Why did the smoke escape?

I am running an L297/L298 driver board with 2 PC power supplies(24V) and using the Automation Direct motors (276 OZ-IN, NEMA 23, BIPOLAR, 2.8A/phase, 1.1ohm). I had been working with it for about 5 minutes trying to figure out why it would run along just fine and then stall for no reason when all of a sudden I heard a loud pop(at first it sounded like it came from one of the motors) then I saw the smoke coming from my "Y"axis driver (L298). I'm sure my combination of parts is not the best match but have put two things together that just don't work or is this just a heat problem? After it popped I could touch the heatsink but it was hot

Thanks for any help
John

did you limit your current to 2A, max of the l298?

Too be honest I built the boards from the Book "CNC Robotics" so I'm not sure if it is limited. I'll apologize in advance for my electronic ignorance but what would that limit current limiter look like?

I had made the perhaps faulty assumption that since he was using a PC supply he had taken care of that detail but perhaps not...

I suggest that you get hold of the application notes for the IC's being used. Try getting them from National or SGS/Thompson

The current is set by Rs1 and Rs2 in the SGS L297 application note.

It also shows how to use other critical components that should make the chips an otherwise "bolt together" project. The A/N also outlines how and when to heat sink the L298 - another part that is easy to let the smoke out of...

Could be heat, could be over current. Without the schematic, it is hard to tell what really happened. Here are some questions troubleshooting ideas that I would start with:

Did it ever work (consistently) before? When it stalled was it attached to anything, or did it just stop rotating on its own? How long after the stall occured did the smoke get released? How big is the heat sink and what does "hot" mean, could you leave your fingers on it or would you get a blister if you tried? Were the motors hot? Is the rest of the components intact?

Do you know the value of the sense resistor? Also are you sure that vRef was right? If it was too high, I would imagine that it would have gone pop right away though...

Basically, before you get another chip and try it again, make sure that you have every set up and checked properly with a VMM. Then look to see what the current limiting on the board is tuned to. Also make sure that adequate heaksinking is provided and that the heatsinks are attached properly to the chips.

The current limiter on the L298 is a sense resistor on pins 1 and 15. There could be current limiting external to the chip though as shown in the application notes for the chip.

Are the outputs of the chip paralleled? If not the max rating on each output of the chip is only 2A, your motors are 2.8A/phase.

Also make sure that there are Schottky EMF-protection diodes on the outputs.

mjarus.

I built the driver boards several months ago and tested them with these motors (but no load) and they worked fine at 12V once I got the speed settings right in Mach 3. At that time the heat sink got hot fast but it was a very small heat sink so I turned everything off when it got hot to let it cool before continuing. When I put the motors on the router setup I removed the old heatsink and attached a piece of 1.5x1.5x6" angle aluminum. When it went pop I could keep my finger on the heatsink and it was uncomfortable but wouldn't have blistered. The motors were not even warm and as far as I can tell every thing else is intact including the L298 on the other two axes.

I will have to try to figure out which resitor is the sense resistor it is either 4.7kohm(between the sen1 and isenA and sen2 and isenB or it is the .5ohm on the ground side of the fast recovery diodes going to the motor

I'm not sure what chip paralelled means but from everything I have read in the last few hours I should have chosen either my motors or my driver with a little more caution (or at least a more through understanding or electronics, I've never found a book that explained it in a way that made sense to me)

I will definitely be looking at a heatsink with a fan if I figure out how to limit the current with the existing boards.

sincerely,
john