Hello,
I have searched the forums for similar topic, and found several related posts. However, none address my requirement.
I would like to turn on an electric appliance (refrigerator) using a relay which is normally closed with a time delay. So, when I apply power to my circuit, I want it to energize the relay so it keeps the appliance disconnected from power, and when delay time elapses, it will cut the power from the relay bringing it to connect the appliance to the power.

The reason I am reversing the usual logic is the appliance should be connected to power most of the time so I think that it will be make more sense from a relay life time and reliability to have it most of the time not energized (normally closed), and only energize it when I need to introduce the time delay at start it up.

I thought of building an electronic circuit that upon applying power will turn on the relay (disconnect the appliance from mains power), and will after few minutes disconnect the power to the relay. However, I thought that if the relays takes 100ms for example to be energized, and for a 50 Hz 220V mains supply, this means that the appliance will be connected to the mains powers for 5 cycles (50 Hz = 20ms) before it gets disconnected.

Is there a way to use may be more than one relay in series/parallel combinations to ensure that the appliance is not connected at all to mains power and only is connected (through a normally closed relay) after a specified delay?

 

Certainly such relays are available at some suppliers, that would do exactly what you are asking for. But I am quite puzzled by whatever it is that you are wanting to achieve. So an explanation would help. I believe that DigiKey sells such relays, as do many industrial supply sources.

 

As Mr. Bill2 mentions such relays are an off the shelf item. While there are dozens of inexpensive versions pouring in from China I can point you to several commercial versions. Macromatic is one such manufacturer distributed by electrical / electronic parts houses and here is a good explanation of the relay functions. What is the difference between On Delay, Off Delay, Single Shot, Interval On and all these other time delay functions?

A good quality relay designed for continuous duty really does not care if the coil is energized or not and in most cases the relay coil does not use enough current (watts) to worry about power consumption. You would be interested in the DOM (Delay On Make) or the DOB (Delay On Break) functions and as can be seen from the link there are plenty of functions.

The NCC (National Controls Corporation) manufacturers a Multiple Function Solid State Timer which can do several functions user definable. It contains a DPDT relay capable of switching 10 Amps 1/3 Horsepower 120/240 VAC They come in a wide range of operating voltages as can be seen in the link. At $120 USD they aren'r cheap but they are good stout reliable units. I used them in dozens of control timing designs and I never had one fail in 20 years before I retired and some are in harsh environments.

If you well define your application in detail like what voltages to be switched and at what load currents it would go a long way.

Ron

 

The reason for choosing the mode of contacts closed with the coil not energized is to reduce power consumption and heat, as well as to assure that the device is available even if the relay fails. An industrial duty relay coil does use a bit more power, and thus produces a bit of heat. In addition, the choice allows going to instant on simply by disconnecting the timer power, using a lower current rated switch.

 

Thank you for your responses. Yes, my main reason for having the appliance connected in the normal closed mode is that I am concerned about the reliability of the relay and that if it fails or the driving circuit fails being the worst case scenario, then the appliance will receive power after all, and it will be as if I did not introduce my circuit.

The delay I am looking for is 15 mins. So will need to check if the ready made relays you directed me to cover such long period. I made few measurements this morning with a 12V relay 220V/10A and seems that the time it takes to move from NC to Open is just 4ms, and moving from open to NC again is 12 ms (I guess it is longer as the relay metal contact had to travel in the air back to its original default position for around 1mm, and blip in the blue trace is because the metal plate after touching back, bounces and settle).

With only 4ms of mains power connected to the appliance before the relay disconnect it for 15 mins is short enough for not to damage it (I though it would be 100 or more ms),

I am attaching two screen shots. The yellow trace is for the power energizing the relay, and blue trace is for the signal across the relay load.

 

Thank you for your responses. Yes, my main reason for having the appliance connected in the normal closed mode is that I am concerned about the reliability of the relay and that if it fails or the driving circuit fails being the worst case scenario, then the appliance will receive power after all, and it will be as if I did not introduce my circuit.

The delay I am looking for is 15 mins. So will need to check if the ready made relays you directed me to cover such long period. I made few measurements this morning with a 12V relay 220V/10A and seems that the time it takes to move from NC to Open is just 4ms, and moving from open to NC again is 12 ms (I guess it is longer as the relay metal contact had to travel in the air back to its original default position for around 1mm, and blip in the blue trace is because the metal plate after touching back, bounces and settle).

With only 4ms of mains power connected to the appliance before the relay disconnect it for 15 mins is short enough for not to damage it (I though it would be 100 or more ms),

I am attaching two screen shots. The yellow trace is for the power energizing the relay, and blue trace is for the signal across the relay load.

OK, the relays I linked to have N/O and N/C contacts. While I do not understand your concern for the relay's transfer time here is what happens. When the coil voltage is removed the field will begin to collapse. But with an AC coil you have no way of knowing where the level of the sine was. The field will collapse to a point where it cannot overcome the physical force of a spring and the contacts will have a transitional period of state change. Your images are reflecting a DC coil? Did you want or need a DC coil? When power is removed with an AC coil you have no way of knowing where the sine is? Additionally in the US mains power is 60 Hz (16.666 mSec and in Europe mains power is 50 Hz (20.000 mSec) and since you mention 15 seconds how much does it matter?

Maybe you should explain this again from the beginning and why your concern for realy switching time and exactly what your objective is along with the appliance specifications like current draw and voltage.

Ron

 

Hi Ron,
An old relative of mine has an old appliance (fridge) which is rated at 220V AC, 50 Hz. I do not know the current as it is difficult for him to find the fridge relevant specs (wattage, current) due to age and lack of knowledge. So, here I am making my first assumption that it would not exceed 10 Amps.
Where he lives, the power company has a nasty habit of disconnecting the power in his areas randomly after midnight and bring it back in even in a more dangerous way. It may come, and go and come again in less than a minute. As this is an old fridge, so I am not sure if its internal circuitry has enough safety measure to handle such connect/disconnect of power in such a short period. I also heard from him that due to this, some of his neighbors complained about loosing some of their appliances (drop dead). It seems that even when the power is back, it does not come back at the right voltage, may be higher or lower, and takes some time to stabilize.

His budget is very limited (10-15 USD). Yes that low, because when it converts to his local currency it is considerable amount of money. So, I have to work of a home brew solution and rely on very cheap Chinese components.

The idea I have is to build a home made delay relay that when AC mains comes back, it will keep the fridge disconnected from the mains power for 15 minutes (I saw in your response 15 second, so I went back to my post, and checked again and it was mentioned 15 mins).

Assuming that this time is enough to by pass the mains voltage connect/disconnect and possible fluctuation from the power company, then the relay would connect.

The straight forward way is connect the fridge to the mains through a normally open relay that has a 12V "DC" coil, and through a driver circuit connected directly to the mains do not energize the relay 12V DC coil till 15 minutes has passed then energized till the second day power cut.

The challenge with this is that the relay's coil will remain energized for long hours which I assume would reduce its life time being a cheap Chinese one. The driving circuit being part of the same limited budget will be based on a cheap transformer 220/12V which I prefer not to keep drawing from it current most of time to drive the relay coil for the same reliability reasons of the transformer itself.

Here is my first question. Is this assumption correct? If I buy a cheap Chinese relay from ebay for 2$, would it live for the next 5 years with its coil energized for 99.9% of the time? If the answer is yes, then nothing else is required.

However if the answer is may be yes or the answer is a definite no, then this takes me to my second approach which is to reverse the logic. Energize the relay's 12V DC coil and stress the AC transformer (there are the two main parts that generate heat, and the relay is also mechanical device) as short time as possible by using a normally closed relay which I drive to open for 15 mins, and leave it to rest for the rest of the day.

The challenge here is that as per the screen shots I shared in my earlier post, there is a time that the relay takes from the moment its coil is energized till it actually turn open which is 4ms in the case of the relay I tested. During this period, the mains voltage will be connected to the fridge after which the power will be disconnected for 15 mins and connected back.

So, my question here is: Would a fridge be damaged if connected to the mains power for 4ms (let's say 10 ms) and disconnected or not.

If the answer is no. Such a very short time of power being connected and disconnected to the fridge will not affect it, then I know what to do next.

However if the answer is yes this very short time will still damage the fridge on the long run, then I still need help.

Thank you in advance.

 

Goal:

No power, relay de-energized, appliance off.
1:00 PM sharp, power comes on, relay energizes, appliance remains off.
1:15 PM sharp, relay de-energizes, appliance comes on.

You have two concerns:
First concern: The time it takes the relay to throw the armature. During that time power also reaches and tags the appliance for an instant. I suppose it could be a potential issue, but I suspect it wouldn't. Any circuit breakers may see the sudden change in loads as being an issue with the wiring and may trip the breaker.
Second concern: Relay longevity. That depends on the quality of the device you're using. Cheap relays may fail in months of service while high quality relays may last for decades. In my honest opinion I don't think the relay issue is as big as you concern yourself with.

Cars have lots of relays in them. They seem to last quite some time, even working in adverse conditions like extreme heat and cold. Still, I don't think in all the years I've been driving (since 1974) I've ever had to replace a relay. Yes, I know they do go bad, but it's not an all too common occurrence. I think your second concern is a moot point, which would negate your first concern.

Instead of timing out a relay to preserve its useful life I'd suggest just either using the relay to apply power a moment or two after power comes on - or just plug the darn appliance directly into the power port, so that when power comes on the appliance comes on as well. No need to engineer a solution to a non-problem.

 

Hi Tony,
You got my goal and two concerns exactly right. The only exception is your last paragraph. In my post I referred to the issue which I am trying to solve by this delay in applying power to the appliance which is the issue with the power stability and voltage from the power company:

"Where he lives, the power company has a nasty habit of disconnecting the power in his areas randomly after midnight and bring it back in even in a more dangerous way. It may come, and go and come again in less than a minute. As this is an old fridge, so I am not sure if its internal circuitry has enough safety measure to handle such connect/disconnect of power in such a short period. I also heard from him that due to this, some of his neighbors complained about loosing some of their appliances (drop dead). It seems that even when the power is back, it does not come back at the right voltage, may be higher or lower, and takes some time to stabilize."

If this was not there, as you said I would just plug the appliance directly to the mains source without an issue.

So, it seems from your reply the relays are not as "unreliable" as I thought, and accordingly, it is better to avoid the first concerns which seems to be more serious, and use a normally open relay, and have it energized most of the day's time to keep the appliance connected to the mains.

 

So you want to isolate the appliance for a period of time - your statement was for 15 minutes. When power comes on you can delay the start of the relay just to be sure. Every time the power fails the relay drops out and resets and waits for another 15 minutes. That's easily doable, and if you happen to lose any equipment it will either be the relay or the time circuit you're using to control it. The fridge remains safe.

Now I understand why you want to delay the power to after some period of time when you suspect the power may be stable for a while. Running a relay for the period of time the fridge is running isn't going to be anything you need to concern yourself with because relays tend to last, and they tend to be reliable. If need be, they're cheap to replace.

Just be sure to use a quality relay. If you power it from 12 volts (just using that number as an example) use an automotive relay. They're good for typically 40 amps DC. Not sure off hand how much current they can handle on AC, but I don't think your fridge will draw more than 15 amps continuous running. The START-UP may be a bit concerning though. You may need a snubber circuit on it as well.

 

From what i read is that you want a delay ON Timer. There are a couple of good ones in the Chinese market. Check this out.

The relay has about 9 functions, commonly used are
Delay ON
Delay OFF
Cycle
ON for some time and OFF for some time.

 

Thank you very much to all who responded to my question.

 

So, my question here is: Would a fridge be damaged if connected to the mains power for 4ms (let's say 10 ms) and disconnected or not.

OK, thanks for the details. To answer the above my guess would be yes, most appliances, especially those with a compressor, can be damaged from intermittent power, especially as you describe.

I have seen plenty of assorted timing relays with different functions, pour off the boat from China and for the most part the stuff works. You would want a delay on make. My own home emergency generator is programmed to wait before initiating a power transfer. You may want to think about a modular delay on make relay board which can drive a larger relay to switch power for the appliance, in this case a refrigerator.

Ron

 

There are a lot of decent quality industrial time delay relays around, often available as surplus. The larger ones can be opened up and the contacts cleaned, which would allow them to work for a few more years. In addition, it should be possible to find a surplus industrial relay that is not a time delay relay, and then use the same circuit to reset the relay in a latched-on position. At that point it would stay latched until the next power drop, and then there would be no reconnection until after the power returned. I now fully appreciate the reason for the power delay effort. It is a good idea.
Note that for 60 cycle power one half cycle is 16 milliseconds and so the pull in delay of just a few milliseconds will not matter, and the drop out delay would be after the power is stabilized, and so that would not matter either.

 

As a side note you also may want to consider a zero crossover SSR if relay mechanical switching time is an issue.

Ron

 

One other possibility is to copy the control scheme used in many commercial coolers and freezers, for exactly the same purpose. Also many air conditioners. So if there are surplus sources available in your area that may be a source of great bargains.

 

As Ron suggested, why not use a SSR, which should last the life of the appliance.
It could be turned on by a simple time-delayed signal, such as this powered by a wallwort.

 

The kind of relay you want is an anti-short cycle timer. http://www.airotronics.com/products-antishortcycle.php 15 minutes is rather long.

So, way back when, when HVAC thermostats were simple contacts, we'd get a storm and a power glitch. The refrigerant system doesn;t like starting under that much load and the surge current destroys the fuse holder.

The anti-short cycle timer fixed it.

In this case, it could be added to the 24 VAC HVAC control circuit. It does prevent the compressor from coming on some delay after a power failure, but will come on right away after that power up sequence finished and the control says to.

 

In this case, it could be added to the 24 VAC HVAC control circuit.

In this case, it's a refrigerator.

 

In this case, it's a refrigerator.

Yep, poor choice of words. Fridges generally switch the line voltage. All of that adds complications and cost. There are also low voltage lock-out relays. Frequent glitches can cause electronics to fry. Old fridges may not have much. The thermostat and a motor driven defrost timer.

Now there might be electronic refrigerators for those tired checking their email with their toasters <G>.

ASIDE: With a current project, I'm putting together, I had a recent conversation with the manufacturer and learned some stuff. The "DIN terminal block based signal isolators" don't have a lot of drive capability. e.g. can drive a 300K load. That actually surprised me. My question was what mode, potentiometer (2, 3 or 4 wire) and what wire would be best intentionally cut with a switch designed to handle a 300 W tungsten load.
So, the best one is the one carrying 200 uA of current and 16-30 VDC and use a contact enhancer.