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Question about power delivery through standard 12v port

Discussion in 'General Electronics Discussion' started by Maskot, Aug 7, 2013.

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  1. Maskot


    Aug 7, 2013

    I was watching this and noticed that he changed the positive voltage and ground wires so that when he plugs in the charger, it charges the battery without giving power to the other components.

    Can someone give more details about this, he didn't quite explain it clearly enough for me to understand. I would appreciate either an explanation of where the wires go or some type of drawing.

    If I did do this, would I need to switch the ground and positive voltage wires of the charger.

  2. john monks

    john monks

    Mar 9, 2012
    From what I gathered from the video without the charger plugged in the battery is connected to the electronics of his raspberry pi. When the charger was plugged in the positive end the the battery was disconnected from the electronics and at the same time the charger was connected directly to the battery. So to answer your question there is a switch in the jact that only disconnects the positive end of the battery from the electronics isolating the electronics while the battery is being charged.
  3. Maskot


    Aug 7, 2013
    Thanks for your reply, I understand what he was doing, I just didn't understand exactly how to hook it up. I plan on using a battery that uses the same 12v port to receive and output power. Instead of disconnecting this every time I wanted to charge it, I wanted to do what he did in the video, I was original going to use a physical switch to change from the components to the 12v plug, but this would be much easier and you wouldn't have to think about it.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Start by looking at the diagrams at

    Many equipment-side sockets for this type of connector has three connections: one for the inner contact, one for the outer contact, and one for a "normally closed contact", which makes contact with the outer contact when no plug is present, but disconnects when a plug is inserted.

    (What actually happens is that the plug has a curved spring contact that touches the outer metal on the plug. As the plug is pushed into the socket, this spring contact is forced outwards. This disconnects the spring contact from the normally closed contact; normally, when no plug is present, the spring contact pushes against the normally closed contact, but not when it's pushed away from the normally open contact by the barrel of the plug.)

    In the design in the video, the normally closed contact provides the battery voltage to the circuitry. When no jack is present, the normally closed contact will supply the circuitry from the battery, but when a plug is inserted, the normally closed contact will disconnect, and the circuitry will lose power.

    In his case, he was switching the positive side of the battery, so he was forced to put the positive polarity to the outer contact, because that's the contact that the normally closed contact touches.

    This convention, positive to outer contact, is actually the original way that this power connector was wired, back in the dim dark days of early transistor radios and cassette players, on which it was originally and widely used. The switching operation was different then, because the power adapter was used for powering the unit instead of using the batteries, which were not rechargeable in those days. So the normally closed contact connected to battery positive, and the outer contact was the positive supply to the circuitry. But I digress :)

    If you want to use the normal polarity, i.e. negative to outer contact, you would have to switch the negative terminal of the battery and circuitry. The inner contact would connect to the circuitry positive and the battery positive. The outer contact would connect to the battery negative. The normally closed contact would connect to the circuitry negative. Provided that the charger is isolated, and you use a plastic recessed socket, this should be workable.
    Last edited: Aug 7, 2013
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