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Thread: Exceeding Vin voltage - what will happen?

  1. #1
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    Exceeding Vin voltage - what will happen?

    Hi,

    I am using a Teensy 3.6 and wondering, what will happen if the VIN voltage of 6V is accidentally exceeded.
    In the circuit diagram I can see the LP38691 LDO and a fuse F1 with 500mA (a "polyfuse" i guess).

    Vin voltage is specified to <6V, the LDO is specified for <10V or absolute maximum 12V.

    I want to use a MH-mini360 adjustable step-down converter to use a 15V supply.
    Of course i would adjust it's output to about 5V before connecting.
    But it could happen, that the adjustment pot has got a loose contact,
    so that the voltage accidentally goes up to 15V.

    Will the poly fuse in this case clamp the voltage and protect Teensy (and a device connected to the USB-host port)?

    By the way:
    Is it correct to leave the pads "H" and 5V connected and just cut the pads VUSB and VIN, when the Teensy is powered by the step-down-converter and
    a device is connected to the USB-host port while occasionally the PC has to be connected to the micro-USB programminig interface?

    Greets, Martin

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by MartinZ View Post
    what will happen if the VIN voltage of 6V is accidentally exceeded.
    VIN also connect to the TPD3S014 chip. It could be damaged if more than 6V is applied.


    Will the poly fuse in this case clamp the voltage and protect Teensy (and a device connected to the USB-host port)?
    No, it works based on current, not voltage.

    If too much current flows, it will heat up and increase resistance. It doesn't "blow" like a traditional fuse. It just becomes a higher value resistor, so a fairly large current (but not as huge as without the PTC fuse) continues flowing as long as the fault condition exists. The change is based on temperature.

    But if the large current is a result of too-high voltage physically destroying something inside any of the chips, the PTC fuse only serves to limit the amount of heat (and possible burning) which might result. For that sort of scenario, by the time the current becomes large enough for the PTC fuse to begin increasing resistance, it's already too late. The damage is already done. The fuse only serves to limit how much current can flow into the already-damaged chip.



    By the way:
    Is it correct to leave the pads "H" and 5V connected and just cut the pads VUSB and VIN, when the Teensy is powered by the step-down-converter and a device is connected to the USB-host port while occasionally the PC has to be connected to the micro-USB programminig interface?
    You should leave that as-is. It you cut it, then any USB device you connect to the host port won't be able to get power from Teensy. It would only make sense to cut if you wanted to power USB devices an alternate way, or try to use that 2nd port in device mode (which has never really been supported, though WMXZ published an unofficial library...)

  3. #3
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    OK, thank you for the clarification.
    So, i guess i would have to use something like this to protect Teensy from
    accidentally increasing voltage at the DC-DC-converter's output:

    Code:
                                                additional Polyfuse 
                                                     150mA
                  _________                        _______                                             ______________
          + -----|         |--------------- +  ---|______|---------|------------------------ + -------|              |
                 |   DC-DC |                                      _|_                                 |              |
         15VDC   |         |          5VDC                        /_\  5.6V Zener                     |  Teensy 3.6  |
                 |         |                                       |                                  |              |
         - ------|_________|---------------- - --------------------|------------------------ - -------|______________|
                                 accidentally
                                  up to 15V

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Yes, if you want to protect against higher voltage and you can't just use a converter which doesn't go too high.

    But how well this actually performs depends on the specs of those parts. Many PTC fuses allow a steady current much higher than their rated current, and the exact amount can vary with temperature. If the over-voltage condition is sustained, quite a lot of power will be dissipated in the zener diode. Likewise, the zener voltage varies with current and temperature, so don't assume you'll always get a precise 5.6V. While you might get lucky, my gut feeling is this circuit won't perform well enough unless built with pretty extreme parts. An ordinary PTC fuse and zener diode probably won't work well.

    There are lots of ways to protect against overvoltage. Turning off the DC-DC converter (if it has an enable pin) or severing the input power are probably much more practical, even though the circuitry wouldn't be as simple.

  5. #5
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    The chances of the zener surviving long enough would be better with a resistor in series with the polyfuse.

  6. #6
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    Perhaps i'd better adjust the step down converter to 5.5 to 5.8 V and use a 5V low drop regulator behind it.
    The low drop 5V Regulator can easily cope with up to 15V and has got a thermal protection.

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