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connecting AC and DC lines using a single connector ?

Discussion in 'Electronic Basics' started by aman, Jul 22, 2005.

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  1. aman

    aman Guest

    I have a question which might seem a little silly but I still need to
    ask. I need to make some connection between two of my PCB's. I am
    using like a 25 pin connector for this. Is there any problem if I use
    some of the pins of this connector to make AC connections (120V) and
    some for DC ? I mean is there any noise issue in the DC line if I do so
  2. Chris

    Chris Guest

    Hi, aman. I'm assuming you're talking about a standard DB-25
    connector. like the ones used for RS-232. This is a bad idea for
    several reasons.

    First, these connectors are basically meant to conduct low current, low
    voltage signals. The pins are optimistically rated for 1 amp, which I
    suppose is OK, at least at the beginning. However, as you have multiple
    insertion/extraction cycles, the connection tends to get resistive. As
    it does, the heat increases as the square of the voltage drop across
    the connector, and you've got plenty of volts to turn into heat. You
    can easily melt the thermoplastic around the pins with a resistive
    connection, with predictably disastrous results. And a fuse probably
    won't protect you until after the connector sockets have melted into
    each other. Actually, the fuse will respect this by politely refusing
    to blow until the hot pin socket has come into contact with a
    ground-referenced logic signal, smoking your electronics. It will
    probably chuckle to itself while this is happening, though, and not in
    a nice way.

    Next, if there's any chance of the connector being unplugged while
    live, the arc right next to your low voltage signals will want to cause
    problems. Along these lines, you should also be aware that the soft
    metals used in the connector pins/sockets are not made to take the heat
    of the switching arc. You could easily weld them together. And in the
    event of a line voltage fault condition, overcurrent could damage the
    pin/socket long before the fuse goes.

    You want to look at the possibility of a wire breaking in the connector
    shell, too. Do you want a line voltage wire with a squib of wire at
    the end floating around in your shell? Got a metal shell? Got

    The problems get magnified if you're using a cable with multi-conductor
    wire. The current going through the line will lead to inductive
    coupling (will probably smoke your stuff with an overcurrent
    condition). The proximity of the wires in the cable will cause
    capacitive coupling (will probably smoke your stuff if there's a line
    surge). And most conductors in these cables aren't specified for line
    voltage. There will be a leakage current you'll have to deal with.
    Any or all of these will kill digital logic, or worse, cause
    intermittent glitches which will drive you nuts.

    By the way, you're not planning on having a male pin sticking out with
    120VAC on it, are you? Hope not.

    I think you've got a start on understanding in how many ways this is a
    bad idea.

    Sorry to be the bearer of bad news. Just put in another connector, or
    a terminal block, or just about anything else. It's not worth it.

    Good luck.
  3. If you are dealing with 120 VAC, I would be worried about the voltage
    rating of the connector - I don't think most small multi-contact
    connectors will be rated to handle that voltage.

    Generally, I try to keep line voltage well isolated from signal and
    low voltage DC supply wiring.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  4. John G

    John G Guest

    If you need to ask this question you do not understand enough
    electricity to be designing boards with 120volts AC on them.
    You should be using a Wallwart to supply a lower voltage or if this
    thing is to switch mains power you need some help from a professional

    Sorry to be harsh but electricity travels at the speed of light and
    kills just as fast.
  5. PeteS

    PeteS Guest

    Although I agree with the previous posts about the lack of experience,
    I will note that there are connectors specifially designed for the
    purpose you want (although they aren't particularly cheap).

    The usual culprits (AMP/Tyco, Molex, FCI, Samtec, JST et al) all make
    wire to board connectors that have some high voltage / high current
    contacts in the same shell as low voltage/low power contacts, with
    appropriate ratings on the pins (including pin to pin breakdowns and
    arc ratings).

    I suggest you look at those (and other manufacturers I obviously
    missed) websites to see if there's something suitable and then see if
    it's available from your favourite mail order supplier.


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