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How can I splice a barrel jack extension cord (DC) without blowing anything up?

Discussion in 'General Electronics Discussion' started by Blue_Dragon360, Oct 13, 2014.

  1. Blue_Dragon360


    Oct 13, 2014
    TL;DR: I need to splice two different barrel connectors together, to create an extension cord of sorts.

    First I would like to state that I am quite new at electronics in general, so I apologize if I'm missing some concepts entirely xD

    I've been trying to deal with this for the past few days, and I've decided to ask online because I am completely stuck. I am attempting to put a Raspberry Pi inside an old NES case (not an original idea, but I thought it would be a fun opportunity to learn more about the hardware section of electronics), and I am having trouble with the power source.

    The USB hub I'm using (which powers everything inside the case) has a barrel jack DC adapter thingy. The USB hub's manufacturer assured me that the barrel port size was 2.1mm x 5.5mm, so I went ahead and ordered an extension cord for that size, as well as the USB hub. I was planning on using the female end of the extension cord as a port running from the inside to the outside for the adapter, plugging the male end of the extension into the hub (and the male end of the outside power adapter into the port I'm making on the NES, of course).

    Like so:
    HUB<-----< <-----=
    Where | is the NES wall, <> are female and male ports (depending on where they're pointing in relation to the wire) and = is the DC adapter (and connecting cord).

    Sadly, it's not that simple. Problem is, the manufacturer didn't know what the heck they were talking about. The port was nowhere near 2.1mm x 5.5mm when I bought it, so I went to RadioShack to see if I could figure out the right size. I believe it is a weird proprietary size, so I'm unable to find an extension cord to act as a port.

    I've decided to remedy this by splicing the power adapter's male end (the proprietary one that came with the hub) to the extension cord's female end to create a more standard extension and conversion. I will then order a 5v 2.1mm x 5.5mm adapter to power it on the outside of the NES.

    Here is where I need help. I have no idea how to obtain the polarity of all the components, and splice the right wires together so that nothing explodes. I have a multimeter, but I don't know how to use it to measure the polarity in this situation.

    NOTE that the end result of the extension cord will hopefully look and function like this:
    Where "<" = Weird (male) barrel port, cannibalized from the 5V power adapter that came with the USB hub (plugging into the matching port on the USB hub)
    and "[" = Standard (female) 2.1mm x 5.5mm, cannibalized from the extension cord that I foolishly ordered based on information from the hub's manufacturer. This acts as a port on the outside of the NES case, where I can plug in a standard 2.1mm x 5mm DC adapter.

    Is there a way to do this?

    Thanks for any possible help!
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome to our forum.

    Typically the wires inside a cable are color coded. Obntaining the correct polarity for an extension cord therefore is as simple as matching like colors.
    If the wires are not color coded, set your multimeter to Ohm range (this little greek guy here: Ω) and select the smallest range available (typically 200 Ohm). Connect the wires of the extension cord first by twisting the wires, not yet soldering them. Now measure from both ends of the extension cord (from male to female connector). Each connector has two contacts: one inner and one outer. Measure resistance from each contact of the male connector to the corresponding contact of the female connector.
    If they match (i.e. male outer to female outer, male inner to female inner), you're set and can continue soldering the joints, insulating them and you're done.
    If tey don't match (i.e. male outer to femal inner and vice versa), you open the twisted connection and swap wires. Re-check and you should be set.
  3. Blue_Dragon360


    Oct 13, 2014
    Thanks for the answer! I'm still confused in a couple areas though...

    The wires for the old adapter's cord are color coded, but the ones from the extension cord are not. On the extension cord, however, there are some odd markings. One wire has letters and the other has dashed solid lines. Here are the letters it says:
    CSA 242699 AWM I A 80°C 300V FT1 =F= LM
    And in another spot, says:
    E315656 <weird symbol, kind of like a backwards R next to a U> AWM STYLE 2468 80°C 300V 20AWG VW-1 LinoYa
    Does this mean anything?

    Also, it's my understanding that switching one of the wires I'm connecting will reverse that polarity, and while it will still transfer energy, the output at my hub will not be correct. I'm guessing that your trick with the multimeter matching inner/outer places to positive/negative wires will give me that information, but I'm running into an issue. The proprietary port is so small that the multimeter's prong things won't go into the middle. Is there a way to do this without accessing it's middle?

    Thanks for your help!

    EDIT: after doing some research, it appears that the wire with the line on it is positive. So I guess I'll connect positive to positive and negative to negative, get a 2.1mm x 5.5mm inner negative adapter, and hope for the best!
    Last edited: Oct 13, 2014
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    The funny UR symbol is the Underwriters Labs trademark. Underwriters Labs tests components and products for safety, for the insurance industry. That symbol means that they have type tested that cable - probably for heat and fire resistance and voltage withstanding properties. Other markings are approvals from other labs, and file numbers for those approvals. All the other markings are manufacturers' part numbers and specifications for the cable.
    How about using a very thin piece of wire, wrapped around your multimeter probe, to touch the contact? Or something very thin and conductive like those old-style razor blades?
  5. donkey


    Feb 26, 2011
    when you have a chance I would like to see setup of this in photo form on our project log page, or PM me a link to wherever you are posting it.
    what sort of hub you using?
    what sort of connections you got?
    what sort of adaptor you using?

    the reason I ask this is after looking at a USB hub recently I found the voltage in was higher than 5volts, The reason was the power pack was not regulated. so the USB hub itself had an inn built regulator so voltage remained at 5v even with peaks of the mains
    also depending on what you connect to the HUB you could draw quite a bit of amperage so if you draw too much the power pack might not handle it and things like your pi will turn off.
    so before going to the hassle of cutting and splicing check you have a regulator in the HUB or in the power pack or a little surge might end up causing issues, if the HUB has the regulator then you might need more than 5volt in
  6. Blue_Dragon360


    Oct 13, 2014
    Hi there! I don't currently have any photos of my project, but I can take a few and post it. :)
    1. I'm using this hub: Pretty basic hub, the reason I ordered it was because it was relatively small and could fit inside the case. I stripped off the casing of the hub, and attached the circuit board directly.
    2. What exactly do you mean by connections? I'm just soldering/heat shrinking all of the wires I need to connect.
    3. It's a basic 100-240V AC input, 5v - 2.6A output.

    It does look like the power pack is regulated, and the USB hub doesn't have an in-built regulator. Although I'm not at all sure of this, if you think it might have definitely let me know! I'm relatively new to all of this stuff.

    I shouldn't be drawing too much power I don't think, only a low-power wifi adapter, the pi, and a couple controllers at maximum.

    I'll post some photos in a bit!
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