On 09/06/2016 11:29 PM, Danny Miller wrote:
On 9/7/2016 12:53 AM, Gene Heskett wrote:
On Wednesday 07 September 2016 01:41:24 Dave Caroline wrote:
That looks like a board I would not recommend look close to the white
sockets near the axis outputs, it has series resistors for the driver
optos, these of course are in the driver as well, this means half
current through the opto and failure to work.
I was working with a free issue board where I met that problem before
So have I, Dave, and I wound up bypassing that particular opto in the
board I used to drive my G0704, but this one doesn't have the opto's
that I know of. Because they are slow, I've found them far more trouble
than they are worth as a safety measure. I'd much druther trust a good
grounding system, star topology of course.
Thanks Dave.
I similarly dislike optos. I have doubts that any properly made drive
can even produce dangerous "spikes". They seem pointless.
My understanding is that the optos are there to protect the driver
inputs and sometimes do signal voltage conversion. The cable from the
controller to the driver can be long and travel close to noise sources.
This makes the cable susceptible to induced voltages which can push the
signal too high or even negative, which will blow out the input if not
protected.
Aside from noise, usually the output at the controller has a current
limiting resistor for protecting the signal driver. These drivers are
usually higher current devices (maybe 24ma) so the limit resistor is
sized for the higher current. On the other end, the limit resistor is
there to protect the opto's LED which can take less current (about 10ma
is common). The LED is a current device so signal voltage is secondary
the resistor just needs to be sized appropriately.
Induced noise voltage can get quite high so sometimes a Zener diode is
used to clamp the input at a max. voltage to ground. LED's have very low
reverse breakdown voltages, so it is common to put a regular diode with
a high breakdown voltage in series to protect the opto when the noise
pushes the input negative. This diode will have a forward voltage drop
that will need to be considered when sizing the limit resistor.
Optos used to be slow, but there are modern optos that are very fast.
One just needs to look at the datasheets.
I attached a very rough schematic of what might need to be on an opto input.
I work with a 7i92 card which is ethernet, and thus already offers
galvanic isolation through the ethernet. If there were a huge surge, it
wouldn't propagate back to the PC. The PC ain't a high-dollar item anyways.
The AM882 drives all have differential opto inputs/outputs themselves,
there's no case for even hypothetical "spikes".
Danny
Dave Caroline
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Cheers, Gene Heskett
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