reminder be careful when measuring circuits

[Guest sstandfast]

here's a quick reminder to everyone out there to be careful when using multimeters to measure values from whatever circuit they're working on.  Here's what just happened to me:

I was playing around with some simple circuits, flashing led's and stuff on my bread board when I forgot a simple but important rule about using meters to measure circuits; no matter what you use to measure a circuit, you always change the circuit.

To set the stage for you, I had an LED indicator on my breadboard that told me whenever I had power applied.  It was pretty simple, I had one end of the LED tied to ground with a 330R resistor between the LED and the supply.  Well the LED was only rated to something like 25mA and like 2V or something like that and my supply is capable of 5V @ 2.5A.  Well I was using the amp meter on my multimeter to measure currents through the circuit when I forgot that whenever you use an amp meter to measure current, it is the equivaliant of placing a short circuit across whatever two leads you are measuring.  Well, I was measureing the current through the dropping resistor for the power indicator when the LED started to glow brightly, VERY brightly.  Then all of a sudden, it EXPLODED!!!!!!!  It was kinda cool but at the same time it startled me.  Fortunately it was just a cheap LED that I burnt out and not something expensive like a PIC.  But it was enough of a wakeup for me to be more careful when measuring circuits.  Anyway, I thought you guys might get a kick out of that story.

Shawn

[Steven_C]

Hey Shawn!

 It's good to hear you are having fun with electronics! Blowing up that LED must have been cool! Did you know that a measure of current is actually a reference to the number of electrons passing through a conductor? and voltage is a reference to the number of electrons waiting around at a 'potential source' that want to pass through a conductor (ie not moving, but waiting to move)? and these electrons (potential) can only move (from one pole to the other) when there is a conductor to pass through!

 So, current can only flow if there is a conductor to flow through! And the current will be of high value if the total resistance of the circuit is low, and the current will be lower if the resistance is higher.

 So when you think about it, if you want to measure current, the meter will need to be IN LINE of the circuit, to give a measurement of the number of ELECTRONS passing through a CIRCUIT over a given TIME PERIOD (like ......how many formula one cars go over the finish line in one second.....well sorta!) So, always measure current through a circuit by breaking the circuit, and placing the meter in series!

BTW....

 to measure voltage across a component, or circuit, or part of a circuit, put your meter across the component

also, your LED burnt(!) out because the resistor was designed to limit the current (Poor conductor, not as many electrons pass through in a given amount of time), and an ammeter actually has its own resistance when it is placed in series with a circuit, which needs to be low (so it doesn't affect the circuit and give false readings)............and when you put it across the resistor, you effectively bypassed the resistor, and let the LED draw too much current, and it therefore blew up! ;D he he!

you don't need to be sorry to me for asking too many questions!! can't you see I love raving on about this stuff!!! (unless it gets beyond my knowledge/experience!! ;))

[Guest logicat2001]

I'm new to this board and have spent a few weeks reading through it's various postings. I'm really excited to begin my first MIDIbox project and already have three PIC's on their way.

I just want to say hello to everyone and thanks in advance for all the good ideas and great attitudes I've found on this uCApps.de Forum.

Threads like this one are so helpful for newcomers; I just had an "aha!" lightbulb moment and several things were just made sparkly clear while reading through it.

Best to all & can't wait to begin,

Anton

[gillesdeshays]

It makes me remember my electronics debut ... at School ...

We often put some 10µF / 16V aluminium condenser in the 220V plug ... and switch on the "emergency red mushroom" on the desk ...

--> A nice flash, a good bang ... and a rain of little particles of metal in all the classroom   ;D ;D ;D ;D ;D ;D ;D ;D

And a teacher really angry !

Don't try this at home !

[Guest remix_me]

Actually I did something similar in my car the other day, my rear wiper decided it only wanted to work some of the time, usually after a few slams of the boot/trunk.

I tested all the connections and could find no fault so ended up taking the wiper motor and relay assembly out to have a look at it, after taking the assembly apart and putting it back together I decided I would test the relay circuit by checking how much voltage was getting to the wiper motor from the relay circuit with the multimeter. As soon as I connected the contacts, to my surprise there was a slight puff of smoke and the wiper motor jumped to life. :o

Well that made me think faulty relay, but since putting the thing back together its worked for the past fortnight.

A nice lucky accident with the multimeter.

[Steven_C]

oh the joys of electronics! when I was 14, I put scissors

in the active pin (240v mains) and shorted them to the metal grounded socket panel in Science class, and blew the tip off the scissors, and I reappeared out of the smoke screen cloud a few seconds later! The teacher just laughed! (Hey kids, don't try this at home or anywhere else, you might die!)