Re: [CAD_CAM_EDM_DRO] Allegro 2961 uptdate

lonnietoons@... wrote:

> I have the PWM drive up and running and am doing some testing with
> a dyno, torque and speed are very good. I did run into a problem
> though and wondered if anybody had
> any ideas on how to handle it.
> Everything works very well when I turn power on first to the 5 volt
> logic power supply wich turns on the 5804 that does the stepping.
> This at the same time brings all 4 PWM chips high(High = off) on the
> input pins (these are then tied to the 5804).
> The problem is when I turn on the 40 volt supply first then turn on
> the 5 volt it blows out some of the PWM chips. The 2961's have an
> enable pin that I have tied high to the 5 volt supply,(High on these
> enables the chip).
> Does anybody have any ideas on how to handle this, other than to be
> sure I always turn the 5v supply on first (a recipe for disaster)
> One idea I had was to use a 5v zener off of the 40volt to keep the
> inputs of the 2961's high when the 5v supply is off line.

One way - I have something like this on my servo amps - is to have
the motor DC power interlocked to the logic power. There is a fail-safe
relay in the safety chain of the CNC system in each servo amp.
When there is no logic power, the relay is de-energized, which equals
the fault state. (Normally open contacts are wired in series for all
the fault relays.) Only when power is OK, there is no blown fuse or
overcurrent condition, then the relay energizes, closing the contacts.
When all relays in the chain are closed, then the computer is able to
command the system to come out of e-stop. When it does that, it
closes a set of sequential relays that charge the cap bank in the servo
amps through a resistor and then closes a direct contactor to supply
DC to the amps. The charge-direct relay circuit is on my web page at
http://pico-systems.com/EMC.html as the 2nd schematic. (The first
is the safety control, but it is somewhat specific for the Servo-to-Go
card and its very wierd watchdog timer.)

I needed all this complexity because I have some fairly large caps right
on the servo amps, and because the high-side current sensing resistor
is BEFORE the cap. So, if I just closed a contactor to the DC supply, it
would look like a short while the cap charged up, and trip the overcurrent
shutdown. It would also fry the relay contacts. This circuit also does
the reverse on power down, removing the direct connection and then
connecting an energy dump resistor. This resistor will brake the motors
in an emergency stop condition or power failure.

Jon