Re: gEDA-user: Power relay question

[Dan McMahill <dan@xxxxxxxxxxxx>]

On 7/28/2011 5:52 PM, Rob Butts wrote:
>    This is a dumb question but I'm having a mental block.
> 
> 
> 
>    I have a 12 volt dc motor that I want to run from the push of
>    a momentary pushbutton which will run until a limit switch gets hit.
>    Digikey has a power latching relay PB1088-ND (cheap) that I can't tell
>    if it actually latches once energized.  (I attached the relay document)
> 
>    Is relays and switches even the best/cheapest solution?  Suggestions?
> 
>    Power:
> 
>    quantity of 2 12 volt batteries available
> 
> 
> 
>    Input:
> 
>    momenary 12 volt pushbutton #1
> 
> 
> 
>    Requirements:
> 
>    12 volt dc motor #1
> 
>    12 volt dc motor #2
> 
> 
>    The application:
>    Stage 1: momentary signal from pushbutton #1 starts motor #1 that
>    runs until a limit switch is hit triggering stage 2
>    Stage 2: motor #2 runs until another limit switch is hit and remains
>    stable
>    Stage 3: momentary signal from pushbutton #1 reverses the polarity of
>    power to the motor #2 running it until back to it's start position and
>    triggering stage 4
>    Stage 4: reverse of the ploarity of the power to motor #1 running it
>    until it is back to the start position
> 

This is slightly more complex, but not terribly different from a dumb
waiter that I cared for many years back.  At some point someone replaced
the many decades old relay controller with a solid state circuit which
was sensitive to powerline disturbances (like those created when an
commercial dishwasher motor shuts off).  I ended up just rebuilding the
relay based thing and it worked like a champ.  The basic approach was to
use a combination of normally open and normally closed switches as well
as DPDT relays.  The idea is this.  You have a relay with one end of the
coil connected to the supply.  Energize the relay via a momentary
contact pushbutton switch to ground on the other end.  In parallel with
that momentary contact switch, put one of the relay contacts so when the
relay is energized it keeps itself energized (i.e. I made a latching
relay).  Now add a normally closed switch in series with that parallel
combination of normally open switch and relay contact.  When this second
switch is activated, you de-energize the coil and the relay opens up.
So in my case, the 2nd switch was a limit switch at the end of the
dumbwaiter shaft.

Since the relay in DPDT, you have the second SPDT switch available to
connect to whatever you may need (motor).

Good things:

- simple in concept.

- fairly immune to power supply noise

- switch debouncing is a non-issue

- easy to construct as you don't even need a circuit board and the
soldering is on large terminals.

- kind of neat to build an electromechanical state machine!

Bad things:

- physically large

- much higher power consumption (but maybe you don't care.  with the
dumb waiter it was a non-issue)

- less flexible

- if the cycle time through the state diagram is short and it runs all
the time you can wear out the relays

- relays are not so cheap (not super expensive but a set-reset latch
from a relay is much more expensive than a chip)

- they make noise but then again, you're controlling a motor

- your friends may make fun of you for not using the latest programmable
solution ;)




_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user