getting power though prolific uart converter

Discussion in 'General Electronics Chat' started by ep.hobbyiest, Mar 3, 2018.

  1. ep.hobbyiest

    Thread Starter Member

    Aug 26, 2014
    127
    1
    Hi,
    i have one following doubt on connection,

    i have one arduino pro mini board and i have connected it to external power supply and connected prolific usb to uart converter (RX,TX and Ground pins only).

    When i disconnect my external power supply then my board is getting power through USB-UART converter even though i didn't connect Vcc pin to pro mini through USB-UART converter.

    How to avoid such power supply problem and why this is happening?
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
    5,991
    1,132
    hi ep.
    I have an identical set up and I get the same problem, tried some simple fixes with no success.
    Finally resorted to unplugging the Prolific USB converter when not working on the project.:rolleyes:

    Interesting to hear if anyone has a quick fix.
    E
     
  3. LesJones

    Well-Known Member

    Jan 8, 2017
    1,497
    356
    I think I have seen a similar thing with an FDTI FT232RL (USB to serial board.) when connected to a PIC12F1840. The PIC continued to work when it's power was removed. As the static state of the TX from the FT232RL is high I think the input protection diode on the PIC input is feeding this input signal to the PIC Vcc pin. As there is a decoulping capacitor between Vcc and ground this powers the PIC during the time the serial input is low.

    Les.
     
    Last edited: Mar 3, 2018
  4. MrChips

    Moderator

    Oct 2, 2009
    17,137
    5,296
    I am assuming that the problem arises because TTL TX data logic is at a positive voltage when the UART is idle. This TX data line powers any CMOS device through the internal protection diodes at the RX input of the device.

    upload_2018-3-3_9-42-58.jpeg

    Current flows from the TX data line into the upper protection diode to Vcc, thus powering the CMOS circuit.

    You can try using a 3-state bus driver such as 74HC126. This is usually a quad device, i.e. four drivers in one 14-pin package. You can get 1-gate devices such as 74HC1G126 which is one gate in a 5-pin SMD package.

    [​IMG]

    Connect the OE pin to Vcc and both to the Vcc of the receiving device. When OE is low (i.e. power is removed), the output pin Y goes into a high-impedance state.

    (I have not testing this solution.)
     
  5. ep.hobbyiest

    Thread Starter Member

    Aug 26, 2014
    127
    1
    Well Explained MrChips.

    But i have doubt here, If we connect Converter IC and PIC micro's RX,TX and ground then as you explained that way PIC will running, because power line will get connected.

    Suggested chip is to avoid supply to external circuit if i am not wrong. But this way we can avoid supply to external circuit excluding Controller.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    17,137
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    Do not connect to the USB power line.
    HC126 OE pin is connected to PIC Vcc
    HC126 Vcc pin is connected to PIC Vcc.
    Prolific Vcc is connected to USB +5V.
     
  7. LesJones

    Well-Known Member

    Jan 8, 2017
    1,497
    356
    ep,
    I don't understand your last sentance. I can see no reason why Mr Chips solution should not work. An alternative solution that I was thinking of suggesting was to connect a diode (Preferably a schottky.) between the output of the USB to serial converter and the input of the PIC (Or in your case an Atmel device.) with the cathode to the converter output so it can only pull the input low. A pullup resistor would then be required between the input pin and the microcontroller's Vcc pin.

    Les.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    17,137
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    That should work too and seems like an easier fix than using a 74HC126.
     
  9. ep.hobbyiest

    Thread Starter Member

    Aug 26, 2014
    127
    1
    Might be your point is right and i didn't get yet.
    I ll try to explain my point again,

    Here i have attached simple raw connection between usb to uart bridge and controller. In this case i left open converter's VCC open. i didn't connect that.
    rawconnection.png


    I have connected external power supply to controller. When i disconnect external power supply and kept Converter module connected to PC then that time my controller keeps on running. Even though i didn't connect VCC.

    No didn't connect VCC.
     
  10. MrChips

    Moderator

    Oct 2, 2009
    17,137
    5,296
    We understand the problem. You don't need to connect Vcc to Vcc.
    The connected circuit is getting its power from the TX data line, and that is not desirable.
    You need to block current from flowing via the TX data line. A diode as Les suggested should do it. Then you need a pullup resistor on the input side of the receiver.
     
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