mosfet problem

Discussion in 'General Electronics Chat' started by rjjenkins, Dec 2, 2011.

  1. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    I have a very simple circuit that isn't quite doing what it should.
    I am using a radio control receiver module to drive a mosfet - a BUZ72 which drives a LED (for testing purposes)

    So the output from the receiver module goes to the gate
    The source goes to ground
    The drain goes to an LED (the other side of which goes to VDD)
    And there is a 10k resistor between the gate and ground

    I'm running this off 4 x AA batteries

    The output from the receiver toggles (i.e. when I press the button on the transmitter it goes on; when I press it again it goes off.) All this works fine.
    But the LED doesn't quite follow this. It goes on, but when I press the button again, it doesn't always go off: often it flashes off and comes on again.

    This also happens when I replace the LED with an actuator.

    Any ideas please?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Datasheet for BUZ72 is here: http://www.datasheetcatalog.org/datasheet/siemens/C67078-S1313-A2.pdf
    Do you also have the receiver module ground connected to the BUZ72 source terminal?

    Do you not have a current limiting resistor between the LED cathode and drain, or the LED anode and Vdd? If not, you are greatly stressing the LED. You may be clamping the battery voltage until Vgs is near the threshold. You would need about 200 Ohms if Vcc is 6v and you are using a red LED.

    What is the Vgs when the output of the receiver goes high?
     
  3. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Yes, the module ground is connected to the source terminal
    The LED is a green one with a resistor incorporated I believe but in any case if I replace it with the actuator the same thing happens (i.e it won't always turn off, but goes off and on again)

    The Vgs is about 5.8V when the output is high
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    And the LED goes on and off without regards to the Vgs?

    Something is changing to cause it.

    Might help if you used a logic level MOSFET.
     
  5. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    The vgs follows the LED
    The problem is worse with the actuator (i.e. it quite often doesn't go off on the second button push, or rather, it goes off and on again)
    Pushing the button quickly seems to help - which suggests switch bounce, but there isn't any indication of that from the output of the receiver.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    How much of a load can your output source?

    Try a 1k resistor between gate and ground, that should turn the MOSFET off with no input. 10k should too, but there aren't many other parts to change, other than adding the resistor to the LED, as Wookie mentioned.
     
  7. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Thanks. Doesn't seem to make much difference with different resistor values. Connecting the LED from the receiver output to VDD does seem to help. Not sure why
     
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    What transmitter module are you using?

    Depending on the transmitter it sends more than one time the same code which could then toggloe the receiver circuit. I had a similar problem with some transmitter / receiver modules. You have to make sure that the receiver detects only once the code you sent.

    In order to avoid a problem like this you could use two different outputs from the receiver module (in case you have 2 available). Then you use two buttons on the transmitter side, one for ON for OFF.

    In the description of the transmitter/receiver module there should be described how long it takes after a button press/SEND command to have it received. Limit the time you apply your SEND command to the transmitter in order to avoid multiple detections on the receiver side.

    The very best would be to have a transceiver on both sides, the transmitter would then have a feedback and would know how long it has to resend the same command.

    Of course, your problem could be something completely different.
     
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