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Help with open collector output

Discussion in 'General Electronics Discussion' started by starscream, Sep 27, 2015.

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  1. starscream

    starscream

    17
    1
    Sep 27, 2015
    Hey guys, i was hoping you guys could maybe shed some light to me on how i can wire my open collector output receiver to switch on a 12v dc motor as I only have a very basic knowledge of electronics.

    The receiver and transmitter are paired fine and an on board LED indicates a successful transmission but i don't understand how to wire the screw terminals. How do i go about wiring up a receiver such as this in order to switch on a simple 12v motor. Do i have to add a relay? I have trouble understanding how open collector outputs work. If you guys could help me work this out i would appreciate it very much.

    [​IMG] motor.
     
  2. Alec_t

    Alec_t

    2,759
    735
    Jul 7, 2015
    When the open collector output is active it provides a low-impedance path to Gnd (conversely, a high impedance when inactive). How you use that depends on the current the output can sink (your datasheet or spec should tell you) and how much stall current the motor draws (usually a lot more than the running current). You will probably need to use the output to switch a power transistor, or relay, to act as the motor-switch.
     
    starscream likes this.
  3. starscream

    starscream

    17
    1
    Sep 27, 2015
    Okay so i did some experimenting and i found that when the receiver is activated i get about 3v dc coming out of the positive supply terminal and output terminal. So i could use a 3v dc relay to switch higher voltages using the output from the receiver correct? Thanks for the help Alec.
     
  4. Alec_t

    Alec_t

    2,759
    735
    Jul 7, 2015
    If you use a relay, don't forget the reverse-biased freewheel diode across its coil to protect the receiver from back-emf spikes!
    The relay pull-in coil current would have to be within the rated output capability of the receiver.
     
    Last edited: Sep 27, 2015
  5. starscream

    starscream

    17
    1
    Sep 27, 2015
    okay so i drew up a diagram of my plan. [​IMG]

    Red symbolizing positive and black symbolizing negative. So 3v dc will output from the receiver upon activation which will trigger the electromagnetic switch in the 3v relay which will then close up the circuit of the 12v battery and motor and provide the necessary 12v's dc to power the motor. But i need to add a freewheel diode to protect my receiver from the 12v battery which could damage my receiver?

    If that is the case where will i need to add the diode in the circuit? Once again thanks for helping me out.
     
  6. davenn

    davenn Moderator

    13,501
    1,830
    Sep 5, 2009
    yep, add it as I have shown,
    practically, the diode leads can just go into the screw terminals on the
    controller board along with the wires to the relay

    cct1.jpg


    Dave
     
  7. GPG

    GPG

    452
    66
    Sep 18, 2015
    Have you got a link to the receiver data?
     
  8. starscream

    starscream

    17
    1
    Sep 27, 2015
  9. GPG

    GPG

    452
    66
    Sep 18, 2015
    The out put is rated for 4A max. What current does the motor draw? You may not need a relay. Are you using the 12V for supply to the receiver.
     
  10. starscream

    starscream

    17
    1
    Sep 27, 2015
    The motor draws 390mA at full load. The power supply to the receiver is a standard 9v alkaline battery.
     
  11. Alec_t

    Alec_t

    2,759
    735
    Jul 7, 2015
    In that case I'm puzzled as to why you found only 3V between the V+ and output terminals when the receiver was activated :confused:. A 3V relay may not be suitable.
    I'm further puzzled by this bit of the receiver datasheet
    "Outputs:- Open Collector: Mosfet output maximum-switching 2Amps/40VDC"
    Mosfets don't have collectors!
    I read 2A max. I wouldn't try switching the motor directly from the output.
     
  12. Old Steve

    Old Steve

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    169
    Jul 23, 2015
    Like Alec, I'm puzzled as to why there's only 3V between +V and output when it's on. If your 9V battery is connected between +V and ground, then when it turns on you should be seeing virtually the full 9V between +V and the output terminal, with an open-collector output. (Actually open-drain, if it's really a MOSFET.)
    Just out of interest, can you tell us what the labelling is on the part I've circled below:-

    RC Receiver.JPG

    There are a few confusing details on the datasheet - they say MOSFET output, then say open-collector, which refers to a transistor and not a MOSFET, then elsewhere they say "Relay output on the receiver can function in either momentary or latching mode."
    They don't seem to be able to make up their mind if it's a transistor, a MOSFET or a relay.
     
  13. GPG

    GPG

    452
    66
    Sep 18, 2015
    Misread. I see no reason not to drive the motor directly if the stall current is within limits. Or if running a 12V motor using a common supply
     
    Last edited by a moderator: Sep 29, 2015
  14. davenn

    davenn Moderator

    13,501
    1,830
    Sep 5, 2009
    that's the problem, we don't know the stall current, could easily well exceed 2A
    I wouldn't risk damaging a nice receiver board
     
  15. 73's de Edd

    73's de Edd

    2,805
    1,176
    Aug 21, 2015

    .


    Sirs:


    [​IMG]
    (As it was " channeled " down to me by Paul Harvey . . . . . . good day.)

    That looks like quite a pricey item to buy . . .unless it was "gifted " to you . .to be using it for just a simple
    RF remoted aspect of a mere whiz-bang function of activating a motor.

    My marked up photo . . . . . exhibit 1 tells all:

    [​IMG]




    This lets you readily see two principal IC semiconductors in the FINE RED LINED area


    Old Steve . . . . . . . sez
    Just out of interest, can you tell us what the labeling is on the part I've circled below:-



    The FINE YELLOW LINED area has the Siliconix Si 4906DY dual mosfet, used for system switching duty.


    Alec_t . . . . . . sez
    In that case I'm puzzled as to why you found only 3V between the V+ and output terminals when the receiver was activated.

    Old Steve . . . . . . sez
    Like Alec, I'm puzzled as to why there's only 3V between +V and output when it's on. If your 9V battery is connected
    between +V and ground, then when it turns on you should be seeing virtually the full 9V between +V and the output terminal,
    with an open-collector output. (Actually open-drain, if it's really a MOSFET.)


    Note:
    The FINE BLUE LINED area is the regulator area

    That area is fed by the Maxim MAX5024 power supply chip which has an option of 3.3 or 5V output.


    Aside:
    I am seeing the RED buss for the long wire "aerial" to the left or its optional SMA hard connector for accepting a whip.
    About all I am not seeing is there being a little SMT bipolar xstr, in just about the lower hidden area that the RED star is
    marking on the u/p xtal.
    With it doing yeoman duty in being a sensitive superregenerative receiver for the system.

    Sooooooooooooo . . . . . with a 300 ohm pull up resistor/load that should be providing adequate drive for whatever SEPARATE ancillary power switching device is to be utilized for the motor.

    Of course, instead of having a pull up resistor, that could also be fulfilled with a like load loop as being provided by a micro reed relay, an optical isolator,etc .

    Thasssssit . . . . . .

    73's de Edd



    .
     
  16. Old Steve

    Old Steve

    734
    169
    Jul 23, 2015
    Yes, but if it's an open-drain output, the drain will be pulled to 0V when the 3.3V / 5V is applied to it's gate, so measuring between the drain, (output), and the +9V supply terminal should result in a reading of ~9V.

    I thought that was a MOSFET package in the fine yellow-lined area. That was why I asked for the numbering.
     
  17. Old Steve

    Old Steve

    734
    169
    Jul 23, 2015
    73's de Edd, you say:-

    "That area is fed by the Maxim MAX5024 power supply chip which has an option of 3.3 or 5V output."


    Besides what I said about the open-drain output, that 3.3V or 5V can't be used to switch the SI4906DY dual MOSFET, either - it requires 16V to switch effectively. They are not 'logic-level' MOSFETs.

    The datasheet:-
    http://datasheetz.com/data/Discrete...ETs - Arrays/SI4906DY-T1-E3CT-datasheetz.html
     
  18. GPG

    GPG

    452
    66
    Sep 18, 2015
    The 16V is max gate voltage. Threshold is .8 - 2.2V Easily switch several amps at 5V. Rds on .05Ω at 4.5V (each). Looks like they are wired in parallel Given that the min supply voltage min is 7V I think the output is 5v
     
    Last edited: Oct 1, 2015
  19. Old Steve

    Old Steve

    734
    169
    Jul 23, 2015
    Oops, you're right. I accidentally looked at the wrong figure, instead of the gate threshold voltage. I thought it seemed high for a threshold voltage, usually about 10V with a standard MOSFET. It is logic-level. Makes more sense now. (Dunno how I managed that. :rolleyes: )

    Still, what I said about the open-drain is true.
     
  20. starscream

    starscream

    17
    1
    Sep 27, 2015
    Okay so ive figured out my i was using a 9v battery that was almost out of juice and hence why i was getting a lower output voltage than the assumed 9v supply. Put a fresh 9v battery in and it seems to give me a full 9v's output. Using a relay with the freewheel diode in place i can successfully control my 12v motor!
     
    Old Steve likes this.
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