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LM7805T Question — Parallax Forums

LM7805T Question

MacGeek117MacGeek117 Posts: 747
edited 2005-07-25 22:56 in General Discussion
My next project will need me to connect a LM7805T to a·car battery. Can the 7805 handle the voltage?
bugg

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Comments

  • NewzedNewzed Posts: 2,503
    edited 2005-07-23 19:58
    The 7805 can handle up to 25 volts input comfortably.

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  • Robert KubichekRobert Kubichek Posts: 343
    edited 2005-07-23 19:58
    bugg said...
    My next project will need me to connect a LM7805T to a car battery. Can the 7805 handle the voltage?
    bugg

    You would be better off with a dc/dc converter, as you are loosing a lot of power in the 7 volt drop, unless you need the heat? smilewinkgrin.gif

    Bob N9LVU scool.gif
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-07-23 20:06
    What if I used a 7809 as a stepping stone from 13 Volts?
    bugg

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  • KenMKenM Posts: 657
    edited 2005-07-23 21:05
    Do you know how much current will be drawn from the regulated 5 volts?

    I have several applications using a 7805 run from a vehicle. I don't recommend stepping down to 9 volts, then 5 volts.


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    Ken

    Post Edited (KenM) : 7/24/2005 5:48:11 AM GMT
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-07-24 00:30
    200, 300 mA. A SX and 12 parallel LEDs. #350-00006
    bugg

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    Post Edited (bugg) : 7/24/2005 12:41:38 AM GMT
  • ForrestForrest Posts: 1,341
    edited 2005-07-24 01:49
    Here's an example of a +5V power supply for a vehicle www.seetron.com/an_vpwr1.htm
    - use Parts List #2 for a 7805 circuit
  • KenMKenM Posts: 657
    edited 2005-07-24 05:52
    If running at 300 mA you will likely need a heat sink on the regulator.

    Taking a worst case scenario of power the regulator must dissipate....

    P = IE

    Worst case E = 9.7· (14.7 - 5 = 9.7)

    Worst case I = 0.3 Amps

    9.7 * 0.3 = 2.91 watts
    bugg said...
    200, 300 mA. A SX and 12 parallel LEDs. #350-00006
    bugg

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    Ken
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-07-24 21:40
    Hi,

    here is a picture of the DC/DC converter PCB I have designed. Input voltage range: 8...30 V, output voltage: 5V @ 500 mA with short circuit and over-heat protection. Just, one LM2574, a throttle coil, one Shottky diode, and two elcaps. If you like to see the schematic, just let me know...

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  • JavalinJavalin Posts: 892
    edited 2005-07-25 10:30
    Hi G
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-07-25 16:09
    Hi James,

    here comes the schematic.

    You can find more information about the LM2574 in www.national.com/ds.cgi/LM/LM2574.pdf.

    The reason for my specific PCB design is that I needed it as replacement for the Rohm BP5220 DC/DC converter, I've been using in one of my industrial projects. Instead using a separate PCB for the converter, you could install the few components on your main PCB (this is what I'm doing in the meantime).

    I like this approach because of the better efficiency (i.e. less power dissipation = heat).

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  • JavalinJavalin Posts: 892
    edited 2005-07-25 18:56
    Cool, thanks.
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-07-25 22:56
    Thanks for all the help, guys! 150mA would be a better figure.
    bugg

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