# A circuit that detects low current usage and activates relay

#### imani9009

Joined Nov 8, 2013
56
Hi
I want to design a circuit with ic's not with microcontroller that can detect when a mobile phone is 100% charged and requiring a very little current and disconnect the mobile phone charger in this condition with a relay.

#### DickCappels

Joined Aug 21, 2008
9,317
Maybe that can be done, it would depend on how the phone behaves when it figures it is charged.

Most if not all modern phones take care of themselves while being charged. Why to you want to do this?

In the meantime, have you monitored the current going into any particular phone while charging and do you know what the typical charge current and charged currents are?

Would you consider a circuit that did not use integrated circuits? Discreet transistors might do it well and easier.

#### ThePanMan

Joined Mar 13, 2020
494
If you wish to reduce the amount of current being drawn then consider how much current a relay would draw. Likely it will draw more than a charged phone would. I'd say "Let it be."

#### Ian0

Joined Aug 7, 2020
6,293
A little experimentation should find a relay that will stay closed if you pass the charger current through the coil. Then all you need is a bypass switch in parallel with the relay contacts to start it up.

#### imani9009

Joined Nov 8, 2013
56
Maybe that can be done, it would depend on how the phone behaves when it figures it is charged.

Most if not all modern phones take care of themselves while being charged. Why to you want to do this?

In the meantime, have you monitored the current going into any particular phone while charging and do you know what the typical charge current and charged currents are?

Would you consider a circuit that did not use integrated circuits? Discreet transistors might do it well and easier.
I have check while 100% the current reduces to 6 mA and when being charged it would draw more than 200mA

#### Ian0

Joined Aug 7, 2020
6,293
You would have to leave it switched on for 2200 hours to pay for the cost of the relay.

#### AnalogKid

Joined Aug 1, 2013
10,125
You would have to leave it switched on for 2200 hours to pay for the cost of the relay.
That's less than 3 months. If the goal is cost savings, there is a (admittedly small) case for that.

ak

#### Ian0

Joined Aug 7, 2020
6,293
I didn’t check that it was a 230V supply. If it’s a 120V supply, then it’s 6 months, and probably a year or two to pay for the rest of the parts and a box to put it in.

(6mA is a lot for a 230V supply, as legislation limits standby power to 500mW)

#### DickCappels

Joined Aug 21, 2008
9,317
I don't know the household electricity rates in Pakistan, so I would not count on electricity savings estimates so far.

Before coming up with alternate solution should we not hear the reason @imani9009 wants to do this? Sometimes people just want to do things for the experience of doing them.

So far it sounds like you want a realy to close when the current drops below 6 ma, the contacts (I assume) need to handle very little current (200 ma) at 5V (or do you want to switch the 230 VAC 50Hz power line?). This will probably require a start button to initially close the relay. How does that sound?

If you want to switch the 5 volts from the charger you can save some cost by using a transistor instead of a relay.

#### imani9009

Joined Nov 8, 2013
56
I don't know the household electricity rates in Pakistan, so I would not count on electricity savings estimates so far.

Before coming up with alternate solution should we not hear the reason @imani9009 wants to do this? Sometimes people just want to do things for the experience of doing them.

So far it sounds like you want a realy to close when the current drops below 6 ma, the contacts (I assume) need to handle very little current (200 ma) at 5V (or do you want to switch the 230 VAC 50Hz power line?). This will probably require a start button to initially close the relay. How does that sound?

If you want to switch the 5 volts from the charger you can save some cost by using a transistor instead of a relay.
It is not about saving of electricity, it is about saving the cell phone battery. Many times it happen we plug in the mobile phone and forget to turn the charger off when it gets 100% and after that it is charged it still getting some current from the charger that could harm the mobile phone. For example we plug in the mobile at night and check the phone in the morning.

#### Ian0

Joined Aug 7, 2020
6,293
It is not about saving of electricity, it is about saving the cell phone battery. Many times it happen we plug in the mobile phone and forget to turn the charger off when it gets 100% and after that it is charged it still getting some current from the charger that could harm the mobile phone. For example we plug in the mobile at night and check the phone in the morning.
If the battery was still getting current from the charger after it was fully charged, it would explode.
So you can conclude that it isn't.
The charger may be taking some current from the mains, but that doesn't necessarily mean that the current is going into the battery.

#### panic mode

Joined Oct 10, 2011
2,301
why do you think that has detrimental effect on the phone's battery?

#### sparky 1

Joined Nov 3, 2018
731
Back in the day when cell phones were less sophisticated you could set a voltage trip point that activated a relay.
There were problems with Lithium batteries getting hot puffing up and there were a few that exploded.
Seen as a liability issue in trending consumer market led to need for innovation, cell phone companies dropped analog to stay competitive.
The later cell phone charging system also wanted low current operation. The battery chemistry and design needed uniformity.
One approach used thermistors, next algorithms, then other logic was included until it grew into what it is now.
A simple answer based on old uncommon analog charging system does not really explain about difficulty in reverse compatibility.
A discussion could address how to interface the newer charging system, the programming and protocols.

An example of cell phone charging system using an On Semi IC. This IC also indicates battery status they call it a fuel gauge IC :
LC709204F - Battery Fuel Gauge LSI [Smart LiB Gauge] for 1-Cell Lithium-ion/Polymer (Li+) with Low Power 2 μA Operation (onsemi.com)

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