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Important notice for PSM owners regarding EXT power ! — Parallax Forums

Important notice for PSM owners regarding EXT power !

RaymanRayman Posts: 14,887
edited 2011-01-25 16:34 in Propeller 1
I've just discovered a condition that can damage the PSM device when using External power greater than 6 VDC.

So, as described here, please only use 4 to 6 VDC REGULATED power supplies with the external power jack !!!!

There are 2 things that can conspire to damage the FTDI USB chip on the PSM:

1. Although some power supplies say 9 VDC output, I've measured up to 13 VDC coming out of them.

2. The USB/EXT switch momentarity connects all three leads together when switching back and forth between USB and EXT.

These two factors can combine to momentarily put much more than the rated 6 VDC MAX on the FTDI chip and I've been able to reproduce damage to the FTDI chip using a "9" VDC supply.

Side note: You can actually still use a regular 9 VDC supply as long as you do not operate the USB/EXT power switch while power is applied.

Thanks,
Ray

Comments

  • RaymanRayman Posts: 14,887
    edited 2011-01-22 05:31
    More notes:

    I believe my other products that use an FTDI chip (like the PSB) are unaffected because they have a 5 VDC regulator between the EXT power jack and the FTDI chip.

    Some PSM units appear to be immune to this power surge. So, some FTDI chips appear more resilient than others...

    I'm very surprised with this power switch now! You can actually move the switch into a certain center position and see that all three leads are connected together...
  • wjsteelewjsteele Posts: 697
    edited 2011-01-25 06:38
    You know, I ran into this but didn't realize what it was. I had to replace the FTDI chip on mine when I was using a 9 volt supply. I'll back down to a 5 volt supply from now on.

    Good catch!

    Bill
  • RaymanRayman Posts: 14,887
    edited 2011-01-25 13:42
    I think may have been a few people affected. I'm thinking that most people use USB with it or there would have been more returns, I think.
    It's a little embarrassing and I'm still trying to figure out why I didn't detect the issue earlier...
    I think it's because some FTDI chips are immune and others aren't.
    Also, it seems that having the USB connected protects the FTDI chip. I wasn't able to recreate the problem until I unplugged the USB...
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-01-25 15:14
    That's why I hate the un-reg wall warts. I have traveled thousands of miles fixing these sorts of problems.

    Clasp them firmly in your fist (unplug first) and throw!
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-01-25 16:03
    Rayman,

    Most SPDT switches are "break-before-make", but you might want to make sure that's what you've got. I suspect, however, that the problem is not with the switch but with the charge on C10 that drains back through the PMOS transistor to the FTDI chip as soon as the switch is flipped. A diode to block current flowing back from cap would probably solve the problem.

    -Phil
  • RaymanRayman Posts: 14,887
    edited 2011-01-25 16:34
    That's what I thought it had to be too. But no, the swtich actually does connect both ends in the middle. You can actually put it in the middle and measure it connected..

    But you're right that even if the switch was perfect, I could still have this issue due to the charge on the 10uF cap...

    In retrospect, it was a mistake not to have a 5V regulator between the power jack and USB/EXT switch.

    Blocking diode is probably a good idea too...
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