View Full Version : (Un)plugging a power cable.

03-04-2016, 09:33 PM
I have a four-wheel scooter powered by a 24 volt battery. The user manual is adamant about not (dis)connecting a device while power is available to the charging cable. I can only assume that this is because something electrical could cause a spark, or maybe "irritate" the connection in some damaging way.

I'm wondering if this same concern applies to mobile devices and laptops? Do we have to be cautious; i.e., plug the cable into the device before plugging into the wall when connecting, and unplugging from the wall before unplugging the device when disconnecting?

03-04-2016, 10:21 PM
Sure does. Never just disconnect power supply and with all computers eject (dismount) external drives, Kindles and all other devices before unplugging. Can cause serious corruption.

04-14-2016, 09:12 PM
I am super late noticing this post & have a quick note to add. Hopefully it is helpful.
Some background of where my experience with such things is: I have over 40 years experience in designing and installing electrical systems for critical power situations. I work with voltages from 1 volt to over 13,000 volts. (Sometimes higher on the discharge side) I am experienced with AC and DC systems.

So, without going into a huge amount of math & Ohm's law stuff, which is tempting, here are a couple of rules-of-thumb:

For your scooter's charger, the reason that the manufacturer wants the line side (the plug-it-in-the-wall side) disconnected before the load side (the scooter) is most likely to protect the circuitry in both the charger and the scooter. While the unit is charging or maintaining, the power is kinda-sorta steadily flowing. When you unplug, there could be a very brief (1/60th or 1/120th of a second) surge or ripple. This could damage the electronics. By unplugging the line side, the ripple or surge will be absorbed by the mass of the transformer that is inside the charger. In addition, there is a small resistor across the DC output positive and negative sides. This also helps get the flow of electrons back to the transformer before they wreck stuff. (That's not why it is there. It is there to chop the part of the AC sine wave that remains after rectification by .... Oh Wait, I was gonna be brief.)

On items where the low voltage plug is smaller than the line side (like a MacBook magsafe), it is usually best to pull the wall plug first for one additional reason- There could be a small arc. The big honkin wall plug can take it better than the magsafe connector.

Maybe A Plan? Put in a switched outlet where you plug in the charger. That way nothing but the switch gets the abuse.

This is simplified, so always feel free ask if you have questions or I confused stuff.

Hope it helps,

04-14-2016, 10:01 PM
Nice explanation Paul. Some day when I have time, I'll tell you the story of two techs I knew (when I was working with big iron) that accidentally plugged a three phase AC power line into the wrong connector. :Oops:

04-15-2016, 08:56 AM
+1 for Paul's explanation. Very well said.:)

I would add one thought. I think when it comes to portable computers (Apple portable computers with the magnetic Magsafe connection). Apple designed the magnetic connector purposefully so that if a user accidentally tripped over the portable computers power cord…or moved the portable computer while plugged in…that the magnetic connection at the computer would harmlessly break free (as opposed to what used to happen before the magsafe connector)!!!:(

So either (as Paul mentioned)…there still is some electrical risk (arcing) with disconnecting the magsafe connector first (assuming Apple didn't build any electrical safeguards into the computers I/O board...magsafe board). Or maybe Apple did a risk assessment & determined that the chance of an electrical issue happening (if the magafe connector was disconnected first) was so much smaller...than the risk of power cord damage or power port damage...when someone trips over the power cord (pre-magsafe).

I'm sure there are lots & lots of folks who can attest to the fact that tripping on the power cord of a laptop computer (while plugged in & not a magsafe type connector)…can cause all kinds of SERIOUS laptop computer damage. Power cord damage, power port damage…and of course…the computer that's sitting on the dining room table falling 2.5 feet to the floor…and almost any sort of "bad" damage occurring!:(

- Nick

04-15-2016, 05:57 PM
Ask a simple question ...
Fabulous reply, Paul! Thank you! Although I'm just a simple computer programmer, I have a feeling that "chopping the sine wave" translates to "converting AC to DC" -- how'm I doing?

04-15-2016, 09:42 PM
Thanks for the compliments guys! It was hard (really hard) for me to keep it brief. When I'm falling asleep while driving, one of my wife's tricks is to ask me an electricity question. Heck, I could drive from Detroit to L.A. on "How do generators work?" She's really smart! (And nice, too)

I hope the guys who plugged in the 3 phase didn't get hurt. That's a good reminder to wear our arc-flash gear if there ever was one! I used to teach my apprentices that if you made a mistake the result could be B.B.D.D (Blind, Burned, Deaf or Dead).

After reading your post, Nick, I looked at my magsafe connector for burns. None found, so it must be good quality. I've accidentally yanked it many times.

You got it right, Bud. If you put a frequency meter on a DC output from the power supply and cut out the resistor (and capacitor) you would see ca 120 Hz across the positive & negative. The capacitor and resistor 'smooth' it to zero Hz. (Actually the capacitor does more of the work, but I left it out of my explanation to keep it semi-simple.) The resistor is what helps keep the surge from wrecking stuff.

Enjoy This Day!

04-16-2016, 12:30 AM
Well, of course! That's just what I was thinking ...