connecting AC and DC lines using a single connector ?

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
?

 

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
insurance?

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.
Chris

 

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) interchange.ubc.ca
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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
designer.

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

 

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.

Cheers

PeteS