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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE! |
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16th Dec 2020, 12:35 pm | #21 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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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. |
16th Dec 2020, 1:37 pm | #22 | |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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Re: Advice on mains current sensing, safely
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16th Dec 2020, 1:51 pm | #23 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
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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. |
16th Dec 2020, 2:02 pm | #24 |
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
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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. |
16th Dec 2020, 2:27 pm | #25 |
Heptode
Join Date: May 2010
Location: Warminster, Wiltshire, UK.
Posts: 682
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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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I bet that car doesn't have a suppressor. |
16th Dec 2020, 4:20 pm | #26 | ||
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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Re: Advice on mains current sensing, safely
Quote:
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:
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16th Dec 2020, 5:27 pm | #27 |
Heptode
Join Date: Jul 2016
Location: Portland, Dorset, UK.
Posts: 870
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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. |
16th Dec 2020, 10:50 pm | #28 | |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 760
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Re: Advice on mains current sensing, safely
Quote:
Switch TV on -> current detected -> hifi to disconnect from mains. Switch TV off -> no current detected -> hifi to ??
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17th Dec 2020, 11:03 am | #29 |
Octode
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
Posts: 1,764
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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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17th Dec 2020, 11:49 am | #30 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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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). |
17th Dec 2020, 2:27 pm | #31 | |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: Advice on mains current sensing, safely
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Al. |
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17th Dec 2020, 5:04 pm | #32 | |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
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Re: Advice on mains current sensing, safely
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17th Dec 2020, 5:46 pm | #33 |
Guest
Posts: n/a
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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!
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17th Dec 2020, 6:51 pm | #34 | |
Triode
Join Date: Feb 2011
Location: Southampton, Hampshire, UK.
Posts: 46
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Re: Advice on mains current sensing, safely
Quote:
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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17th Dec 2020, 11:16 pm | #35 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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Re: Advice on mains current sensing, safely
Which raises the question as to just how small the transformer would be?
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17th Dec 2020, 11:56 pm | #36 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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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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18th Dec 2020, 11:17 am | #37 | |
Hexode
Join Date: Apr 2007
Location: W Yorks, UK.
Posts: 406
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Re: Advice on mains current sensing, safely
Quote:
You're unwilling to buy one 5V relay? |
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18th Dec 2020, 12:35 pm | #38 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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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. |
18th Dec 2020, 11:40 pm | #39 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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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 |
19th Dec 2020, 9:20 pm | #40 |
Nonode
Join Date: Jan 2016
Location: Stafford, Staffs. UK.
Posts: 2,529
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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! |