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SX Brownout — Parallax Forums

SX Brownout

Mag748Mag748 Posts: 269
edited 2006-07-13 19:28 in General Discussion
I have an SX project that works fine if I use a 9V 1500MA power supply, but the power regulator that I am using gets extremely hot. I measured the·current usage which turned out to be about 600MA,·So I went and got a 6V 800 MA power supply and tried it out, but the SX does not run. The power going to the regulator is about 6.5 V and the power at the SX is 4.7V. the brownout is set at 4.2 V, so why is the SX not running? I have a brightness control on the LEDs that I am using, and when I lower the brightness, the SX jumps into action. So, I am pretty sure the brownout thing is causeing the problem.

Thanks for any explanation as to what is going on.
-Marcus

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Comments

  • Mag748Mag748 Posts: 269
    edited 2006-07-07 02:07
    The SX seems to be locking up when the voltage drops below 4.95V

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  • Mag748Mag748 Posts: 269
    edited 2006-07-07 02:21
    In the SX data sheet it says that the minimum supply voltage for 20MHz operation is 2.7V

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-07-07 02:23
    Which regulator are you using?· It's possible that 6V is below the minimum regulating voltage of the regulator, and then you're seeing a voltage drop, and possible noise?· Does the regulator require caps?· Do you have them installed?

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • BeanBean Posts: 8,129
    edited 2006-07-07 02:50
    Try setting the brownout to 2.6V.

    Bean.

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  • Mag748Mag748 Posts: 269
    edited 2006-07-07 14:17
    The voltage regulator is a 7805 with the caps installed. I will try setting the brownout to 2.6V next time I program the chip. thanks,
    Marcus

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  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-07-07 17:12
    7805 drops out at 7volts. That is likely your problem.
    You may get it to work somewhat, but you need a LM2940-5.0 for 5 volt operation as it drops out at 5.5volts or so.

    Setting the SX to 2.6V brownout and using an LM2940-3.3 will operate at 3.3volts - BUT you need the 3.3 volt Programing adapter for the SX-key.
    Don't turn off the brownout feature unless you have engineered a very stable power supply. Otherwise, you may just have the SX turn on and off continously.

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  • Mag748Mag748 Posts: 269
    edited 2006-07-07 19:30
    Is it normal for the 7805 to get extremely hot (with the heatsink) with a supply of 12V? this must not be good right?

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  • BeanBean Posts: 8,129
    edited 2006-07-07 20:03
    It depends how much current you are using. If your using 1 amp, then the regulator would have to dissapate 7 Watts of heat (that's alot).

    Bean.

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    ·
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-07-08 21:23
    Following Bean...I still use the 7805 in 1-off projects, and yes...It's going to get hot...The higher the voltage going into it the more power it must dissapate as heat to get 5V.· As Bean said, the more current draw...

    This brings up an interesting point...Running a regulator such as the 7805 at its maximum (not absolute) input voltage and drawing maximum current (1A) a standard TO-220 heatsink is insufficient to keep it cool.· You need something hefty in that case, and in one case I have seen the 7805 go into thermal runaway when it didn't have a heatsink.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • LightfootLightfoot Posts: 228
    edited 2006-07-11 18:58
    I had the same problem with a 7805 and fried an SX before I found the problem. I learned that when I used the center tap on my 12 volt transformer.

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-07-13 19:28
    It goes to show you that you cannot always count on thermal protection, since the speed of the curve (how fast the temperature rises) can affect that.


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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