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DC-DC Isolator

Discussion in 'Power Electronics' started by gillesfan, Nov 5, 2017.

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  1. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Hello Guys

    I was wondering if someone could help me with a circuit?

    I am trying to read the inputs from a slot car track to a computer ADC. My attempts to directly connect the track have resulted in dead pins on the computer.

    My initial attempt being
    Code:
    0-22V DC ------------
                         |
                       10k Ohm
                         |
                         |---------------------150 Ohm-------------- Computer
                         |
                       2K2 Ohm
                         |
    GND----------------------------------------------------------- Computer GND
    
    I suspect the attempt to connect Computer and slot track GNDs was the root of my problem.

    How can i introduce a DC-DC converter which will isolate the grounds?

    Any help gratefully received.


    It is important the output follows the input proportionally.
    I know this would be fairly trivial if the input was AC, unfortunately its not and i really need to isolate the output.
     
    Last edited: Nov 5, 2017
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The motors in the track will generate voltage spikes which may be very high. The reduction in voltage due to the input resistors will help but may not be adequate.

    Put a zener across the ADC input to limit the positive and negative voltages.
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    10,768
    2,425
    Nov 17, 2011
    Having the track isolated from the computer is a good idea. But this is not a dead simple thing. Several options come to mind. The most simple is this circuit:
    upload_2017-11-6_7-39-26.png
    By splitting the10 k resistor of the voltage divider into 2 * 4.75k resistors, you provide a high impedance part in bot ground and power rail. This is not complete isolation but protects bthe computer's input from high currents. Additionally the zener diode as suggested by duke protects the input from voltage spikes while the small capacitor helps to smooth the DC input for better stability when doing the A/D conversion.

    A more sophisticated scheme can be built using a linear optocoupler, e.g. the IL300.
     
  4. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Thank you guys.
    This information is a big help.

    I am currently using simple optocouplers for the timing system, the linear Opto couplers sound ideal. In my ignorance i was unaware of their existence.

    I shall start experimenting with them.

    All the Best
     
  5. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Well i seem to have a system which tracks the changes well but the computer ADC is not seeing it.
    I'll let you know how it goes
     
  6. Alec_t

    Alec_t

    2,909
    791
    Jul 7, 2015
    Exactly what 'inputs' are you interested in ? Just the track supply voltage; or some track-superimposed signals ?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    10,768
    2,425
    Nov 17, 2011
    Upload a circuit diagram. It's hard to check what you're doing without one.
     
  8. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    HI

    I think i have killed my IL300s

    It was working like this yesterday but now it is not.

    In essence, what is happening is that the IL300 seems to be deadening the input of the LM741

    The input goes into the LM741 just fine and comes out with the waveform i expected. A DC offset with half sine on top.

    This is all fine until i connect it to the IL300, then instead of transferring the signal across as it did yesterday it deadens it to a slightly lower DC.

    The same think happens with all my IL300s and did not happen yesterday.
    I have checked my circuit several times but the problem remains.

    [​IMG]
     
    Last edited: Nov 13, 2017
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    10,768
    2,425
    Nov 17, 2011
    The 741 is completely unsuitable for this application. You need a rail-to-rail opamp like e.g. the OPA341.
    Also Track - should be connected to DC gnd.

    Plus you need an output amplifier to generate a useable signal for the ADC from the weak opto-output. See this application note.
     
  10. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Is it likely i have killed my IL300s? Is there any way to check them?

    Diode Checks
    IL300
    Pins 1,2 -- Infinity and 1880
    Pins 3,4 -- Infinity and 770
    Pins 5,6 -- Infinity and 770


    OPA341s seem hard to source are there alternatives?

    To clarify the arrangement
    THROTTLE MONITOR (for calculating fuel consumption and controlling sound)

    [​IMG]

    My electronics knowledge unfortunately is limited (more familiar with coding) i chose the 741 as a direct replacement for 241s in an example circuit i saw.

    Ideally an output of 0- 3V or 2.5-4.5V should result i'm not bothered.

    Any help with what i am doing is hugely appreciated



    The DC polarity was used for the timing system which is as follows


    TIMING CIRCUIT[​IMG]

    Where
    A is 470 Ohm
    B is 1K8 Ohm
    C is 1K8 Ohm

    THis has been stable, but i admit i am struggling a bit. I may have drawn my diode teh wrong way round. It's the right way around in the circuit.

    Any help with these circuits is really useful.
     
    Last edited: Nov 13, 2017
  11. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    I am not interested in anything superimposed its only superimposed because of the timing system

    What i want to do is use the computer to monitor throttle position .
    Its currently sampling the timing every 1/1000 sec, and also two ADCs for which i was going to sample throttle position, average these every 1/20 sec or so and then use that to approximate fuel usage, engine RPM (for playing digitised sound) and detect skid/accidents (when the car leaves the slot, the Voltage increases from the 0-15V range up to about.

    Initially i connected the comouter directlt to the track (with resistors to reduce the voltage to avoid over voltage, and diodes to avoid reverse voltage and capacitors to avoid spikes.
    Unfortunately i still killed a couple of pins on the computer which i suspect must have been to floating GND between track and Computer

    Thus i introduced the opto couplers.
     
  12. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    My test case is using a rectified sine wave source but the actual slot track source will essentially be a varying DC from 0-22V
     
  13. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    OK well with a different circuit i can confirm my IL300s are not dead.
    Thus 1880, 770 and 770 seem to be sensible numbers.

    I will push on with the new circuit and see what happens
     
  14. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    If those are millivolts on the diode test range then they are quite reasonable. 1.8V for the LED and .77V for the photodiodes.

    Bob
     
  15. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Hi i assume that is what they are, it is the diode setting on my DVM, and yes 1880 for the LED and 770 for the photodiodes yes.

    I have a system working now with the 741s.

    It is not ideal and my ADC is exhibiting very odd results when the IL300 is connected that is, when the circuit is not connected to the ADC i get a steady voltage.

    Yet when the ADC IS connected, i get a sine wave at about 500Hz. Both on the ADC resonse AND on a scope connected.

    Any idea why my ADC is overlaying a sine wave? ANy idea how to eliminate it?
     
  16. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Well i am not disatisfied wth the signal until i connect it to my ADC, which seems to turn it into quite a different signal.

    [​IMG]
     
  17. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Revised circuit associated with the traces above

    [​IMG]

    I have replacement rail-rail op amps on their way.
     
  18. Alec_t

    Alec_t

    2,909
    791
    Jul 7, 2015
    Why is the 741 getting its +ve supply via R2?
     
  19. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    And where is the required buffer ampl after the IL300?

    Bob
     
  20. gillesfan

    gillesfan

    17
    1
    Nov 5, 2017
    Ahhh there has to be a buffer amp? i do not have one in there will that make a difference? I have seen circuits with and without i was trying to do it step by step and i have not put a buffer amp in after the IL300 so far

    Well the output is essentially the same except at the ADC where the out put is different but still not what i want.
     
    Last edited: Nov 16, 2017
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