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
Thanks,
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.
DaveC
This is an invalid return address
Please reply in the news group
It is likely that your wireless device has an input circuit with a dc voltage
across the inputs when the input is open (that is how the open contacts
are sensed). One side is likely common with one terminal of the
battery.
You should try putting a power FET device across the input (source
and drain) and using a capacitor from source to gate to hold the
FET on for a period after your relay opens. A resistor across the
capacitor sets the discharge period. I would start with 470 MF and
1meg resistor for 4-5 minutes delay (depends on gate voltage on
turn-off, device specific).
If the battery negative is common to one of the input terminals,
use a N-channel FET (I did something similar with an IRF520,
way over kill on current, but you are not likely to damage it).
If the wireless device voltage is only 3 volts, you will need a
FET designed for logic level gate voltages.
Connect as follows:
Source of FET to - input terminal
Drain of FET to the other input terminal
Capacitor negative (use 16V cap) to Source
Capacitor positive to Gate
1 meg resistor across capacitor
100K resistor to battery +, other side of 100K to one of your relay contacts
Other relay contact to Gate.
Use a grounded soldering iron as the FET gate can be
damaged by static electricity.
If the battery positive is common to one of the input terminals,
use a P-channel FET and reverse all polarities listed above.
When the relay contacts are closed, the 100K resistor uses the
battery to charge the capacitor. Voltage on the capacitor keeps
the FET ON.
When the relay contacts open (on power fail) the capacitor holds
a charge while the 1 meg resistor discharges the capacitor. At
some low voltage on the Gate, the wireless sensor will detect an
open circuit and send the alarm. Experiment with resistance and
capacitance to get an acceptable time delay.
If you shop the parts, likely cost per circuit is
USD $2.00-4 .00.
Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA