Current setting for the 3-axis controller.

The electronics that take the impulses from the computer and make the motors turn.
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Joined: Sat Jun 06, 2015 8:09 pm

Current setting for the 3-axis controller.

Postby owlogic » Sat Jun 06, 2015 8:54 pm

I'm looking at the SLA7078MRP data sheet and have the 3-axis controller board upside down on the bench. You guessed it, I'm troubleshooting and have been digging back into the design and app notes. The Z axis driver is working fine. (Mach4 via a smartBob). But I have two non-functional channels.

With an ohmmeter, I find about 0.15 ohms from pins 5 or 19 to ground on two of the SLA7078s, but the 3rd chip is open on those two pins, so somehow I must have burnt out the current sensing resistors. Will replace that chip, Y axis.

That leaves the X axis, also not working, but the current sensing resistors seem to be okay. It worked for a while then stopped. The voltages on the SLA7078 pins all seem to be okay, including 0.14V on the reference pin, and it is receiving clock and direction signals. Continuity seems good. But no action from the motor which itself works fine on the z axis. Suggestions for further troubleshooting? Replace?

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AJ Quick
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Joined: Sun Dec 26, 2010 11:14 am

Re: Current setting for the 3-axis controller.

Postby AJ Quick » Fri Jun 19, 2015 11:44 am

What are your DIP switches set to? They can cause a driver chip to be set to sleep mode.

Similarly the reference voltage when set super high can cause sleep mode, though this isn't the case here.

Measuring the sense resistance should be a good way to go, but continuity isn't always the best since the internal state of the driver can change what is grounded / powered depending on what state it was on when powered off.

Are there any scorch marks on the chips indicating that they have blown? Were you using any kind of cooling heatsink on the driver chips? 0.14V is about 1 amp, and the chips can get pretty hot even at that low of current.

I'd replace if you can, but that can be difficult to do with the pin package of the chips.

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