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Converting from 30V to 5V or less

Discussion in 'Electronic Basics' started by [email protected], Sep 19, 2008.

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  1. Guest

    I'm using a computer to gather data using LPT1 port. The input pins of the lpt
    accept 0-5V.

    I want to know when three utility motors using 30V start and when they end. When
    a motor starts it gets 30V. Actually they should get 24V but occasional bursts to
    30V are normal. The motors themself can handle the extra voltage.

    What is the simplest way lower the voltage to 5V for my LPT port? I've done
    simple electronics but a simple schematic would be great. I want to know when
    each of the motors start.
  2. Jamie

    Jamie Guest

    Using a Photo coupler is the safest method that I know of.
    Just pull one of the input pins of your port using the photo output

    Since you stated 30 V's maybe the max at times I would use ~ 2 thirds
    of the LED (If) which is ~ 45 ma.
    R = (30-1.2)=(28.8)/0.045 = 640 ohms.
    W = 0.045*1.2 = 0.054 watts.

    Might be advisable to use a zener diode of around 30 volts so
    that it can be protected from reverse voltage exceeding (Vr)
    6 and forward voltages exceeding 30 volts.

    If this is a brushed motor, it can generate inductive energy that
    isn't safe even for the coupler. A cheap zener diode helps this out in
    minor cases and the photo coupler helps in major cases to protect the
  3. Jamie

    Jamie Guest

    Yes, I made a mistake there. got my 1.2 mixed up with my 28.8.No, I didn't miss it.

    I just wouldn't subject the PC port
    to a motor circuit. Although you
    have covered all bases as far as over voltage
    and reverse voltage, you didn't account for
    current loops on the ground path between the devices which loves to
    make PC's miss behave.
    Motors are notorious for generating such currents if
    improper installation is performed.

    To each, his own.
  4. Jasen Betts

    Jasen Betts Guest

    Just changing the voltage is possibly not enough, things like ground
    loops cour damage your pc or stop the other equipment from working.

    the easiest way is use a 24V relay, or to use optocouplers.

    The parallel port inputs have internal pull-ups so you only need to
    connect them to the pc's digital ground or disconnect them - you don't
    need to provide a 5v signal.

  5. Jasen Betts

    Jasen Betts Guest

    He asked for that, but what he wants is a way to get 0V to the printer
    port. the input pins have interal pull-ups.

    For it to read zero the motor must have sufficiently low voltage
    drop to ground the data pin through your 1K resistor...
    If it's a (for example) a brushless DC motor the voltage drop
    may be sufficient to present as logic high with the DC supply

    I'd add a resistor in parallel with the zener to provide a reliable
    path for the off-state signal current, 470 ohms would be a good choice.

  6. Test

    Test Guest

    Thanks guys, I'll try either way.

  7. Test

    Test Guest

    What is a "pull up"? How would I connect this to my printer pins? The 30V is btw
    AC (just found out).

    Right now I have John Field's:

    1/2 WATT
    | |
    [MOTOR] [1N5230B]
    | |

    Just to be safe I'll do this:

    30V----+--[1000]--+------+ +---------+
    | | +---+ +----> pin? in LPTport
    [MOTOR] [1N5230B] | 5V relay|
    | | +---+ +----> pin? in LPTport
    GND>---+----------+------+ +---------+

    My aim is to find out when the motor is powered up. In above the voltage is
    dropped to <5V for the relay not to fry.

    What about the current? I don't know what the current is.
  8. Test

    Test Guest

    Thanks. I was rather surprised for the AC. I was testing teh first simple setup
    with another transformer that was DC.
    I'll have to rethink the system over again.

    Thank you for your time.
  9. Jamie

    Jamie Guest

    This is what you need to make it simple and still safe for
    AC work. You only too use a cap on the port side so you
    don't get a pulse on the pin.

    This is an AC input version much like the one's used in the front ends
    of PLC cards that accept AC as the input type.

    Found at digikey.. as"
  10. I would not bet on the fact that your motors are referenced with the
    same ground. If you connect the wrong wire to your PC parallel port,
    and you have ground issues, you can destroy your mainboard.

    You can test this by using a DC voltmeter between the ground at the
    parallel port and the ground at the motor.


    Bob Monsen
  11. Jasen Betts

    Jasen Betts Guest

    a resistor used to increase the voltage at a point in a circuit whenever
    it's not connected to any input signal.

    what this means is that you don't need to provide 5v to the printer
    port, you need to take it away from the printer port.
    I had suspicion it might be...
    you'll just need to change that a adding a rectifier so
    that the relay gets DC, and a capacitor to stop it switching off while
    the AC voltage altenates, and reduce the resistor so that the relay
    gets enough power....

    An opto-coupler will use less power than a relay and is less fussy
    about how much power it gets which makes the design easier (cheaper
    and more reliable too)

    1N4002 .------.
    --+-------->|---+---[3300]--[|o \/ |] PC
    | | | |
    | 10uF=== .---[| 4N25 |]-- pin 10/12/13
    30VAC (M) 50V |- | | |
    | | | [| |]-- pin 20
    --+-------------+-------' `------'

    the diode can be 1N4002 to 1n4010, the capacitor should be rated
    for atleast 50V (more than 50V is fine)

    with those pins (pick one of 10/12/13) it'll come up as the bit
    6/5/4 on port 0x379.
    which current?

  12. Test

    Test Guest

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