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I would like to make usb-charging sockets from two sides of the bed, and I don't like the idea of power supply working 24/7 so I decided the following scheme to have power ON while me or my wife are charging:

schematic

simulate this circuit – Schematic created using CircuitLab

But the power on my side socket will be ON, when my switch is OFF, if my wife's switch is ON (being continuously charged is not good for battery). So I have the idea to use DPST switch to also switch usb socket off:

schematic

simulate this circuit

Is it OK to use DPST switch between 220vAC and 5vDC circuits? Does it depends on it's characteristics, how to select right on? Would be there some kind of cross talk between circuits?

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    \$\begingroup\$ This statement; "being continuously charged is not good for battery"; is false. Any device with a Lithium Ion/Lithium Polymer battery (iPhone, etc) will stop charging and switch to external usage when the battery reaches 100%. In fact, by unplugging it you are draining the battery and using more of the limited charge cycles of the battery, shortening its lifetime. \$\endgroup\$ – Garth Oates Apr 21 '16 at 10:51
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    \$\begingroup\$ As for wishing to turn off the step down transformer, build a load switch to turn off mains when no current is drawn: e.g. sound.westhost.com/project40.htm . Although my personal opinion is this whole project is a bit bizarre... \$\endgroup\$ – Garth Oates Apr 21 '16 at 11:01
  • \$\begingroup\$ @GarthOates see digitaltrends.com/mobile/… \$\endgroup\$ – kassak Apr 21 '16 at 13:16
  • \$\begingroup\$ While this might seem like a good idea, it is not as practical as you might think for 2 reasons: 1) The power draw of a modern switch-mode phone charger is extremely small. Feel the adapter with nothing plugged in. If it is just at room temperature, it is a modern charger and there is practically no power draw when idle. 2) The resistance from the long wires on on the USB 5V side will significantly reduce the charging rate. Modern smartphones will draw over 1 Amp of current. You would need to use good sized wire if you want minimal effect to charging rate. A lot of trouble for little return. \$\endgroup\$ – Vince Patron Apr 23 '16 at 0:02
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Smartphones chargers are constantly being upgraded to be more efficient. Many major mobile phone manufacturers are working to develop a rating system for charger energy efficiency in no-load mode. For example, Sony's new standard chargers (EP-300, EP-310 and EP-800,etc) have a no-load power consumption ≤30mW. A 2012 study at the Lawrence Berkeley National Laboratory found that an idle charger drew 0.26 watts on average; this figure goes up to 3.68 watts when a phone is attached and charging, and drops down to 2.24 watts when the phone is attached and fully charged. Overall, the cost is a handful of pounds over 12 months. enter image description here

Here is the typical Standby Power Characteristics of the Texas Instruments 5.3V 2A Mobile Phone Charger Design (PMP4432): enter image description here From this I conclude that it will be effecient to have two smartphone chargers connected to two separate wall points each controlled by their own switch.

schematic

simulate this circuit – Schematic created using CircuitLab

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Two-way switching circuit.

Mixing mains and low voltage on the one switch isn't recommended. You can achieve what you want with a two-way switching circuit as shown in Figure 1. The PSU (represented by a transformer in the schematic) will be on if both switches are up or both are down. These type of switches are called "3-way" in North America.

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  • \$\begingroup\$ Thanks for the answer, but no, this is logical XOR switch. My first variant is OR switch, but I wanted to add ability to power off mine socket by same switch \$\endgroup\$ – kassak Apr 19 '16 at 7:22
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Assuming this is a regular DPST switch, and both poles are isolated (which they are by default), then yes this should work. Using a multimeter to test for connectivity between the opposite poles and etc. when the switch is in the on position is a good precaution or else you may fry your usb connectors.

Make sure you buy a switch capable of handling your ac power current, or else it may weld closed. Any switch should work for 5v DC.

An alternate solution is to have a switch on both 5v usb power outlets and one on the main AC power to the converter (assuming I interpreted your question correctly and you wanted the converter to be off when not used) This would be better isolated.

Better yet, use a DPDT switch And have two converters (one for each socket)!

Point is there are many solutions.

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  • \$\begingroup\$ Im on my iphone at the moment so I will add a schematic later. \$\endgroup\$ – Laessen Apr 22 '16 at 21:42
  • \$\begingroup\$ It is better not to have ac power and dc power on the same switch, but is techincally feasible. \$\endgroup\$ – Laessen Apr 23 '16 at 0:13
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Using a DPST switch like that, you need to be very careful about creepage distances between the mains and 5V USB terminals of the switch.

If there's any possibility of mains voltage leaking across the insulation to the extra low voltage side on a damp day, then the arrangement could become very dangerous.

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  • \$\begingroup\$ I would say that the worry and extra work of finding a good, isolated DPST switch is not worth any microscopic savings in cost OP would get from it, especially since if there isn't a load (or battery is fully charged and phone isn't being used) there is virtually no power being transferred from the adapter. \$\endgroup\$ – QuickishFM Feb 1 at 13:27
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Your second circuit is actually a nice little elegant thing, it allows the DC power supply to be turned off when not in use and allows each usb socket to be switched individually.

As mentioned by others though, make doubly certain that the terminals are well insulated and the switch is rated for the voltage and current. Switched can be bought with plastic isolators between the pins molded in to the switch body.

I might actually steal that circuit now...

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