Delay on a mains-powered relay?

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:

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Thanks,

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Reply to
DaveC
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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.

Reply to
majortom

(fire/security)

system

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

Reply to
larwe

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

Reply to
majortom

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,

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Reply to
DaveC

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

Reply to
legg

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
Reply to
NSM

The full specifications for this relay are shown here

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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.

Reply to
Ross Herbert

Someone suggested hydraulic. I was thinking pneumatic. Checkout:

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

Reply to
No Spam

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

WT

Reply to
Wayne Tiffany

I read in sci.electronics.design that DaveC wrote (in ) about 'Delay on a mains-powered relay?', on Tue, 4 Jan 2005:

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

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

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Reply to
John Woodgate

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

Reply to
mroberds

Reply to
Richard Henne

Reply to
Rob Gaddi

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.

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Tony Williams.
Reply to
Tony Williams

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.

Reply to
Frithiof Andreas Jensen

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 | | +---------------+------->

Reply to
Fred Bloggs

power

Not

is

If

Hi. Hmm, sorry but you should Goo.... Altavista this. Use the term 'delay relay'. Here's one:

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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):

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

Cheers.

Ken

Reply to
Ken Taylor

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.

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Reply to
CJT

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

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