# Problem with finding or making component that gives a signal based on electrical current or power consumption

Joined Jan 19, 2022
1
Hello,

What I'm looking for is, just like in the title, a component which can measure the power consumption and turn off the flow when the power consumption is low. It has to be analog. Let me give an example:
We have a TV that uses 100 watts when turned on and 3 watts on standby.
When the TV is turned on, it should work just like normal and the flow should just run as normal.
But when the TV is on standby, it should turn off when it has been like that after a minute or so.
To turn it on again, you should have to go over to a device and turn it on manually.
The device that should turn off the flow is placed in a power outlet, and the TV connected to the device. Outlet < Device < TV.

I've had some problems finding a way to make this work. I can't really find a component that can measure the electrical current and send a signal.

I hope I've given enough information and that it is understandable. I'll try to answer the questions about specifics as quickly as possible if needed.

#### LesJones

Joined Jan 8, 2017
3,701
It is possible to make a device that will shut of the supply when the demand is below a certain power level. The problem is that it can't then switch the power back when an attempt is made to turn the TV back on. Even if the TV just had a simple mechanical switch no current would flow when it was switched on as the device itself would be breaking the circuit. The only way it could work if there was a reset button on the device that would have to be pressed to enable the power for long enough for you to switch the TV back on.
EDIT. I have just re read you post and realised that you are you are prepared to press a reset button. You could detect the flow of current using a current transformer the output from the secondary could be amplified enough to drive a relay and hold it on. You could use an analogue timing device such as a 555 IC for the timing. You need to consider that the device itself would be consuming extra power while the TV was in use. You would have to work out if this used less energy than that used while the TV was in standby mode.

Les.

Last edited:

#### Yaakov

Joined Jan 27, 2019
5,488
The obvious way to do this is not analog, of course. There are existing IoT relays with power monitors that could easily be programmed to do it, without the deadlock issue (for some devices).

To do it in an analog fashion would, as @LesJones points out, require an intervention other than the TV remote. Also, in cases of any devices that use a soft power switch, you will have that same problem even with the digital version.

#### Ian0

Joined Aug 7, 2020
4,824
Must be either an old television or an illegal television. The maximum permitted consumption in standby is 1W in Britain and Europe.
Your second problem would be making sure that the device you design would itself not use more than 1 Watt!

#### LowQCab

Joined Nov 6, 2012
1,914
Trying to save 3-Watts is absurd.
Just un-plug it from the Outlet if things are that tight for You.
Have You calculated how much Money 3-Watts would amount to on your Electric-Bill ???
In the USA, on average, that would be around ~$0.08 cents per-Month. No matter what method You would like to use, the Electrical-savings would never pay for the cost of the parts. . The best thing to do, would be to sell your TV, there's nothing but garbage on it anyway, and You would save a small, but noticeable, amount of Money, who knows, You might even reduce your Bill by ~$10.oo.
.
.
.

#### Alec_t

Joined Sep 17, 2013
12,732
Since this is 'Homework Help', post your best effort at accomplishing the set task, so that we can guide you to a solution.

#### AlbertHall

Joined Jun 4, 2014
11,952
A current transformer, rectifier, and comparator.

#### crutschow

Joined Mar 14, 2008
29,466
In the USA, on average, that would be around ~$0.08 cents per-Month. I think your math is a little off. 3W continuous is a little over 2kWh/mo and where I live in Colorado, that will cost about 25 cents. #### ci139 Joined Jul 11, 2016 1,797 an example: We have a TV What is the actual application and the actual numbers ? #### click_here Joined Sep 22, 2020 533 Sometimes a project is just fun to do. But just to address the cost - I pay$0.25190/kWh and I'd be happy with a pay back period of 2 years. (4 years is also a typical number businesses use)

3W over one day
$3W \frac{kW}{1000 W } \frac{24h}{day} = 0.072 \frac{kWh}{day}$
$0.25190 \frac{}{kWh}0.072\frac{kWh}{day} = 0.018\frac{}{day}$
$0.018\frac{}{day}\frac{365days}{year} = 6.62\frac{}{year}$

Over 2 years that would save $13.24, so that would be my budget. I'd try to see if I could keep it under$10

#### crutschow

Joined Mar 14, 2008
29,466
The lowest power way and likely simplest way to do this would be to use a small current transformer of the type designed to light a LED when it detects current.
Then, instead of lighting a LED, use that to control the LED input of a SSR (solid state relay).

There are articles listed on Google about making such a current transformer.
Also there are some inexpensive ones, as well as SSRs from Ali Express.

A switch shorting across the SSR output can be used to start it.
The SSR will the turn off when the current drops below a low level.

#### Ian0

Joined Aug 7, 2020
4,824
How about a few turns of wire around a reed switch?
But is the power used to switch the triac/relay/SSR whilst it is running less that the power saved by not letting it run on standby? - that depends on how much time it spends running to how much time it spends on standby.

#### ci139

Joined Jul 11, 2016
1,797
? each time your "Standby detector" activates you will need to reset it to be able to turn on your ?TV?
a simple back-light switch adds an operation of manual turning off the power but will cost less the 1$- i assume Last edited: #### MrAl Joined Jun 17, 2014 8,863 Trying to save 3-Watts is absurd. Just un-plug it from the Outlet if things are that tight for You. Have You calculated how much Money 3-Watts would amount to on your Electric-Bill ??? In the USA, on average, that would be around ~$0.08 cents per-Month.
No matter what method You would like to use,
the Electrical-savings would never pay for the cost of the parts.
.
The best thing to do, would be to sell your TV,
there's nothing but garbage on it anyway,
and You would save a small, but noticeable, amount of Money,
who knows, You might even reduce your Bill by ~\$10.oo.
.
.
Hi there,

Well the way i understand it now is that the electric usage is not looked at for a simpler individual device anymore, it is averaged over the whole country or area. 3 watts does not make a difference to be either as an individual, but multiply that times the number of devices plugged in in say the entire USA and it amounts to a hell of a lot of energy wasted.

Here is another example...
My cell phone input current normally drops to about 20ma when charging via USB, but when i charge wirelessly, it stays around 200ma now. At 8v, that's 1.6 watts vs 0.160 watts, quite a difference. if everyone had this kind of wireless charger in the USA that would be a huge load on the power grid and that is something that involves the infrastructure of the entire country.

So maybe 3 watts might not be much, but it has to be thought about in terms of the total number of devices being powered at the same time.

#### MrAl

Joined Jun 17, 2014
8,863
Hello,

What I'm looking for is, just like in the title, a component which can measure the power consumption and turn off the flow when the power consumption is low. It has to be analog. Let me give an example:
We have a TV that uses 100 watts when turned on and 3 watts on standby.
When the TV is turned on, it should work just like normal and the flow should just run as normal.
But when the TV is on standby, it should turn off when it has been like that after a minute or so.
To turn it on again, you should have to go over to a device and turn it on manually.
The device that should turn off the flow is placed in a power outlet, and the TV connected to the device. Outlet < Device < TV.

I've had some problems finding a way to make this work. I can't really find a component that can measure the electrical current and send a signal.

I hope I've given enough information and that it is understandable. I'll try to answer the questions about specifics as quickly as possible if needed.

Hello there,

Yes measuring the current sounds like a way to go. If the TV is 'on' and then turned off, the current will drop a lot. That tells you the TV has been turned off, so you disconnect it from the line with a relay.
Now that it is disconnected, how do you tell when it is turned back on. A possible solution is to excite it with a 50k resistor, and that simply means placing a 50k resistor across the relay contacts so the TV input stays energized. At that point the voltage will be fairly high.
Now when the TV is switched back 'on', the voltage will decrease dramatically (hopefully unless the TV does not allow that) and you can detect that decrease and turn the power back on. The problem is now that the TV might not turn on you may have to push the button again if it has a button.
So it's a little tricky and requires some experimentation with your actual TV set, and if you swap the TV out for a new one, you may have to change the way you detect the various states. For example, you may be able to detect the power LED on the TV for one sensor input to the control circuit.

As to sensing current, the modern way would be to use a Hall Effect based current sensor. They provide isolation and decent current detection.
A current transformer (as others have mentioned) may work too because the sense current will be large enough to excite the core enough to produce a measurable output. I used a current transformer for years to detect input current to various devices and provide a simple LED indication that the device was "on'".

Good luck with it.