Re: [CAD_CAM_EDM_DRO] Re: Pixie error connection

Hello Keith,

The BC517 looks like a good candidate for NPN Darlington. Sold by
DigiKey.

http://www.datasheetcatalog.com/datasheets_pdf/B/C/5/1/BC517.shtml

Wire the relay controls between the Darlington emitter and Pixie ground.
Wire the Darlington collector to Pixie +5vdc.
Wire the Darlington base to Pixie ERROR.

The load due to the relay, as seen by the Pixie 5kohm pullup resistor,
is the Darlington gain times the relay control resistance. We don't know
what the relay control resistance is but the Darlington gain is about
30000. So if this does not work then somebody is going to have to get
some data on the current required by the relay control circuit.

The ULN2803 would do the job for the relay but you cannot wire it as
recommended if you don't want the Pixie to perceive an error. Normally,
the relay controls would be between the Darlington collector and Pixie
+5vdc and the Darlington ground would be wired to Pixie ground. This
would draw most of the current from the pullup resistor and cause the
ERROR voltage to drop to a level that may be detected by the Pixie as a
fault.

So you would probably need to wire the relay control between the
Darlington package ground and the Pixie ground and wire a single
Darlington output to Pixie +5vdc. A diode is not needed since the solid
state relay does not actually have a coil.

Tom Hubin
thubin@...