Re: [CAD_CAM_EDM_DRO] Opto Isolation ??

> From: Hugh Currin <currinh@...>
>
> Thank you for looking over my circuits and commenting. From the great
> comments and tutorials I received I've updated them. If you get a chance
> please take a look:
>
> internet.oit.edu/~currinh/opto.html

Let's look at your circuits from the load end first. You have the opto's
transistor driving the inputs to the PIC on Dan's board and the inputs to
your printer port which are pulled up to +5V with 1k and 4.7k respectively.
The input impedance of the PIC and your printer port are high enough to
ignore for our purposes (100s of k ohms at least). This means that to drag
these load voltages to 0 you'll need to sink 5mA and 1mA through the opto's
output transistor to ground. This is well below the device's maximum output
current capabilities, so were OK there. The next thing to look at is the
Current Transfer Ratio or CTR. It's listed in the specs at a minimum of 80%.
This means that to achieve a collector current of 5mA in the photo
transistor, we'll need to push at least 6-1/4mA (5mA/.8) through the LED.
Since you've used Ohm's law to calculate the LED current at 11mA, it should
work fine. Your printer port outputs are capable of going to less than .7V
while sinking 14mA (and probably more), so the LED should turn off without
difficulty.

This is the problem with optocouplers; the ones with a high CTR are slow and
the fast ones won't drive much of a load without significant input power. I
believe some of the early optos had a CTR of less than 10%.

Disclaimer: I'm not an electrical engineer, nor do I play one on TV!

> >You'll also need a separate DC supply for the switch inputs, and there

can't

> >be any connection between the PC's ground and the DC ground in the

machine.

>
> I need a 5V supply for the driver boards and was planning to pull the
> voltage for the switches from this??

That'll be fine.

> >I'm way less squeamish about directly connecting the PC to external

circuits.

> >Instead of connecting your stuff to the built-in port on the motherboard,

get

> >one of those $8 parallel port boards you plug into a slot on the PC and

use

> >that.
>
> Where are these available? For $8 or $20 I'd get one (or two) just
> 'cuz.

I buy all my miscellaneous PC junk from these guys:

http://www.computergate.com/cgi-bin/start

The Parallel port board is at:

http://www.computergate.com/cgi-bin/prodinfo?sn=3484189435212123&cd=ONPP

It's one of those funky cgi type urls, so it may not work. Just search on
"printer port" from the main page. Their #ONPP is $6.95 (5 for $5.95 each!).
Your basic cheap circuit board made by 8 year old slaves chained to wave
solder machines in China...

Matt