Help with L298N

Discussion in 'General Electronics Chat' started by JRM1, Sep 15, 2008.

  1. JRM1

    Thread Starter Member

    Aug 30, 2008
    18
    0
    SgtWooki
    You helped me with my questions back a couple of weeks ago. The chat was titled Help with 2N3055 Transistor.
    You suggested that I use the L298N to drive my coils. I have pretty much been working on it since then. I've had several stumbling blocks but nothing I couldn't work out until now. I installed 4 L298N's on the board and am driving 8 coils. All of the controls seem to work fine. I put a scope on the outputs and it was switching polarities as desired. However, as soon as I hooked up the coils with the scope still hooked up, the outputs went to a steady state +12VDC. No longer switching.
    I have attached a simple scetch of the basic circuit. I found out that when I copy from AutoCad and paste to paint that it pastes in a lot of white space that I can't get rid of. I drew this circuit in paint. Let me know if it doesn't come through OK.
    The frequency of the incomming pulse is 100ms on and 100ms off so about 5 cps.
    Any suggestions??
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, you can't just hook a couple of inverters together and drive the L298 like that.
    The bridge requires some "dead time"; that is, both the upper and lower sides of the bridge turned off.
    You do this by:
    1) Place a logic low on the Enable input.
    2) Wait at least 2uS.
    3) Change your input states.
    4) Wait at least 2uS.
    5) Place a logic high on the Enable input.

    This will ensure that the upper and lower side of the bridge will never both be ON at the same time. If they are allowed to be on at the same time, you will have a direct short from V+ to ground via Rsense, and you will burn things up.

    Also, you need to have a sense resistor connected. Rsense should be purely resistive, and of a suitable rating for your coils. The idea of Rsense is to allow you to measure the current in your load by measuring the voltage across Rsense. If you don't care about your load's current, you can connect the Rsense terminal directly to ground.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    896
    Sense pins 1 and 15 must be grounded or in series to ground with a current sensing resistor.
    Pin 8 must be grounded.
    Enable pins must be a logic high.
     
  4. JRM1

    Thread Starter Member

    Aug 30, 2008
    18
    0
    I do have pins 1, 15 and 8 tied to GND. I have a logic high tied to the enable pins. I guess I just got lazy and thought that would be assumed. Paint is very cumbersome for me to work with. I'm trying to learn LTspice but have not been very successful in locating components.
    I'll try to place some dead time in there by switching the enable inputs.
    Thanks
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You'll also have to look at the propagation delay from the 1st inverter to the 2nd inverter.

    You will be far better off with too much "dead time" than not enough "dead time".

    You'll need to calculate the propagation delays for everything in the circuit to ensure that "shoot-through" doesn't happen. You can find the propagation delays in the datasheets for the components. Use the datasheets from the manufacturer that are appropriate for the components that you actually have on hand. If you don't, you'll wind up with big problems.
     
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