Hi,

I want to make a circuit to protect my refridgerator from undervoltage. If the voltage gets below 100 volts the refrigerator must be disconnected from mains. I need a continuous working relay in order to do that. Anyone know where to find relays that work continuously without getting hot.

 

You choose a relay which will handle the refrigerator load. Somewhere in the refrigerator data the power should be listed or the required current. How do you plan to detect an under voltage condition? The relay itself will only act as a switch to disconnect the refrigerator from mains power when an under voltage condition exist. Additionally when an under voltage condition exist you want some hysterisis or when mains voltage hovers around 100 volts the thing will chatter on and off. So you want it to drop out at lets say 100 volts and reconnect at for example 105 volts.

Here is an example of what I am getting at including the relay with 10 Amp contacts. OVER/UNDER VOLTAGE | ADJUSTABLE TIME DELAY ON DROP-OUT. Macromatic shown in the link is only one example of a dozen companies who make these things. You also have the option of rolling your own. Personally for stuff like this I prefer over the counter turn key solutions.

Ron

 

Thanks for the answer Ron. I forgot to mention something. Besides that I want to protect the refrigerator from an undervoltage, which is bad for the compressor motor, i also want to protect it against the voltage returning again quickly after it had dropped. This is bad for the refrigeration system and the manufacturer also mentioned this in the manual of the refrigerator.

So when the the voltage drops, the relay must de-energized and cut he mains power off. It has to stay in that situation even if the voltage returns to normal. If the voltage returns to normal, a buzzer will notify me to press a button to energize the relay again. I need a SPDT relay for this application. The relay will be continuously energized so the coil must dissipate the heat efficiently.

 

Relay is a "bad word" here. I think the appropriate term is a contactor to handle the load and "relays" to handle the logic and there are drop out adjustible drop-out relays.

An anti-short cycle timer is designed to work with compressors. e.g. http://www.airotronics.com/products-antishortcycle.php
Used one on an AC system that would blow-up a fuseholder.

let's go one up and look at the Idec Smart relay and you might be able to design your own smarts. Add battery backed up power and it wound not have to be manual anymore.

One option is a Constant Voltage Transformer. The output is somewhat square, but it has some regulation due to a capacitor.

Now, i have a Sorenson 500 W AC regulator, that was surplus cheap like $125.00 back in the 80's. I use it for my audio amp.

 

Thanks "KeepItSimpleStupid". But where do i find relays with coils that stay pretty much cool when energized continuously.??

 

Most relays have continuously rated coils. They may get warm but are intended for use like that.
As long as you allow reasonable air cooling.

 

Most relays have continuously rated coils. They may get warm but are intended for use like that.
As long as you allow reasonable air cooling.

THanks
AlbertHall

 

So when the the voltage drops, the relay must de-energized and cut he mains power off. It has to stay in that situation even if the voltage returns to normal. If the voltage returns to normal, a buzzer will notify me to press a button to energize the relay again. I need a SPDT relay for this application. The relay will be continuously energized so the coil must dissipate the heat efficiently.

All you want / need is a continuous rated coil which most are. Let's take a look at a basic common relay. All quality relays include a data sheet so lets look at one example. The Omron MK-S is a popular common series. Now looking at the AC coil versions we see a 110 volt coil version and we also see under specifications:

Must Operate Voltage: 80% max. of
rated voltage
Must release voltage: 30% min. of rated
voltage at 60 Hz
25% min. of rated
voltage at 50 Hz

You can't use or depend on a relay coil pull in or drop out voltage as a voltage sensor. It is not done that way. This is why they make under / over voltage sensors like the one I linked to. Now when you mention:

So when the the voltage drops, the relay must de-energized and cut he mains power off. It has to stay in that situation even if the voltage returns to normal. If the voltage returns to normal, a buzzer will notify me to press a button to energize the relay again. I need a SPDT relay for this application. The relay will be continuously energized so the coil must dissipate the heat efficiently.

So what you want is a relay latching circuit which when voltage is above 100 vac you can latch on but if voltage drops the circuit is disconnected and will require a manual action on your part to again latch the relay. Latching circuits are very common including having an audible alarm included. Just keep in mind that if voltage drops and you aren't around the system remains off until you reset it. You also need a low voltage sensor be it like what I linked to or similar to detect / sense the low voltage condition and make something happen. You cannot really rely on a relay coil to act as a low mains voltage sensor.

Ron

 

You can also wire the relay using the N.C. contacts to power the frig and only energize the relay coil when the voltage drops.

SG

 

I have a rig I wired into my garage door opener. Sometimes the wife leaves the door open after she's left for work. Using a timer and an old wall wart along with a couple relays and switches to sense when the door is open; if it's open at (lets say) 7:00 AM the timer will come on, and with the door open the circuit will trigger a signal to close the door. As soon as the door moves away from the switches the system locks itself out so that it can't be re-energized should the door return to full open - due to a child or a pet crossing the safety beam. If the door is closed, at 7:00 AM nothing happens.

Since things can stay cold in a refrigerator for hours if the door is not opened you can design a rig like mine to every few hours - re-energize the circuit. If the voltage is still low the refrigerator will remain off. But if normal power has returned then you don't need to be present to reset the refrigerator. All you need is a programmable timer capable of switching lights on and off every minute (on for one minute, off for one minute, and so on) (not that you'd want that), your system can energize a reset relay for one minute every few hours with no harm to anything. Your fridge is protected by whatever circuit you come up with to detect brown-outs (under voltage) and a timer that can reset your system every few hours or so.

 

All you want / need is a continuous rated coil which most are. Let's take a look at a basic common relay. All quality relays include a data sheet so lets look at one example. The Omron MK-S is a popular common series. Now looking at the AC coil versions we see a 110 volt coil version and we also see under specifications:

Must Operate Voltage: 80% max. of
rated voltage
Must release voltage: 30% min. of rated
voltage at 60 Hz
25% min. of rated
voltage at 50 Hz

You can't use or depend on a relay coil pull in or drop out voltage as a voltage sensor. It is not done that way. This is why they make under / over voltage sensors like the one I linked to. Now when you mention:

So what you want is a relay latching circuit which when voltage is above 100 vac you can latch on but if voltage drops the circuit is disconnected and will require a manual action on your part to again latch the relay. Latching circuits are very common including having an audible alarm included. Just keep in mind that if voltage drops and you aren't around the system remains off until you reset it. You also need a low voltage sensor be it like what I linked to or similar to detect / sense the low voltage condition and make something happen. You cannot really rely on a relay coil to act as a low mains voltage sensor.

Ron

Hi Ron,
Sorry for not answering sooner, i came home late from work the last days. I further looked into this matter and yes you are right. It was in my plan to connect a buzzer, to tell when the voltage was normal again and then reset the system. Thanks for the hints. I'll look into it further and come back again with more questions.

 

You got questions? We have answers.

Ron

 

You got questions? We have answers.

Ron

Thank you Ron

 

It's an old Radio Shack slogan. https://en.wikipedia.org/wiki/RadioShack

 

It's an old Radio Shack slogan

Yes it was back in the day. Within the company we would say: "If you have questions we have problems".
SG

 

It's an old Radio Shack slogan. https://en.wikipedia.org/wiki/RadioShack

Yeah, and I liked it so I applied it. Obviously it does not apply to Radio Shack anymore.

Ron

 

Hi,

I want to make a circuit to protect my refridgerator from undervoltage. If the voltage gets below 100 volts the refrigerator must be disconnected from mains. I need a continuous working relay in order to do that. Anyone know where to find relays that work continuously without getting hot.

Why re-invent the wheel. You can buy this on ebay. In addition it shuts off with spikes or hi voltage and delays repower for 2 minutes. The one I purchased shuts off at 95 rather than 100.