Low power consumption relays

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soonguy

New Member
Dear wise people

I wish to make a power failure alert on the radial mains circuit that powers our freezer in its own shed. Our old distrution system had a single house-wide trip, so that we'd know if the power was off to the freezer. Our new distribution unit of course has individual trips, so it would be perfectly possible not to notice the freezer shed had tripped out, many hours or worse. I therefore intend to make a noticeable alarm with lamp and probably buzzer alarm, to come on if there was a trip, using a 240v relay. (Lamp and buzzer would represent a very low load on the relay.) I notice that so many relay coils have quite a power usage. As so many household things are already needing necessary standby consumption, a really economical relay seems a good thing to use for something that is on all the time. The most economical relay I could find, at only 50mw is this one from Radio Spares. But they only come in packs of 10. Please would anyone know of how to buy a single one of this type, or something of comparable power usage? Many thanks.
 
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The power consumption of that relay is given as 1.3vA; the "10mW" is the minimum load power through the contacts, with a 10V circuit.

("Coil consumption" on page 42).

I'd consider a solid state relay such as this, used to "hold off" a MOSFET that controlled the alarm from a battery supply.

eg. Have a one megaohm resistor from gate to battery positive, with the opto relay switch output between gate and source to prevent it turning on while AC is present, plus a small cap across the opto to prevent short interruptions from causing an alarm trigger.

It only need 1mA or less from the AC side, so 240mW worst case with a simple resistor or rather less with a capacitive plus resistor dropper, rectifier and zener limiter.
 
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Is your idea to power the relay coil from the AC going to the shed, and power the lamp and alarm from a different AC feed out of a different "trip" (AC feed from a different breaker)?

What will be the total power needed by the lamp and buzzer combined?

Here is a list from an excellent US distributor. I'm sure someone on this forum can recommend a similar UK source.


Here is a selection from that list that has convenient mounting tabs and connection terminals. The coil current is 15 mA:


ak
 
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Thanks so much. Were your words 'such as this' meant to be a link to a suggested solid state relay? Best wishes, Tony
 
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No consumption :
- Latching relay.
- Use contacts of any relay that will close when de-energized to turn on your warning.
- A green light in parallel to the heating element tells the heater is energized visible from a distance and consumes peanuts of energy compared to the heater.

Edited - Ignore ! Total mental fart
 
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In a freezer?.

The issue with a freezer is that you can't just monitor it's current, as it turns ON and OFF all the time.
 
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Plug a 12v adapter in the freezer outlet, run the 12v to house, place small circuit with 9v battery in house. No drain on battery until power goes out in shed.
 
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Thanks so much Externet. So, will a 240v latching relay make a circuit for whatever warning I want to set up, when the power comes off it? I thought they needed a short pulse, rather than just a cessation of the power to them?
 
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Dear rjenkinsgb,
Thanks again for your wisdom on this. I am really more of an electrician than electronics person. I realise that a normal relay would do what I want, but at a wasteful amount of standby current, and being always on 24/7 is not what really what mechanical relays are for. I do indeed still have an old Omron relay I used years ago to switch off our old gas boiler (not a balanced flue one) whenever the fairly powerful kitchen extractor fan was on, to stop flue gases being sucked back into the kitchen. I see from my meter that its coil is about 50k ohms, which would be wasteful on all the time.

So I see that the much preferable electronic solution is two separate 240V inputs, but it only switches on the secondary circuit when the monitored power goes off. (It seem easiest for me to stick to 240V for the alarm circuit as well, rather than have a battery, so a battery never needs to be checked. And I have a separate mains supply close by, and easy to tap into. Also, there would be no need to cover the situation of a temporary blip in the freezer supply, because a mains blip would also disable the alarm circuit. In the event of an external power outage, we’d know about it almost immediately, if at home.

As our freezer ‘shed’ is actually just what would have been a coalshed back in the day and is now within the house, the door is visible from the kitchen. My plan in any case is to wire a couple of 240V LED green instrument panel bulbs to the freezer socket radial circuit via a 1amp fused spur unit, above the door and visible from the kitchen. (There is also a washing machine on this radial circuit, which could be the cause of a trip.) To have a couple of red LED panel bulbs light up, and a buzzer sound, if that circuit trips, is an extra level of safety.

The diagram of the Vishay optocoupler you suggest is in the image above. Please may I ask: are you saying I should:
  • Put the 240VAC power to be monitored onto terminals 1 and 2.
  • Put the 240VAC power for the alarm lights/buzzer on terminals 5 and 6.
  • Are you suggesting a mega-ohm resistor on the input of the monitored power, terminals, to further reduce power consumption and prolong the unit’s life?
Or am I misunderstanding how that unit operates and should be wired? Thanks so much for your advice on this,

Best wishes

Tony
 
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The optocoupler is a low voltage, low current device.

You would need to feed the LED (pins 1 & 2) through a small bridge rectifier, either a module or made up with four 1N4148 diodes.

The input to the bridge rectifier would be fed 240VA through a 1M resistor from live, with the other input to neutral.

The output would control a MOSFET (holding it off) that was switching a low voltage alarm run from a battery. (Or it could trigger a relay to control a 240V alarm, but from battery power).

I'll draw a circuit out when I get chance.
 
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This is a rough of my concept;
The AC is rectified by the diode bridge, with current limiting via the two 470K resistor, so only around 250uA drawn from the mains and fed through the optocoupler LED.

The transistor is turned on for part of each half cycle, when the LED is drawing current, and repeatedly discharged C1, so the MOSFET gate never gets any appreciable voltage on it and the MOSFET is held off.

If the AC fails, the optocoupler transistor does not turn on so the capacitor charges and the MOSFET switches on, over a few seconds. That sounds the alarm or turns on a relay to switch eg. an AC operated alarm.

 
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I suggest Flame-Proof resistors with insulation rated for 500V (1W min) or better, not for power, but length for insulation IEC 60115-1 4.7 and a reverse diode on input of IR opto LED. They will also act as fuses from direct lightning transients limited by power meter at 6kV spark gap.
 
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