Thread: Signal Inverter
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Old 04-15-2010, 07:04 PM
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Originally Posted by RSmith View Post
I need some assistance, please, anyone, I'm a novice so I hope I can communicate what I need sufficiently for you to understand.
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.

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".
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.

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 mock-up with an led it works as expected as long as the input signal never floats.
CMOS inputs must never be left floating; they must have a current path to either Vdd or GND.

I don't know what kind of liberties I can take in mixing TTL and CMOS.
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.

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.
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.

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.
The small packages are getting tough on hobbyists.

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

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.
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.
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
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