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Furnace bump timer

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Posted by amdx on January 9, 2011, 3:50 pm
 
I put this circuit together over twelve years ago.
I have it installed on my furnace/air condition control.
I have a momentary switch near the thermostat, when I
push the switch the furnace or air will come on for about
4 minutes (plus or minus).  I move an alligator clip
depending on season, you can put in a switch.
I use it rather than turning the heat/air up and down,
for a short comfort adjustment.
 It runs from a 9 volt battery and has very low standby current,
about 80 nanoamps. The battery lasts two or three years.
  Others can modify for different purposes. If you think of another
use, please post it.
                         MikeK
http://i395.photobucket.com/albums/pp37/Qmavam/Furnacebumpcircuit.jpg





Posted by Jamie on January 9, 2011, 4:29 pm
 
amdx wrote:


Something tells me a capacitor across the coil (1,5) should be added to
aid in making sure the armature gets fully moved over with out chatter
on the contacts.

   That's just me... ignore me otherwise.

Jamie



Posted by amdx on January 9, 2011, 6:34 pm
 

Hi Jamie,
 I noted I have a diode on one coil but not on the other.
 I don't remember if the diode was added to correct a problem or
if it was put in as general good practice and just forgot on the other coil.
But the circuit is drawn as has been working for 12 years.
                 MikeK



Posted by Jamie on January 9, 2011, 7:46 pm
 amdx wrote:


   It's most likely the relay you are using has it's mechanics designed
to get past the load swing  point of no return before the contacts that
is maintaining the coil current opens. My thoughts were to have a cap
on the opening coil to help it give a little more time to make sure it
gets over the hump.

  We've worked with those kind of dual coil latching relays before and
had issues when the applied duty cycle of DC current to the coil was
not always long enough and the relay would either switch back or
sometimes get stuck in the middle. The fix was to place a cap across the
coil with a blocking diode. I've seen some that come with these
components already in them.

   If it works, that is all that matters.

  Jamie.



Posted by Josepi on January 9, 2011, 4:32 pm
 Very nice idea and good logic.

I can't help but think the circuit is way over designed with too many
components. It doesn't matter to you because you have it built now.

I would have thought a UJT transistor with a simple RC ramp generator
but....wait! Yeah another contact on the relay would go a long way to
accommodate much simpler circuitry. probably not happening on a latching
relay.

Here is something ese you may consider...steal power from the furnace
circuit. Hook your common to the one side of the contact and to the other
side of the contact make a simple trickle charge circuit with.

furnace connection -> diode -> resistor (say 1K) -> capacitor to common (say
1000uf) -> 9v input.

If the voltage is critical you could insert a 7809 regulator to setady the
voltage at 9 volts from the 30 you may get off the trickle voltage stealing
circuit or you could use a parallel resistor across the ripple capacitor to
drop the voltage down to 9-12 volts. The trickle current has to be light and
the cap. big enough to carry the relay coil snaps.

I have discovered the timer input on my HRV system and gor PB to activate it
for 20-40-60 by holding it down longer for one, two or three blinks of the
feedbak LED on the wall. Short shots of ventilation stuff is nice. You get
it when you want it but don't have to find it running your wallet down a
month later.




I put this circuit together over twelve years ago.
I have it installed on my furnace/air condition control.
I have a momentary switch near the thermostat, when I
push the switch the furnace or air will come on for about
4 minutes (plus or minus).  I move an alligator clip
depending on season, you can put in a switch.
I use it rather than turning the heat/air up and down,
for a short comfort adjustment.
 It runs from a 9 volt battery and has very low standby current,
about 80 nanoamps. The battery lasts two or three years.
  Others can modify for different purposes. If you think of another
use, please post it.
                         MikeK
http://i395.photobucket.com/albums/pp37/Qmavam/Furnacebumpcircuit.jpg







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