Attached are 3 captures from my L297/L298 board using the same motor at 500, 750 and 900 steps/sec on a 36v supply, wired bipolar parallel (coil resistance = 3.2ohm).
This shows the A-enable signal (blue) and the sense voltage (red). The sense voltage is directly proportional to the current in the coil (x 2), so .5v = 1A. At 500stesp/sec the RMS voltage is 0.64v, a current of 1.3A RMS, the peak voltage being 0.9v equivalent to 1.8A. You can see that the current doesn't start to flow (indeed its negative slightly at the start of the cycle which is due to the back emf/collapsing field of the previous cycle) until 0.7mS into the cycle, it then ramps up due to the inductance of the coils (these being high inductance motors i.e. not very fast) until it finally gets to the current limit and the driver goes into chopping.
At 750stesp/sec the RMS current has dropped to about 1A, the peak still being 1.8A. This is because the initial start-up and ramp is a greater proportion of the cycle time.
At 900steps/sec the current never gets to the set limit before the cycle ends... this is the fastest the motor will go at this voltage and the power output wil be low, the RMS current here is about 0.5A.
To go faster with these motors needs about 70v...
Setting Vref using the scope like this is much more accurate than adjusting it statically or relying on the overall current consumption.
This board is ment to be rated upto 36vdc, So I tried using my 36vdc 10A PSU.
I set the board to it's lowest current setting on the DIP switches to make sure I did not burn out or over heat my stepper motor, I am only testing X-Axis atm.
Powered up the board, and all the lights and the fan came on, Turned it all off and, when I next turned it back on, I was greated with a nice pop / bang, with sparks, and blue / white smoke.
Part of the Z-Axis TB6560AHQ had exploded, Oh Joy.
I have since looked at some of the datasheet for the Absolute Maximum Rating.
L7812CV - 35 Volts Absolute Maximum Rating.
L7805CV - 35 Volts Absolute Maximum Rating.
TB6560AHQ - 40 Volts Absolute Maximum Rating.
I will post a pic of the poor TB6560AHQ chip once I have uploaded it to my site.
Here is a pic of what is left of the chip.
My 36vdc PSU is not adjustable.
I now need to start looking at ways to limited the voltage and spikes.
I was looking at using a zener diode and Metal Oxide Varistor to deal with the spikes.
But I still have to look into it more.
Thats pretty impressive... any collateral damage as far as you can tell? I'd strip the chip off the board and then retest on the 13v supply and confirm other voltages are as expected...output of regs, logic levels, etc. and compare the voltages on equivalent pins of the remaining drivers - without motors attached.
No point in replacing it if there is more damage...
Alternatively you could send it back...
I have removed the damaged chip, and have powered it using my adjustable bench PSU, all the lights came on, no more smoke.
So it looks like only the one chip got damaged.
That snubber circuit is very interesting... It's just what I need. I'm running one of those 50V power supplies that Kip found in "unregulated" mode at 43V. My drivers will smoke at 50V, so I'm nervous.
Can you point me towards any online material that would help size the components to dissipate the sort of spikes I'm likely to see?
Sorry to hijack! (maybe Irving's tip will save a few more driver boards... :)
Yes, sadly I wasnt able to find an easy way to turn the volts on those down as they do some strange things with the internal reference line that doesn't follow the application note for the chip they use. I managed to blow one up quite spectacularly!
When I said zener diode I really meant a transient suppressor diode (transorb), such as these from Vishay. But i think clamping to 50v from a 43v supply might be tricky. A zener will clamp, but wont catch transients. you could use a 47v zener as a clamp with a 48v transorb in parallel to catch spikes, with a series low ohm resistor as a current limit between the supply rails. Farnell sell the transorbs, about 30p a go.
Hows it going with that board then Mad professor? any joy with it yet?
I've now got exactly the same board and motors sitting on my table, i'm just waiting for my pci parallel port extension to turn up so i'm dying to find out how much money wasted/saved
Dan Brown: I have not done any more testing since after removing the damaged chip.
I have tested that all the other axis are still working using my adjustable bench power supply, but I can only run the board / motors upto 15volts on this power supply.
But at this lower voltage my current rig is very slow, max of 200mm/min, for the very short time that the board was running at 36vdc I was closer to 450mm/min rapid move, not cutting.
I will be back to testing the current sence resistor and dip switches.
When using my scope with my bench PSU I always have to use my laptop on battery, as if the laptop on mains or a PC is used I get very strange readings on the scope software, so thats something else I have to look into.
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