Re: coolant pump control questions

Hi Mark,
What you need is a relay driver. The easiest method is to buy an
integrated circuit meant to do this. One example is the SN75477P
which is available from digikey.com. It has a built in clamping
diode as well. It can handle up to 300Ma. The SN75477P can drive up
to 2 different relays. Download the datasheet from:

http://www-s.ti.com/sc/ds/sn75477.pdf

Look at page 2 of the datasheet. For the simplest method: You will
use the output of the parallel port to activate pin 1 & pin 2. Since
your relay is 5V and the logic requires 5V, you will tie pin 5
(clamp), pin 8(VCC), and one of the coils leads of the relay to your
5V source. You will tie the other end of your relay coil to pin 3 of
the chip. Be sure pin 4 is tied to GND. When the input at pins 1 &
2 go HIGH, the internal transistor is turned ON, and they sink
current from the relay turning it ON. Bring pins 1 & 2 LOW, and the
internal transistor will turn OFF. The voltage built up in the coil
will be disipated by the internal diode.

Now, if you want, you can use that relay, to power a much larger
relay's coil. If the larger relay you drive is also engaged using a
coil, be sure you have a clamping diode for it too. Make sure you
have proper circuitry to protect the relay's contacts when driving an
inductive load (like a pump).

Jeff
http://www.xylotex.com

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "markpowerplus <mabw@b...>"
<mabw@b...> wrote:

>
>
> Yes , the problem I have is the control side of things, I don't

know

> the range of the PC . By "switching" on the coolant in the software

I

> get a 5 volt reading on pin 1 of the parallel port but it will not
> switch the relay which is a 5 volt 10 ma. When I connect the relay

to

> pin 1 and the other side to earth the volts drop off to almost zero

I

> am guessing here at saying there is not enough current? this is the
> smallest rated relay I could find (it is a reed relay) I could not
> find a solid state relay in 5 volts.
> help please!!!!!!!!
> thanks
> Mark