"burned out" PWM input?

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GordonFreeman
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Joined: 26 Apr 2017, 23:28
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"burned out" PWM input?

Postby GordonFreeman » 26 Apr 2017, 23:53

Hi all, I was experimenting with using my VESC to drive a spindle (60mm outrunner) on a CNC using LinuxCNC. I have a transformer power supply with rectifier and cap setup to run the CNC at around 36 volts. I was also driving the spindle from that, but I think I was getting some missed steps and stepper motor power lost under high spindle load so I tried to use a separate power supply to the VESC/spindle. The LinixCNC software provide a PWM to the some pins on the parallel port and seemed to work. I used a variac with rectifier and cap and when I powered everything up I thought I smelled something melting and unplugged everything quick and the ground wire on the PWM input was melting. Oops, I guess a variac is not isolated in some way that drove power through that circuit and through the parallel port on the LinuxCNC computer.

But anyway, I got it to work using batteries for the VESC, I replaced the melted PWM wires and it all seemed OK. I then got a new switching power supply to run the VESC and that seemed to work ok too. But then it started acting funny, not starting up right and kicking out under speed control changes and eventually not working at all.

But it works fine if I hook the VESC through USB to a computer (BLDC tool) and use that for control. And when hooked to a computer through USB I see no PWM signal on the live readout (in BLDC Tool) even when I hook it up to a conventional rx. So I assume I damaged the PWM input circuit somehow.

Is there a way to diagnose what damage I did to it? I don't see any obvious physical damage.

Thanks,

rew
Posts: 943
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.
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Re: "burned out" PWM input?

Postby rew » 27 Apr 2017, 04:31

When you get a ground wire melting you probably have a ground loop. The signal line that accompanies that ground line is a good candidate for having encountered "out of spec" voltages. If the chances of survival are 50/50, then it's something like 49% will never work again after the incident, 49% will work without a problem even if you exceeded the spec, and a small percentage (2% in my example) will take some time to fail. It seems you got the latter option.

I think the "PWM INPUT" uses the input capture feature of a timer. So simply using an unused pin is not trivial. :-(

GordonFreeman
Posts: 2
Joined: 26 Apr 2017, 23:28
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Re: "burned out" PWM input?

Postby GordonFreeman » 09 May 2017, 07:26

OK, What are my next "easiest" options to control it, I2C? etc (I don't even know what that is). I can use a quadcopter control board, but I've never used anything but PWM.

It's going in a new RC project as I have purchased an actual VFD and spindle for the CNC.


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