Problem- Confused about magic smoke event..

[skullworks]

I'm cross posting this from the Yahoo form since there were no new posts for 16hrs.

Last night we finished up the wiring on a 3 axis system based on the first rev
G251's.

As per Mariss "---IMPORTANT APPLICATION NOTICE---"

Problem: Eliminating obnoixious hissing, sizzling and squealing sounds. The G251
drives make unpleasant noises in multiple axis applications. This doesn't hurt
the drives and they will not have positioning errors but the sounds are very
annoying.

The G251 drive is not a chopper. It is a synchronously clocked pulse width
modulated (PWM) drive. It is inherently silent by design, it should make no
hissing, squealing, grunting or whistling sounds or any sounds at all.

Cause: The G251 drives are not optoisolated. Signal GND (term 12) from each
drive must connect to the parallel port GND (pin 25) to complete the step and
direction signal path. Each G251 drive's Signal GND has a slightly different
voltage potential because of motor phase currents and wiring resistance back to
the power supply.

This small but inevitable (<100mV) potential difference between drive grounds
causes significant (>100mA) ground-loop currents to flow between the drives.
This high frequency (20kHz) AC current is sufficient to disrupt the operation of
the of the motor phase current regulators and make them audibly noisy. The
drives are still perfectly accurate but now make annoying noises.

Cure: Install a 100 Ohm 1/4W resistor in series with each drive's Signal GND
(term 12) to parallel port GND (pin 25) circuit connection. This completely
cures the audible noise problem and renders the drives utterly silent while
stopped or turning at slow speeds.

The 100 Ohm resistors are too small to have any effect on performance but are
large enough to reduce ground-loop currents 1,000 to 10,000-fold (<<100uA). Such
small currents are now completely unable to interfere with the normal, silent
operation of the G251 drives.

(end quote)

The BOB in use was a PMinMO.com 4X072105a 4axis board. the 100 Ohm resisters
were added to the signal ground line going to the G251 pin 12, the resistors
were at the bob end of the wires. Enable (pin 11) was not used. So only Step,
Dir and sig gnd were attached to the G251's.

The bob was feed +5v from a isolated power supply. Para port pin 1 was driving
Aux1, pin 14 Aux2 and these were functioning properly.

When we fired up the Stepper power all was fine, and we had it on for about 90
seconds before attempting a jog move. as soon as I tried to jog 1 axis all 3
resistors on the signal ground lines smoked. Power was cut in 3 seconds. All
damage was confined to the resistors, wires and board traces are good with no
signs of heating.

So now I'm at a loss as to what to check next. the G251's appear fine, didn't
get warm at all. The motors were attached and current set resistors were in
place.

I'm turning this problem loose on the hive mind in hopes of finding a cure or
workaround.

Thanks.

[Senna]

I was really hoping there would be a reply from Mariss on this as I'm at this very moment wiring up my controller employing G251's. I have the 'IMPORTANT APPLICATION NOTICE' stapled to my schematic and the 100 Ohm 1/4W resistors are on the way.

Since you cross posted was a reply and remedy made on the Yahoo Forum...???

Senna

[jalessi]

It sounds like the ground wire from the power supply to the G251's terminal 1 was not connected.

[skullworks]

Easy. You lost your GND connection from the power supply to the drives. You
continued to have a GND connection to the BOB.

Power supply current Flowed from supply+, through the drive, out the SIG GND
pin, through the 100 Ohm resistors, to BOB GND and back to the supply. The
resistors then burned (1A thru 100 Ohms = 100W).

The path was supposed to be from supply+, through the drive, out the SUPPLY GND
pins and back to the supply.

Mariss