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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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Old 15th Dec 2020, 3:22 pm   #1
GMB
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Default Advice on mains current sensing, safely

I need to build a passive thing that works out that our new TV is on to power up the HiFi for the sound (the TV, being modern, has crap sound).

The obvious solution is to sense the mains current being drawn by it since, being modern, it's power consumption is several times that of a valve-based TV (unbelievable - about 400W - don't need the central heating these evenings).

My thought was a current transformer but I am aware that these are possibly dangerous in the situation of a short circuit. So just before the fuse blows the current transformer maybe becomes a ball of plasma? This idea is based on it stepping up somewhat so that there is little voltage drop of the mains - not that it will be much of an issue as the mains here is always on the generous side of the right answer.

It will have to extract enough energy to power a relay.
Has anyone actual expereince of doing this or ideas to make it safe?
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Old 15th Dec 2020, 3:24 pm   #2
Nickthedentist
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Default Re: Advice on mains current sensing, safely

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

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Old 15th Dec 2020, 3:42 pm   #3
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Default Re: Advice on mains current sensing, safely

The posh answer is a hall-effect current sensor plus associated electronics. However, if the TV really draws that much current, you could probably just put a series resistor in the mains supply, and apply the voltage across the resistor to a bridge rectifier and then a relay coil - a 5 or 6 volt type ought to be OK. Calculate the resistor value to give enough voltage drop across the relay when the TV's drawing current.

This will only work if the TV's current draw is fairly constant. If it varies a lot, you may well need a shunt regulator (zener plus power transistor) across the relay coil.

Cheap and cheerful, but it might just work.

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Old 15th Dec 2020, 3:43 pm   #4
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Default Re: Advice on mains current sensing, safely

Does the telly have any USB sockets which are supplied with 5V when it's switched on?
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Old 15th Dec 2020, 4:30 pm   #5
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Default Re: Advice on mains current sensing, safely

I used a small current transformer, & a couple of transistors & a relay. This was to turn on my workshop vacuum cleaner when the bandsaw was switched on.

The current transformer must have a load on the secondary side.
I don't have the circuit unfortunately, as I just made it up as I went along.
Fairly simple though. Used if for about 3 years now, with no problem.

Dave.
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Old 15th Dec 2020, 4:30 pm   #6
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Default Re: Advice on mains current sensing, safely

Quote:
Not technically entirely passive
.. so fails the requirement. They also turn on mains things so another PSU required.

Quote:
The posh answer is a hall-effect current sensor plus associated electronics.
But again not passive.

Quote:
just put a series resistor in the mains supply
Not keen on a hot part, also I want the relay elsewhere so needs to be an isolated feed.

Quote:
Does the telly have any USB sockets which are supplied with 5V when it's switched on?
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). The TV outlets have the wretched "double insulated" aspect - i.e. an electrostatic voltmeter sees them as mains.
I already have issues with this and the audio which is about to be improved to remove the last traces of hum.

So still looking for the ideal solution - making a safe current transformer.
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Old 15th Dec 2020, 4:33 pm   #7
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Default Re: Advice on mains current sensing, safely

Quote:
I used a small current transformer, & a couple of transistors & a relay.
Yes, but how did you make the transformer safe to deal with blow-outs? (and how did you make it).
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Old 15th Dec 2020, 4:51 pm   #8
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Default Re: Advice on mains current sensing, safely

In the past I've done something similar using a bridge rectifier in series with the mains, a suitably-rated transistor wired as a 'super-Zener' across the DC terminals of the rectifier, and a reed-relay in parallel.

At low currents (a few milliamps) there's insufficient voltage to turn-on the Zener but there is enough to pull-in the reed-relay. At higher currents the Zener turns-on and clamps the voltage the relay's coil 'sees'.

The reed-relays I used were rated as having 2.5Kv isolation between the coil and the contacts.
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Old 15th Dec 2020, 4:55 pm   #9
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Default Re: Advice on mains current sensing, safely

I did exactly this in the first lockdown. The usb idea is good but my TV powered the usb outlets all the time. I did look at detecting audio via the headphone connector and toslink too.
In the end I went for a current transformer on the mains feed followed by rectifier, smoothing, threshold detectors and some careful timing before a relay, switching power to the audio system. The threshold/timing has to take care of several aspects such as normal running with light and dark pictures, black screen whilst updating, loading Netflix, loading schedules etc. I ran the various scenarios and noted the output and time delays from the rectifier. This is in a small box, built around op amps and cmos 555s, continually powered and works well. I did dabble with an arduino based unit for fun but gave up on it.
The small current transformer just has a load resistor and a vdr across it before the rest of the circuits.
You can get small current transformers, on a mini pcb intended for Arduino etc, in the usual places.

Ken
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Old 15th Dec 2020, 5:09 pm   #10
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Default Re: Advice on mains current sensing, safely

Your only problem with the current transformer solution seems to be what happens to said transformer in the time between your TV going dead short (is that a frequent occurrence ?) and the fuse blowing (ought to be quite quick in the case of a dead short). If the current transformer primary is wound with something a good deal lower resistance than the fuse wire then it shouldn't ignite before the fuse opens.

However if this really is keeping you awake at night then could you protect the current transformer with a limiter across the primary ? Let's say it normally runs at 1.6A RMS (your telly's drawing 400W) and something over a volt RMS on the primary, and this will transform up to 24V RMS and 70mA RMS ojn the secondary to drive one of these https://cpc.farnell.com/schrack-te-c...vac/dp/SW02586. The peak primary voltage should then be less than +/-2V.

If you took one of these bridge rectifiers https://uk.farnell.com/vishay/pb3510...pb/dp/1815637# and cross-wired + to - then the two ~ terminals would each have two pairs of series diodes connecting them - one pair 'pointing' one way and one the other. This device passes no current in either direction until the voltage reaches about +/-1.4V. If it tries to go much above that then the current through the diodes rises very quickly and the device looks more and more like a short. It's rated for 35A continuously and 350A pulsed with a fusing I2t of 500A2s. That should be stronger than any reasonable mains fuse. If 1.4V isn't quite enough, given the 'normal' voltage across the current transformer, then you could cross-wire another bridge and put it in series with the first one. That would give you 2.1V, using just one diode pair in the second bridge, or 2.8V using both of them.

Cheers,

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Old 15th Dec 2020, 5:47 pm   #11
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Default Re: Advice on mains current sensing, safely

You could alternatively just search for "smart power strip" on your favourite search-engine and have something delivered to your home the next day for a price which - when you consider your time as a billable resource - makes designing/building your own seem an exercise in economic silliness.

Why spend time/effort re-inventing the wheel when you can buy one off the shelf?
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Old 15th Dec 2020, 6:23 pm   #12
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Default Re: Advice on mains current sensing, safely

Current transformers don't mind a short on the secondary, but they object to being open-circuited.

I'd suggest a CT with permanent load consisting of a bridge rectifier feeding an electrolytic capacitor and your 24V relay coil.

If your TV takes 400W, that's about 1.6A, and the relay needs 50mA, then you need a current ratio of 32:1.

So, I'd suggest a 240V:9V mains transformer, (ratio 26.7), rated at about 20VA. Connect the bridge rectifier to the 240V winding. Connect the 9V winding in series with your TV. It's now working as a current transformer. And the insertion drop will be around 24V / 26.7 which is less than a volt.

And of course, you get your isolation. Twice, once in the transformer, once in the relay.
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Old 15th Dec 2020, 6:27 pm   #13
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Default Re: Advice on mains current sensing, safely

I found that the smart ones were quite dumb ! The two I tried had to be “programmed” to recognise ON and STANDBY/OFF. They did this ok but got confused by “gone into standby but I’m updating software/schedule” or “I’m on but screen is black whilst I load Netflix etc”, as examples. Maybe there are better ones about.
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Old 15th Dec 2020, 8:02 pm   #14
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Default Re: Advice on mains current sensing, safely

Hi GMB, there was an article on the design of small, non precision CT's in Elektor about 7 years ago based on a small (EI42?) lam stack.
A Kalee says, no problems with CTs being short circuited, and this simple design could easily be protected against an open circuit. It would be possible to fit a burden resistor across the sec winding, but as the normal failure mode of a bridge is to go s/c, again no problem.

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Old 15th Dec 2020, 8:26 pm   #15
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Default Re: Advice on mains current sensing, safely

How about a LDR or photo sensor on the ON led light or even the screen ?
Feed this into a simple circuit driving a relay for the amp.
Another idea.Use an old gash AM radio near the back of the tv.When the tv fires up the hash
out of the earphone socket rectified to drive a relay.
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Old 15th Dec 2020, 9:00 pm   #16
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Default Re: Advice on mains current sensing, safely

I did it using the USB method on my kitchen telly. A 5V relay switching the mains inside my powered speaker (usual precautions of exporting a double insulated power rail to a class 1 appliance taken).
Not keen on any appliance that leaves the USB 5V on when it's in standby, even my lounge telly with the usb hard drive only powers it up in standby if it needs to 'use' it for something. None of mine do, I have a Logik, a Samsung and a Hisense and they all have usb-powered Roku boxes on them and they all power down when you put the telly into standby.
Maplin used to do a sort of double adapter thingy that went between the tv and the wall socket, providing a second 13A outlet switched in sympathy with the load, although I think (can't really remember) that they issued a recall on them.
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Old 15th Dec 2020, 10:05 pm   #17
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Default Re: Advice on mains current sensing, safely

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.

There is also the possibility of switch-on surge (mainly on a cold start) which must not cause any issues.

I also want a passive solution, so if nothing is happening then as little as possible is powered.

The much-advocated ready made solutions all require me to the build a PSU for the relay control and some seem to draw power themselves (why?). I am not, as perhaps people are assuming, trying to turn the mains on for the HiFi. My Hifi is super-clever in that its PSU stands by using a rechargeable battery and senses power demand from the preamp to initiate the start-up sequence. This allows different reasons to start it automatically. The TV is just the latest one - but brings the annoying problem of being double-insulated. The audio comes from its headphone socket and I have recently had reason to swap the cable to the PC for zoom talks - another double insulated thing. Plugging the audio jack generates a loud burst of mains hum - so it seems to need a lot better electrostatic isolation than managed so far.
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Old 15th Dec 2020, 10:25 pm   #18
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Default Re: Advice on mains current sensing, safely

A little rouse I've done in the past is to wind a few turns of enamelled copper wire around a reed switch, making a very simple current-operated relay. Sometimes they use commercial versions of these to tell a shunt motor controller when the field current has disappeared, so avoiding a runaway motor.

Or make a current sense relay out of 2 bridge rectifiers wired so you get 2 4-way strings of anti-parallel diodes. Use this to shunt the coil of a 6VAC relay (RS do them). The volt drop across the diodes pulls in the relay.

Just a couple of ideas?
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Old 15th Dec 2020, 10:57 pm   #19
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Default Re: Advice on mains current sensing, safely

Probably just me, but I'm confused by your reference to a primary and secondary winding on a current transformer.

In all the current transformers I have used, there has only been one winding, the "primary" being the active line of the mains cable passed through a hole in the transformer assembly.

Therefore, the "primary" is the mains lead and not likely to be doing anything weird in the event of a short circuit (assuming everything is properly rated and fused of course).

As mentioned above, a suitable burden resistor must be applied to the current transformers windings to prevent it generating large voltages in overload situations.

Using a normal transformer "in reverse" for this type of application is not recommended, not only for the dangers it presents but it also has house insurance implications.
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Old 16th Dec 2020, 9:52 am   #20
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Default Re: Advice on mains current sensing, safely

Quote:
In all the current transformers I have used, there has only been one winding, the "primary" being the active line of the mains cable passed through a hole in the transformer assembly.
So that's a 1 turn primary and the other winding (secondary). Just because it isn't much to look at doesn't mean it is not there! But is that enough to extract enough energy to operate a relay? I assume that these are normally just for measurement.

Quote:
Therefore, the "primary" is the mains lead and not likely to be doing anything weird in the event of a short circuit
A short circuit is very weird! It means for the time it takes a fuse to melt you have the entire mains across the primary (however small it may be). In that very brief time you have a lot of energy flying about so my proposed device must be safe. Not so hard when a current transformer is just for measurement I guess but in my case it has to be coupling enough to extract significant energy normally.

My problem is that I can see potential problems but do not have a feel of just how bad they are.
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