Help pls L297/L298 board problem

[crazydaddy]

Am testing an L297/298 driver board that I constructed, pretty standard chopper design. It works fine with two different examples of small unipolar stepper motors. A larger stepper drives ok whilst my test clock is pulsing it but as soon as the clock stops the L298 output goes into an illegal state with all 4 outputs on (as indicated by 4 leds tied to each output)! Clock back on and off she goes no problem. Have tried a larger capacitor across the supply to no effect. Have tried another example of the same 34 frame motor and get same problem.

Output from the L297 at idle seems to be correct with no more than two inputs to the L298 being driven.

Tried another L298 chip, same problem.

Any ideas? Suggestions for diagnostics? Help would be appreciated.

Cheers

[crazydaddy]

Doh! 'Problem' solved. It was just me being stupid and not thinking about the voltage across the motor coils driving the leds. The current drawn when the motor is idle (no clocks) is correct. The board is working perfectly it seems! Sorry guys.

[James Taylor]

I don't suppose you'd be able to tell me where you got your 297 and 298s. A web search thus far has failed to provide any places in NZ that has them...

[vratko28]

this is because l297 is making PWM on output and not inhibit of l298, look at datasheet of l297..

from PDF:

PIN 11
CONTROL
Control input that defines action of chopper. When low chopper acts on INH1 and INH2; when high chopper acts on phase lines ABCD.

this is the reason of all leds lighting, if you put an osciloscope on each output you should see sawtooth on each line.

from PDF:

An input called CONTROL determines whether the chopper will act on the phase lines ABCD or the inhibit lines INH1 and INH2. When the phase lines
4/11
are chopped the non-active phase line of each pair (AB or CD) is activated (rather than interrupting the line then active). In L297 + L298 configurations this technique reduces dissipation in the load current sense resistors.

L297 i chopping NON-ACTIVE PHASE that is why leds are ON!

[mojalovaa]

I have a problem with the drive for stepper, using L297 and L 298 chip, I have this problem as they can not spin the stepper speed of 30 revolutions per minute, as soon as I put the settings to a higher rpm, there is simply no strength to run by myself, I have connected no load but can not be idle even ones that run. Stepper is 12V, 0.84 A 1.8 degrees per step. Does anyone know what could be. Test it with the K-CAM.

[pminmo]

You need to set an acceleration profile, you just can't start a stepper and have it go from 0 to x instaneously.