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Old 16th Dec 2020, 12:35 pm   #21
Terry_VK5TM
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Default Re: Advice on mains current sensing, safely

There are two classes of current transformer - sensor and protection.

Sensor types are basically low voltage (mV), low current output.

Protection types have higher voltage/current outputs and more stringent construction styles.

I doubt however you will find one at a suitable price that will operate a relay directly.

If you absolutely, positively must use a step down transformer as the sense element, the current rating of the low voltage winding that the mains would be run through must be higher than the fuse rating in the plug eg 13A fuse - 15A minimum current capacity of the low voltage winding.

Personally, I would just get up close and personal with the audio systems remote control.
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Old 16th Dec 2020, 1:37 pm   #22
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Default Re: Advice on mains current sensing, safely

Quote:
the current rating of the low voltage winding that the mains would be run through must be higher than the fuse rating in the plug eg 13A fuse - 15A minimum current capacity of the low voltage winding.
But what about the secondary?
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Old 16th Dec 2020, 1:51 pm   #23
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Default Re: Advice on mains current sensing, safely

It's down to I²t ratings.

The 240V:9V transformer that I advocated, 20VA rating, has a 9V winding capable of carrying 2.2A continuously. For your TV at 1.6A, that's fine.

If your TV goes short-circuit, then it's a moot point whether the 3A fuse in your plug or the 9V winding fails first, while the umpteen amps are flowing. I'd put my money on the fuse clearing. But, if worried, you might want to use a higher-rated transformer.

What about the 240V winding on the transformer, our "secondary"? Well, while umpteen amps are flowing in the 9V winding, there will be 9/240 x umpteen amps in the 240V winding (it might be a good idea to use a chunky Zener or surge suppressor, rated at 30V or more, which won't interfere with normal operation but will absorb current in this scenario).

If wire gauges for the transformer are properly chosen, the primary and secondary current densities will be about the same, and there's no reason to think that either is more vulnerable than the other.
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Old 16th Dec 2020, 2:02 pm   #24
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Default Re: Advice on mains current sensing, safely

Short-circuit currents available at appliances plugged into domestic mains sockets are typically on the order of 200A - 2kA. The latter would represent a short flex fed from a socket near the meter cupboard. If the primary conductor is 0.75mm² or more it is capable of withstanding the I²t let-through of a 13A BS1362 fuse, so need not consider it further.

The secondary short-circuit current into a resistive load might not increase proportionally to the increase in primary current if the core saturates. Clearly, the value of primary current that will cause this depends on the size of core, number of primary turns and hence how far up the curve you are working it at normal load current. For a measurement CT that requires a high dynamic range with good linearity, there has to be a lot more core (and hence more energy transfer to the burden resistor under S/C conditions) than in your case where only a threshold detection is required. You could push a small core closer to the knee with the TV load and possibly get with clamping only the secondary to stop the extreme case of the relay coil opening causing an O/C voltage hazard.

One thing to watch out for with a completely passive device is relay chatter damaging the controlled load. All relays have inherent hysteresis between operate and release currrents due to the decrease in reluctance as they operate, but on a small AC relay it might not be enough to prevent rapid switching cycles if the controlling load varies about the threshold. There might be advantage in rectifying the secondary and using a DC relay.

Last edited by Lucien Nunes; 16th Dec 2020 at 2:08 pm.
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Old 16th Dec 2020, 2:27 pm   #25
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Default Re: Advice on mains current sensing, safely

I take it your amp is not remote controlled. If it was I do it this way.
I have a remote. It come from America, it learns up to 4 different remotes. The beauty is it can be programmed to switch 4 different things on and off instantly or anytime you like with it’s built in timer. Ideal for my mum as it switches the tv on then the free sat box then her favourite channel then the amplifier.All by using one button. Then turns the whole lot off by one button . Andy
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Old 16th Dec 2020, 4:20 pm   #26
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Default Re: Advice on mains current sensing, safely

Quote:
I take it your amp is not remote controlled.
Correct, because that would mean it had to be powered all the time. I am very resistant to things being powered all the time because the costs are surprising and there is an obvious risk of failure.

Actually the main purpose of the proposed current-sense startup is actually to turn the HiFi off - because in doing it manually it keeps getting left on (it is not near the TV).

Quote:
I have a remote. It come from America, it learns up to 4 different remotes.
Not sure that works for our TV as I don't think the remote uses IR (it's one of the "magic wand" type).
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Old 16th Dec 2020, 5:27 pm   #27
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Default Re: Advice on mains current sensing, safely

I did had a product that consisted of a single power outlet connected to a box that had a 5V relay inside it connected to a USB lead, designed to be used with a computer but in a lot of cases pointless as a lot of desktop PC's leave the USB ports powered so you can use them for charging and the like.

If you wanted to keep the 5V supply completely separated then there chips/devices for isolating power supplies.
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Old 16th Dec 2020, 10:50 pm   #28
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Default Re: Advice on mains current sensing, safely

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Originally Posted by GMB View Post

Actually the main purpose of the proposed current-sense startup is actually to turn the HiFi off - because in doing it manually it keeps getting left on (it is not near the TV).
Is that -
Switch TV on -> current detected -> hifi to disconnect from mains.
Switch TV off -> no current detected -> hifi to ??
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Old 17th Dec 2020, 11:03 am   #29
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Default Re: Advice on mains current sensing, safely

If I was doing this I'd use a current clamp sensor on one side of the mains feed and use an Arduino board to do the logic, with either a solid state relay to turn on the hifi, or a transistor + traditional relay. Perfectly safe and non intrusive
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Old 17th Dec 2020, 11:49 am   #30
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Default Re: Advice on mains current sensing, safely

To clarify: when the TV is fully powered I want the HiFi signalled to start up, and when the TV goes to standby I want the HiFi signal withdrawn so it either shuts down or reverts to other uses.

Reminder: solutions involving an active device that sits about sensing fail to meet the other requirement of having zero power consumption when the TV or HiFi is not being used.

I should also mention that the HiFi was built by me in the 70's. It is an adaption of the original Sinclair Stereo 60 system - quite a bit modified from the original. The automatic power aspect came about because of trying to avoid a nasty switch-on bang in the speakers. I went the extra mile so that its own mains power is relay-switched and the speakers too, and now works on demand from the preamp. This allowed it to be used as an on-demand audio system attached to our old house audio network. I am restoring this to its original state as it had a useful unplanned future-proof aspect in that it solved the problem of digital devices failing to synchronise for multi-room use. The Hifi has zero power consumption when on standby (that's a real zero, not just low).
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Old 17th Dec 2020, 2:27 pm   #31
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Arrow Re: Advice on mains current sensing, safely

Quote:
Originally Posted by Nickthedentist View Post
There are cheap, ready-made devices which do this. Although I like a challenge, I think I'd just buy one off-the-shelf.

Quick Google search... Not technically entirely passive, but: https://www.amazon.co.uk/Energy-Powe.../dp/B00VKU57D4
Which is what I use here, in two locations: no problems.

Al.
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Old 17th Dec 2020, 5:04 pm   #32
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Default Re: Advice on mains current sensing, safely

Quote:
Which is what I use here, in two locations: no problems.
But is no use to me as they just switch on a mains device. They are also not pasive.
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Old 17th Dec 2020, 5:46 pm   #33
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Default Re: Advice on mains current sensing, safely

Over 30 years ago Labgear developed an amplified set top TV aerial which sensed the line output field using a small coil which then turned on the battery powered amplifier. I knocked up the proof of concept prototype switch part in 10 minutes using a coil, a transistor, a diode and a relay. Of course the real product "evolved" into using an ASIC!
 
Old 17th Dec 2020, 6:51 pm   #34
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Default Re: Advice on mains current sensing, safely

Quote:
Originally Posted by GMB View Post
To clarify: my concern on safety relates to what happens if the TV or cabling tries to short the mains for some reason. The current transformer primary experiences 240V applied to a winding that was designed for a couple of volts, with the secondary thus trying to deliver many kilovolts to its circuit. It has to be able to experience this without causing a problem.
It's not nearly so bad as this. Once the voltage drop across the primary of the current transformer (whether a measurement CT or a reversed 240:9V transformer) gets above a few volts (perhaps a few hundred mV for a measurement CT, maybe 12V or so for the 9V transformer) the core will saturate and the while thing will stop behaving like a current transformer. Yes, brief spikes will be seen on the output near the zero crossings of the primary current, but the burden resistor (or a zener across the relay if driving one directly) will absorb this just fine because the energy that can be transferred through the undersized core is limited by the total flux swing and the supply frequency. During this time the average power in the burden resistor / relay / clamping zener is fairly modest, and the duration is limited by the 3A fuse blowing because the current transformer is not appreciably altering the fault current.

There is good reason to go with a low power relay, and no more core than you really need to drive it, but the energy that passes through the current transformer during a fault is limited, and calculable, so you can design the burden to survive it.
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Old 17th Dec 2020, 11:16 pm   #35
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Default Re: Advice on mains current sensing, safely

Which raises the question as to just how small the transformer would be?
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Old 17th Dec 2020, 11:56 pm   #36
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Default Re: Advice on mains current sensing, safely

Can you share the details of the relay with us ? I imagine they'll determine the details of the transformer needed to drive it.

Cheers,

GJ
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Old 18th Dec 2020, 11:17 am   #37
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Default Re: Advice on mains current sensing, safely

Quote:
Originally Posted by GMB View Post
it's power consumption is several times that of a valve-based TV (unbelievable - about 400W - don't need the central heating these evenings).
Wait, what? What modern TV consumes 400 watts?

Quote:
Originally Posted by GMB View Post
Brilliant. I should have thought of that.
But I am seeing snags: I mainly have a supply of 24V relays (maybe should have mentioned that).
You're unwilling to buy one 5V relay?
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Old 18th Dec 2020, 12:35 pm   #38
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Default Re: Advice on mains current sensing, safely

I have a load of relays, many 24V with coils in the range 400 to 1000 ohms.

Modern OLED TVs with big screens have huge power consumptions it seems - I admit I didn't expect that! You can feel the heat coming off the screen when standing near it!

What I do not yet know, because I have mislaid my power meter, is how variable it is. I presume that if the screen is black then the consumption could be very low as I guess it is all about generating light from not entirely efficient OLEDs.
I will need to check this out.
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Old 18th Dec 2020, 11:40 pm   #39
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Default Re: Advice on mains current sensing, safely

Looking/thinking similar to #16. Remote controled socket ( either a pair or a dual socket out of the remote socket).
https://cpc.farnell.com/c/electrical...-mains-sockets
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Old 19th Dec 2020, 9:20 pm   #40
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Default Re: Advice on mains current sensing, safely

I use a device on my PC that seems to fit what you want. It's whole reson for being is power saving. I use a hifi amp for the audio from said PC, plus the monitor is fed from this. Only once the PC is on and drawing enough current, does it switch on the sockets for the amp and monitor. Is this the kind of device you are looking for? If so I'll crawl under the desk and find the name.

OLED TVs aren't that power hungry unless there's something wrong. I'm sat here watching ours, the whole house is only running 800W. And we have a fair bit of (admittedly LED) lighting inside and outside. The pond pump and UV, the various routers servers and bits including cameras, and of course the amp under the TV providing decent sound. I think when all lights are off, central heating not running we don't get much below 400W. Just tried switching off the TV and dropped from a fraction over 800W to a fraction below 700W. So I'd say 120W ish, including the audio amp (which the TV switches for me).

Now the Plasma that preceded it that was a beast. And yes, you really could feel the heat, the mutliple fans were also a clue. I don't think the In-Laws we gave it to realised!
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