Re: [CAD_CAM_EDM_DRO] Re: Pixie error connection

Hello Keith,

I jumped into this thread well after it started so I am still playing
catchup with the end goal.

I don't know what you are controlling with the relay but, since you want
to control it with the error signal, I presume that you want the relay
to be as safe as if the control inputs got disconnected or the Pixie
board lost power.

In that case, you need for the relay to be active only when the open
collector error signal is off or high. In that situation you only have
1ma source from the 5kohm pullup resistor tied to +5vdc.

Solution number 1 is to find a solid state relay that can be activiated
by as little as 1ma. You can test this with a 5kohm resistor in series
with a +5vdc supply and the control inputs.

Solution number 2 is to amplify the 1ma of available current from the
5kohm pullup resistor to whatever current the relay requires to
activate. A 2N2222 NPN transistor, wired as an emitter follower, may be
sufficient. The transistor has a gain of something like 60 so can, if
necessary, source up to 60ma for the relay control input.

To wire the NPN transistor as emitter follower:
Connect the Pixie +5vdc to the NPN transistor collector.
Connect the NPN transistor emitter to the relay control positive.
Connect the relay control negative to the Pixie 0v or ground.
Connect the NPN transistor base to the Pixie open collector ERROR.

If you connect the relay control between +5vdc and the NPN collector,
instead of between the NPN emitter and ground, the relay will also work.
However, the low impedance with the NPN emitter connected directly to
ground will likely be seen by the Pixie as an ERROR because the voltage
at the open collector will now be very low.

If the relay needs much more than 50ma at the control inputs then I
would use a Darlington NPN instead of a 2N2222. I would still wire it as
an emitter follower for the same reason given above.

Tom Hubin
thubin@...

*******************

toolspindoctor wrote:

>
> Mark,
>
> Radio Shack has TIP3055 NPN Transistor and some others as well. This
> one has a power dissipation of 90W.
>
> * Mfr.#: TIP3055
> * Case type: TO-220
> * V(CE): 1.1
> * V(BE): 1.8
> * I(C) Max: 15.0A
>
> Would this fit the bill?
>
> Thanks
> Keith
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mark Vaughan" <mark@...>
> wrote:
> >
> > Hi Keith
> >
> > Yes one of these will probably do the trick, though it seems
> pointless to
> > buy an array when you only have one relay to interface to.
> >
> > Most simple darlington or hi gain npn transistors should do the job,
> hence I
> > commonly use MPSA13's because I have bucket loads of them. You can
> connect
> > the base via a resistor to the pixie card, 10K resistor would
> probably be OK
>
>