Delay on a mains-powered relay?

[DaveC]

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
[email protected]
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[Ken Taylor]

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

Hi. Hmm, sorry but you should Goo.... Altavista this. Use the term 'delay
relay'. Here's one:
http://www.automatictiming.com/pages_div/timedelayrelays.html

The more run-of-the-mill parts places don't sell them, but relay specialists
do (not sure what the situation is in your area).
Farnell also have these (they're possibly easier to use):
http://au.farnell.com/jsp/endecaSea...=null&comSearch=true&manufacturerDisplay=true

(They operate USA too, I just didn't have the link handy)

Cheers.

Ken

 

[majortom]

What would be the purpose of setting off ur alarm every time power
fails??
Not being nosey, it's just that the only alarm panels (fire/security)
I've seen had some form of
battery backup, whether it be Nicad, Lead Acid, etc. to keep the system
running
normally under power failure.

 

[[email protected]]

Not being nosey, it's just that the only alarm panels
(fire/security)

I've seen had some form of
battery backup, whether it be Nicad, Lead Acid, etc. to keep the

system

Yes, but they still dial home to report AC loss.

 

[majortom]

k, then yeah the brit guy above pointing toward the time delay relays
might be a way to go...

 

[legg]

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

 

[NSM]

| 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 normal method is a relay with a hydraulic delay. This means you don't
need a no fail power supply. However these days, if you have backup power,
there are solid state equivalents.

For your method you would need a large cap run off say, 12 VDC - maybe 1000
uF and up.

N

 

[DaveC]

So the answer is... no.

If power fails at the remote site, so does the control circuitry and its
power. So a traditional delay relay won't work.

The relay, powered by mains, simply opens the wireless sensor's sense circuit
when power goes away. I just want it to do so after waiting a few minutes to
see if power comes back on.

The alarm isn't for security purposes, just for letting those responsible
know that certain things have occurred, one of which is power fail. The
sensor is a wireless device run on a lithium battery, and is a packaged deal
so has no spare power available. The relay's not near the alarm CPU, so that
power isn't available, either.

Suggestions for a simple mains-powered circuit that will open a pair of
contacts about 5 minutes after de-energizing? If it's to be battery-powered,
it needs to be super-low drain. I rather keep battery replacements down to
every 2 years, at most.

An idea that just struck me: the presence of mains power could keep this
circuit de-energized. When power fails, the battery is connected to the
circuit which starts the countdown. When zero is reached (ie, 5 minutes have
passed), a pair of contacts would open. And the energizing circuit
(triggering these contacts) would only have to be a one-shot; once the alarm
is triggered, the circuit could de-energize. That way the battery is utilized
only briefly, during power-fail situations. How might I construct such a
circuit?

Thanks,
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.

DaveC
[email protected]
This is an invalid return address
Please reply in the news group

 

[Ross Herbert]

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,

The full specifications for this relay are shown here
http://my.execpc.com/~indelect/potter/pbku.html

Your problem is that you are using the relay coil to directly monitor
the mains voltage and since the relay has a release time of around
10ms it doesn;t take much of a glitch to cause the relay to release. I
don't think there is much you can do to delay the release of the relay
itself unless you go through a major re-design process. The simplest
method of achieving a delay is by connecting the relay coil across the
output of a full-wave bridge rectifier and then connecting a large
electrolytic cap (say 4,700uF) across the coil. However, the low
resistance of the 120Vac coil is a problem and you would need to
replace the 120Vac coil with a 110Vdc coil of 10,000 or 6720 ohms
resistance (see spec data) in order to achieve a significant delay.
You might be able to get around 10 - 20 seconds delay (at a guess)
using the above method depending on the contact spring tension and the
coil used (10,000 or 6,720 ohms) but you may need to experiment with
even larger capacitor values.

In order to achieve delays in the minutes range you would need to have
some ancillary solid state delay circuit (with its own fail safe power
supply) which becomes initiated whenever a mains failure occurs and
only after the time-out period of this circuit is the alarm sent to
the monitoring folks. If a mains failure is a simple glitch or one
lasting several minutes (shorter than the time delay period) then the
circuit would be reset to await the next failure. As you can see this
adds complexity and the need for a separate fail safe dc supply for
the electronics and this may not be a viable option.

 

[John G]

So the answer is... no.

If power fails at the remote site, so does the control circuitry and
its
power. So a traditional delay relay won't work.

The relay, powered by mains, simply opens the wireless sensor's sense
circuit
when power goes away. I just want it to do so after waiting a few
minutes to
see if power comes back on.

The alarm isn't for security purposes, just for letting those
responsible
know that certain things have occurred, one of which is power fail.
The
sensor is a wireless device run on a lithium battery, and is a
packaged deal
so has no spare power available. The relay's not near the alarm CPU,
so that
power isn't available, either.

Suggestions for a simple mains-powered circuit that will open a pair
of
contacts about 5 minutes after de-energizing? If it's to be
battery-powered,
it needs to be super-low drain. I rather keep battery replacements
down to
every 2 years, at most.

An idea that just struck me: the presence of mains power could keep
this
circuit de-energized. When power fails, the battery is connected to
the
circuit which starts the countdown. When zero is reached (ie, 5
minutes have
passed), a pair of contacts would open. And the energizing circuit
(triggering these contacts) would only have to be a one-shot; once the
alarm
is triggered, the circuit could de-energize. That way the battery is
utilized
only briefly, during power-fail situations. How might I construct such
a
circuit?

Thanks,
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.

DaveC
[email protected]
This is an invalid return address
Please reply in the news group

I never cease to be amazed at the questions asked in this NG from people
who do not have a clue from grade school experiments about electronics
but want to design or modify industrial situations where some manager
should be putting up the money to employ a competent engineer skilled in
the particular area.

Relying on poorly informed advice is asking for trouble and a sure
course to the courts for a damages claim.

 

[John Woodgate]

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?

Impractically big. If you get one big enough, Feerguy will want to hear
from you. (;-)

If not, what're my options?

Back-up battery and inverter. A standard UPS would probably do. Do you
know how much power the system consumes?

 

[[email protected]]

The relay, powered by mains, simply opens the wireless sensor's sense
circuit when power goes away. I just want it to do so after waiting a
few minutes to see if power comes back on.

Can you use a solid state relay or a DIP relay, instead of the
presumably larger relay you're using now? If so, the coil current
will be much less, and you might be able to get away with something
like a wall transformer, a big capacitor, and the relay. The time
delay won't be very exact, but it may work for your application.

Matt Roberds

 

[Frithiof Andreas Jensen]

How can I delay the de-energizing of these relays?

Use a Thermal Relay - maybe called "Delay Relay". Best approach is to find
out what the format/socket is, then one of the suppliers of process control
gear will have something that plug into it.

 

[Tony Williams]

How can I delay the de-energizing of these relays? 5 minutes
without power is OK.

Use something like this maybe. View in fixed width font.
____
+---------+ +------|AC +DC|---+-----+ +---->Out
| | | | | _|_ |
)||( | Bridge | +|C | | o |
120Vac )||(18Vac |Rectifier| === | RL|----|N/O
)||( | | | |_ _| o |
| | | | | | |
+---------+ +------|AC____-DC|---+-----+ +---->Out

A 120:18V transformer on the mains, bridge rectifier,
capacitor smoothing, and 24V comms relay.

Relays have an On/Off hysteresis. Once it has been
energised, a 24V comms relay will not then release
until the voltage has reduced to below about 8V.

24V sinking to 8V is not far off T = R*C.

A standard 24Vdc high sensitivity comms relay has a coil
resistance of about 2900 ohms. If C= 47000uF then the
the relay will be held closed for about 136 secs after a
mains failure.

A 47000uF (at 35vdc working) is not cheap, and the thing
is a little clunky, but easy to put together and not much
to go wrong.

 

[Fred Bloggs]

If power fails at the remote site, so does the control circuitry and its
power. So a traditional delay relay won't work.

The relay, powered by mains, simply opens the wireless sensor's sense circuit
when power goes away. I just want it to do so after waiting a few minutes to
see if power comes back on.

The wireless "sensor's sense" ciruit is most likely something with a
weak pull-up of say 100uA, so that shunting the relay input terminals
with C=100uA/(3V/300secs)=10,000uF 10WVDC should get you close:
View in a fixed-width font such as Courier.

+--------[100]--+------->
| |
o | +
/ === to sensor
o | 10,000u
| |
+---------------+------->

 

[No Spam]

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,

Someone suggested hydraulic. I was thinking pneumatic.
Checkout:
http://ecatalog.squared.com/catalog/173/html/sections/21/17321027.html

Not cheap but do have a 5 min +/- 12% version.

 

[Wayne Tiffany]

I don't know what is powering the relays, but can you use a UPS to keep it
powered, but then time out?

WT

 

[Dimitrios Tzortzakakis]

No.These relays are designed to work in ac.There's no way in storing ac in a
capacitor and diode circuit.Only expensive inverter designs.You need some
engineer to construct a dedicated circuit, maybe with battery, for your
purposes.

 

[CJT]

I never cease to be amazed at the questions asked in this NG from people
who do not have a clue from grade school experiments about electronics
but want to design or modify industrial situations where some manager
should be putting up the money to employ a competent engineer skilled in
the particular area.

Relying on poorly informed advice is asking for trouble and a sure
course to the courts for a damages claim.

Engineering in the U.S. has never had the political clout needed for
effective enforcement of registration laws. A PE license carries with
it almost no recognition in a world where just about anyone can get
away with calling themselves an engineer (except where public funds are
involved). Doctors and lawyers, and even beauticians, have done a
better job getting their expertise recognized IMHO.

 

[CJT]

Can you use a solid state relay or a DIP relay, instead of the
presumably larger relay you're using now? If so, the coil current
will be much less, and you might be able to get away with something
like a wall transformer, a big capacitor, and the relay. The time
delay won't be very exact, but it may work for your application.

Matt Roberds

It's an AC relay. Just how do you intend to attach that "big capacitor?"