Signal Inverter

Discussion in 'The Projects Forum' started by RSmith, Apr 15, 2010.

  1. RSmith

    RSmith Thread Starter New Member

    Joined:
    Apr 14, 2010
    Messages:
    3
    I need some assistance, please, anyone, I'm a novice so I hope I can communicate what I need sufficiently for you to understand. I'm interfacing an off the shelf board that puts out a PWM (Dalf board from Robot Power) signal that I'm attaching to a Powerex hybrid IGBT driver. The Dalf board's PWM signal is 5VDC CMOS and the input on the IGBT driver is TTL. The documentation from Powerex says the PWM signal has to be inverted before it goes to the driver because the IGBT module is active when the PWM signal the driver gets is "low". The Powerex documentation also suggests using a CD74HC04E CMOS hex inverter. The hex inverter comes in a 14 pin DIP package and on a breadboard mockup with an led it works as expected as long as the input signal never floats. I don't know what kind of liberties I can take in mixing TTL and CMOS. I have not wired up the Dalf board, hex inverter and IGBT driver yet, only the hex inverter on breadboard with an led and 5VDC power supply. I would like to get a single channel inverter so I don't have to be concerned with what to do with the unused inputs and outputs. I have contacted Digikey and Mouser both and they say all they have in a single channel inverter is in a smaller package, so small I can't easily handle it. I also ran across logic to logic optocoupler buffers and inverters in a package that can be handled and used on breadboard and also gives an additional degree of isolation. The optocouplers I've looked (Fairchild 740L6001) at come in TTL and Cmos outputs (there may be others) but only LSTTL input, here again I don't know what liberties I can take in mixing. I also don't really have a good grasp on what additional components I should be adding to this interface circuit in the way of pull up/down resistors and/or filter capacitors before and/or after the inverter be it the CD74HC04E, 740L6001 or something similar. Sorry for the long windiness. Thanks in advance, Rod S.
  2. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,094
    Location:
    In the vast midwest of the USA; CST
    Hello Rod. Welcome to AAC.

    Just a suggestion; it makes text easier to understand if it's broken down in paragraphs. Some of us are a bit "long in the tooth:, and seeing a large blob of text usually makes me want to go look at something else instead.

    Well, since we don't have the documentation that you do, you will need to find out what the fan-out of the Dalf board's output signal is. If it's 4000-series CMOS, you will have very little current to work with. If it's 74C or 74HC, you'll have a bit more to work with. The docs should say what the fan-out is.

    CMOS inputs must never be left floating; they must have a current path to either Vdd or GND.

    We can't tell you, because we don't know what KIND of CMOS you are working with. However, rather than a 74HC04, a 74HC14 would be better, as it is a hex Schmitt-trigger inverter. The Schmitt trigger greatly reduces the likelihood of false triggering, and "squares up" an input with slow transition times quite nicely.

    Don't worry about the unused OUTPUTS; just leave them disconnected. It is the unused INPUTS that you need to take care of. If you fail to provide a path to either Vdd or GND, they may oscillate at high frequencies, causing hard-to-diagnose problems.

    The small packages are getting tough on hobbyists.

    I'd avoid optocouplers unless you really need the isolation. They can be mighty slow to turn on and off.

    You didn't post a schematic of your circuit as it currently exists.

    We don't have any documentation for the parts you're using, except for the 74HC04's which are industry standard components, and the optocouplers which you provided part numbers for.

    Post your schematic, most preferably in .PNG format, complete with part numbers and reference designators (ie: R1, C3, U1, etc.)

    Use the "Go Advanced" and "Manage Attachments" to upload the image from your computer to the forum.
  3. RSmith

    RSmith Thread Starter New Member

    Joined:
    Apr 14, 2010
    Messages:
    3
    Thanks SgtWookie,
    I see your point and will break future posts into more manageable text.

    The Dalf board actually uses a PIC microcontroller (PIC18F6772) http://ww1.microchip.com/downloads/en/devicedoc/39646b.pdf it has two I/O pins 6 and 8 configured as outputs.
    From the documentation I can't tell if it's 4000-CMOS or what and maybe since it's a PIC it doesn't apply. I did a search in the PDF for "RG3" which is part of the pin's names but still did not find what I would consider the exact info. your'e asking for.
    I did find a description of the PWM pins as "Schmitt Trigger input with CMOS levels". But I would not ask anyone to wade through over 400 pages of text to find what we need. I would search more if I knew what else to look for.

    Could the description "Schmitt Trigger input with CMOS levels" relate to the 74HC14 being a better choice as you suggested?
     
    This is a PDF on the Dalf board http://www.embeddedelectronics.net/documents/Latest/dalf1_om.pdf see page 120, of particular interest is pins 6 and 8 on the left side of the PIC chip.

    This is a short PDF of the IGBT driver http://www.pwrx.com/pwrx/docs/db_00004c.bg1a.pdf

    Sorry I don't have a schematic at this time but as far as wiring the Dalf board to the hex inverter and then to the IGBT driver not knowing any better I would run the PWM signal line to a input on the inverter then run the corresponding inverter output to the driver. tie unused inverter inputs to ground and supply 5VDC and ground from the Dalf board to the inverter. the Dalf board inverter and drive would have common grounds.
    The IGBT would have an independent power supply.
  4. beenthere

    beenthere Retired Moderator

    Joined:
    Apr 20, 2004
    Messages:
    15,815
    Location:
    Missouri, USA (GMT -6)
    Would a single logic level FET like the VN10LP work as the inverter?
  5. RSmith

    RSmith Thread Starter New Member

    Joined:
    Apr 14, 2010
    Messages:
    3
    I also ran across this in the documentation for the PIC

    Maximum output current sunk by any I/O pin..........................................................................................................25 mA
    Maximum output current sourced by any I/O pin ....................................................................................................25 mA


  6. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,094
    Location:
    In the vast midwest of the USA; CST
    RSmith,

    I received your E-mail. Generally, if you have not received a reply to your thread in several days, simply make another reply at the end of your thread to bring it up in the list. The forums are not interactive; it may take awhile to receive a reply. However, if it's been several days, people have likely forgotten about it.

    PIC uC's (microcontrollers) logic outputs can drive either TTL or CMOS if they are using the same voltage for Vcc/Vdd.

    Page 120 of the document you linked to had nothing to say about pins 6 or 8. I could spend a lot of time wading around that document trying to dig out the particulars. Sorry, I simply don't have that much time.

    If pins 6 and 8 are PWM outputs, you don't really need the 74HC14, you could probably use a 74HC04 - as long as the logic levels used by the destination are also 5v.

    If the destination is using a different voltage for Vcc/Vdd, you may need to use a 74HC05 (open-collector/open-drain outputs) and pull-up resistors. The Dalf board, the 74HC04/05 and the destination board all need to have the same ground potential.
Similar Threads
Forum Title Date
The Projects Forum Video signal to IP based signal Oct 9, 2014
The Projects Forum 0-10vdc analog control signal - split to a relay Sep 29, 2014
The Projects Forum Counting Signals Before Turning On a Load Sep 23, 2014
The Projects Forum Amplify and transmit RF signals Sep 14, 2014
The Projects Forum Create a simple High pass filter to sense AC signal Sep 3, 2014

Share This Page