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Power supply design issues. — Parallax Forums

Power supply design issues.

Turnbull2112Turnbull2112 Posts: 65
edited 2009-01-11 02:09 in BASIC Stamp
I recently had some boards made and I am using a LM1084 regulator for the power supply. (24VDC to 5VDC)·I am switching some LEDs and a 5VDC coil relay with a BS2. The relay is being switched through a 2N4401 transistor. The LEDs work fine but when the relay output goes HIGH the voltage regulator gets very very hot. I have already fried 2 BS2's here and I'm getting frustrated. Are there any existing power supply schematics available? I have attached my schematic for review.·I would greatly appreciate any feedback and patience as I am still learning here...
Rob
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Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-01-05 20:28
    Turnbull2112,
    ·
    Using a Linear regulator and going from 24V to 5V is going to generate some heat...· Even if your 5V circuit is only drawing 100mA the Regulator will need to dissipate about 1.9 Watts of heat with the 19 Volt difference.· If you can you would be better off using a lower voltage supply, or have a secondary regulator in a cascade design similar to how it has been done with the Propeller so that you·spread the heat load out a bit.
    ·
    i.e.
    ·
    Supply the 24V into a 15 Volt regulator, and then supply the 15V into the 5V regulator.· In this scenario each regulator will need to dissipate 1 Watt of heat rather than 2 Watts for a single regulator.
    ·
    ·
    Alternatively a switch-mode regulator will be more efficient and not run as HOT, but it might end up costing more in additional components.· Here is an alternative that might work.· http://cache.national.com/ds/LM/LM2679.pdf
    ·


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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Mike2545Mike2545 Posts: 433
    edited 2009-01-05 22:52
    Turnbull2112, I agree with Beau, but you could just use a transformer with a lower secondary output. The fewer volts a regulator has to reduce the better. The LM1084 has an absolute max rating of 25 volts, you are pushing it a bit hard with 24 volts. Try using a 12.6 CT from the shack, just use the center and one of the outer output leads to get 6.3v, your regulator will breath a bit easier.
  • Turnbull2112Turnbull2112 Posts: 65
    edited 2009-01-10 23:57
    You guys nailed it! I dropped the supply voltage down to 12 VDC and everything calmed down. I was hoping to keep the supply voltage 24 VDC but I will go with the step down transformer approach. Thanks again. Stamped out another issue...
  • remmi870remmi870 Posts: 79
    edited 2009-01-11 02:09
    what are you using to supply the power to your board, what is the input voltage? may i recommend a old pc power supply, it already is set up for regualted and overcurrent protected 5v, 12v and even 3.3v to supply the power to your project, they are cheap, safe (all self contained ready to plug in) can run on 110 and 220v.
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