Issus with 2nd L297/L298 pcb

Discussion in 'The Projects Forum' started by nerdegutta, Jan 7, 2014.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009

    I've made a Bi-polar step motor controller based on the L297/L298. Schematic is attached.

    I made one PCB, and it works fine. There was some small issues with tracks to close to one another, but I fixed it with a knife. According to the DMM there are no connections, or shorts on any wire/trace.

    So I made another PCB, based on the same transparent printout. I'm making PCB's with the photoresist method.

    With the 2nd PCB I cut the tracks with the knife, and started soldering. Checking for solderblobs and such.

    Starting with the vias, and moving on to the powerblock. Making sure everything was fine before moving on to the next component.

    When all was finished soldered on, I "started" the board, with a 555 to generate clock signals. The LEDs connected to the L297 output blink correct. But when I connect the O'scope to the stepper connector. I didn't get the results I wanted. See PNG of O'scope.

    Channel 2 is correct, and channel 1 is a mess. What causes this? I've tried with 2 different L298. (Heatsink is removed to get better picture.) I've also tried to connect a Bi-polar stepper, but it do no run right. Just like it is skipping every other step, and according to the O'scope, that is correct.
    Last edited: Jan 7, 2014
  2. SgtWookie


    Jul 17, 2007
    Where are your .brd and .sch files?

    What is your desired current flow through your stepper motor's windings?

    You have Vref grounded. This will result in the chopper circuit being continuously tripped.
    The range for Vref is 0v <= Vref <= 3v. However, you must avoid exceeding the maximum power rating of the L298, which can get a bit tricky, as the total voltage dropped across the bridge increases with current.

    You have R7 and R8 shown as 0.5 Ohms each, as shown in the datasheet:
    So, when 1A current is flowing through either of them, there will be 0.5v dropped across them. If you wanted 1A to be the maximum current, then you would set your Vref to be 1v using a resistive divider. A 1k and a 3.9k resistor in series across GND & +5v would be plenty close enough.

    Use a LARGE heat sink on the L298!! If you're near 2A current flow, the L298 will drop around 2v on the low side and around 1.8v on the high side; for ~3.8v total drop. That is 3.8v x 2A = 7.6 Watts; a large amount of heat to get rid of.
  3. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
    I've updated first post. I know some of the traces are a bit to thin and too close.
    I've tried with 3 different motors. Nema 23, Nema 17, and a AEG. Max current will be appx 3A. L298s datasheet says it can handle up to 4A

    I've used the circuit in the datasheet as a starting point. Googling the rest. I cannot see a 1k and a 3.9k resistor in any of the schematics I've read.

    I have a heatsink, and a 5v fan cooling it. When it test runs, the PCB that works, the heatsink is not too hot. It's "touchable".

    But why does one PCB work, and the other not? Is the working one just a "lucky punch"?
    Last edited: Jan 7, 2014
  4. Alexander Rivera

    New Member

    Mar 2, 2015
    Hi Very Good Day
    We are trying to build a 3D printer, our driver system are based on L298 and L297, we are using ceramic resistance 3 Ohms 5 Watts.
    We really appreciate your comments and suggestions.
    We are looking for sponsors and researchers and customers.

    ID e-waste de Colombia Alexander Rivera