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12V 5Ah SLA Rechargable Battery for PEK — Parallax Forums

12V 5Ah SLA Rechargable Battery for PEK

murphmurph Posts: 4
edited 2012-07-31 07:12 in Propeller 1
Hello.

Was wondering if there would be ANY reason to worry about using my 12V sealed lead acid rechargable battery, or even my huge car battery, to supply input to the LM2940(5.0) DC regulator on my PEK board. The data sheet specifies that the battery is well within range for supply input, and a .47 uF electrolytic capacitor is installed across the supply input for my wall-wort.

In theory, everything looks OK, but don't want to risk damaging my expensive components (drawing less than 1A total).

Is the LM2940 going to be a reliable "Doorman", protecting everything downstream indefinately?

I'm thinking that I might be missing at least something somewhere.

Thanks for your assistance.

Comments

  • evanhevanh Posts: 16,109
    edited 2012-07-30 16:58
    In terms of noise, as long as the car isn't still attached then you're good to go. In terms of safety, you'll want to add a fuse inline with one of the battery leads just in cast you short it out. In terms of heatsinking remember that it's W=VxI. So, at the full 1 amp and a drop of say 8 volts, from 13 down to 5, your LM2940 is an 8 watt heater.
  • murphmurph Posts: 4
    edited 2012-07-30 22:29
    The 12V battery should give me some decent "hang-time", but the 8W heater is certainly an unwanted side-effect. I've made heat sinks with wing nuts and washers before and they seem to work pretty dang good, but they are as ugly as hell. They should hold me over temporarily until the prototype is finished being tested.

    The data sheet after all states that the voltage regulator was designed for vehicles, so the whole story would make perfect sense.

    Thanks for the response:)
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2012-07-31 01:47
    When the reg gets used in a vehicle there would be no shortage of space for a decent heatsink. You could make a pre-regulator regulator to knock off some of the voltage overheads and get that heat somewhere else or venture into the switch mode sort.

    Remember that that battery on full charge would be 13.8 V to 14.4 V depending on the charger scheme. The advice of putting in a fuse (very near the battery terminals) is very apt, the surge capability of these batteries is swift and brutal.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-07-31 07:12
    The LM2940 was specifically made with protection features useful in an automotive setting (reverse polarity protection, etc.)

    And NEVER expect any rechargeable battery to be limited to the 'nominal operating voltage'. Charging generally requires a higher voltage to rearrange the chemistry to a full charge.

    And be sure to follow the capacitor specifications for the LM2940 as other voltage regulators have very different requirements.

    If it is used in an actual automobile you may have to resolve specific problems as the starter solenoid, the horn, and switching headlight levels put significant transients into the supply.
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