On Tue, 4 Jan 2005 18:09:21 -0800, DaveC wrote:
I've got several remote alarm sensors triggered by 120vac relays. When power
fails, the alarm calls the right folks.
Problem is that even the small power glitches get these guys out of bed. Not
a good thing.
How can I delay the de-energizing of these relays? 5 minutes without power is
OK. Will a series diode and shunt cap do the trick? If so, what size cap? If
not, what're my options?
The relay is similar to this one:
http://tinyurl.com/6pppw
Assuming power is available to make the call, your control circuit
obviously doesn't depend on 120VAC being present, to function. It is
also unlikely that your remote sensors run on 120VAC, as this could
represent an unneccessary safety hazard.
Therefore the delay is best performed in the device's control section,
using the power provided to perform the control function.
5 minutes is an incredibly long delay to ask of a de-energized relay.
It also could represent a signifigant security breach, if ignored. Why
power loss should affect remote sensors is a question you should raise
with the vendor.
You should get the liable vendor or service rep to set the product up
to meet realistic expectations that don't compromise security or
convenient function.
...............................................
Imagine if the delay was required of the hardware relay alone -
assuming a 100mW coil , that drops out at 45% of the label voltage;
5 minutes hold-up requires 30j minimum of stored energy.
(.1 x 60 x 5)
If the initial stored voltage was 165V, and drop-out was 75V, the
storage cap required would be 700uF
If the initial stored voltage was 12v and dropout was 5V4, the storage
cap required would be 3 Farads.
.................................................. ..
This example used an extremely sensitive device - your referenced part
consumes 80 times the power in the example and would require 80 times
the stored energy, 80 times the capacitance.
The obvious source of stored energy for any controlled delay is the
controller's supply.
RL