop the input voltage about 0.7V and protect the LM7808 from connecting the power backwards. The LM7808 would need a heatsink (and the usual filter capacitors ... look at the datasheet).

    A voltage divider works very poorly for dropping voltage when there's any significant amount of current involved and even more poorly when the current can vary a lot as in your case.

    A Zener diode could work, but is relatively inefficient, particularly when the current drawn is very variable.
  • everesteverest Posts: 141
    edited 2009-07-25 15:59
    Mike,

    Thanks! This is actually not an automotive application, but a lot of the same principles apply. My project enclosure has a fan, so I'll pick up a heat sink and position the regulator near the inflow. Thanks!

    -Jeff
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