Pages: 1 [2]   Go Down
Author Topic: My "where do babies come from?" of electronics.  (Read 1522 times)
0 Members and 1 Guest are viewing this topic.
Pittsburgh, PA, USA
Offline Offline
Faraday Member
**
Karma: 58
Posts: 4020
I learn a bit every time I visit the forum.
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

And if something happens to enough of those leds, the rest will start to die in a manner that would make a funny video.....

Logged

Examples can be found in your IDE.

UK
Offline Offline
Faraday Member
**
Karma: 92
Posts: 3969
Where is your SSCCE?!?!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

So I learn best by trial and error. I got out the breadboard and a bunch of LEDs. first I setup up about 12 LEDs and a 1k resistor for each (what I have in mass quantities) and connected them in parallel. gave it some power. they werent as bright as they can be but they lit up. next I tried them in series. I believe I was able to get about 4 to light up. I had one 1k resistor on that circuit. then I decided to live dangerous and dropped LEDs all along the voltage and ground rails. 117  leds give or take. connected a 9v with no resistors and they all lit up nice and bright. I don't know what I learned but I like it.
What you learned is that little 9V PP3 batteries can't give much current, which is why all the LEDs didn't spontaneously combust.
Logged

Why not visit my eBay shop? http://stores.ebay.co.uk/Majenko-Technologies
Replacement for the Arduino IDE: UECIDE - Proper serial terminal, graphing facilities, plugins, overhauled internals.
Java isn't bad in itself, but it has enabled morons to write programs.

Valencia, Spain
Offline Offline
Faraday Member
**
Karma: 118
Posts: 4554
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

117  leds give or take. connected a 9v with no resistors and they all lit up nice and bright. I don't know what I learned...

Did you try holding the battery in your hand while you were admiring the LEDs...?

Logged

No, I don't answer questions sent in private messages...

Kentucky
Offline Offline
Newbie
*
Karma: 0
Posts: 45
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Here's something that may help you a lot, once I realized this, things made a lot better sense to me:

Current in a circuit is (generally) not "set" by the power supply, it is set by the load (resistor, motor, hair dryer, etc)

The load "pulls" or "draws" current.  Think of household wiring.  You may have an outlet in your bathroom on a 15 amp breaker.  Let's say a hair dryer on high "pulls" 10 amps.  Now if you plug in TWO hair dryers on high, they would attempt to draw 20 amps, thus tripping the 15 amp breaker.

Now, power supplies have a maximum amount of current they can deliver, as you saw in your huge 9v/LED experiment.  The LED's wanted more, but the 9v simply couldn't deliver.  BTW, you probably need a new 9v now.  Better yet, toss that thing and use better batteries (like some AA, C, or D in series, or a wall wort).  Batteries have an "Amp Hour" or "Milliamp Hour" (mAh) rating that you can use to actually calculate how long a battery will power your circuit! smiley-grin

Note that LED's are not the same as a light bulb though.  Without a resistor, an LED will pull so much current that it will burn itself up.
When learning electronics, if you don't blow some stuff up here and there you're not learning properly LOL smiley-grin
Logged

BHZ, MG, Brazil
Offline Offline
Sr. Member
****
Karma: 10
Posts: 346
Android developer; Arduino enthusiast
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

117  leds give or take. connected a 9v with no resistors and they all lit up nice and bright. I don't know what I learned but I like it.

If your 9V source wasn't a battery (that has a very limited current capacity), but instead was a bench supply, a wall wart or any other source of AC-DC adapter, you'd have learned what it means to divide V/R when R=0: you get inifinite current (well, not infinite, but very high). All of your leds would give a very bright glow, right before exploding or simply dying. If your LEDs had held the high current (very unlikely), you might have had a blown fuse in the power supply.

So you learned that 9V batteries have an internal resistance that limits its maximum current.
Logged

Learn to live: Live to learn.
Showing off my work: http://arduino.cc/forum/index.php/topic,126197.0.html

Offline Offline
Newbie
*
Karma: 0
Posts: 14
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Ah. It's starting to come together. That makes sense BackWoodsBrewer.
I was actually pretty surprised that none of them did blow. On one of the smaller experiments the wire on the battery accidentally touched the lead of an led past the resistor and it was bright so that's what "sparked" the idea for the as many led's as I could fit. I have blown a led with this exact 9v just for kicks.
I literally just tested the battery with the multimeter and its putting out 8.8V
Logged

Valencia, Spain
Offline Offline
Faraday Member
**
Karma: 118
Posts: 4554
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I literally just tested the battery with the multimeter and its putting out 8.8V

As much as that? I'm surprised, I would have said more like 8V. The battery must be very new.

But again...try holding the battery in your hand for a minute while you admire all those LEDs.
Logged

No, I don't answer questions sent in private messages...

UK
Offline Offline
Faraday Member
**
Karma: 92
Posts: 3969
Where is your SSCCE?!?!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Those 9V batteries are capable of putting out a couple of amps, but only for a few minutes before they're completely drained.

The typical internal resistance of a PP3 is between 1Ω and 3Ω depending on manufacturer and chemistry.  Let's take 2Ω as a good mean value.

At 2Ω and a nominal 9V voltage, I=V/R, so I = 9 / 2 = 4.5A.  That's the short circuit current of a typical PP3.

Pass that through 117 LEDs in parallel (assuming the "perfect" LED - exactly the same voltage drop as all other LEDs and zero resistance) - 4.5/117 = 38mA.  That's normally above the IF of a typical LED, but won't normally cause the LED to blow immediately.

Now, the world is not perfect.  So what you have is equivalent to a perfect 9V power supply, with a 2Ω current limiting resistor, and a bunch of LEDs, each with its own VF.  What that means in practice is that some LEDs will get a slightly higher current than others.  You probably noticed that the LEDs may not have all been the same brightness.

The ones with the most current will blow first.  That will mean there's a greater proportion of the current available to the rest of the LEDs.  The next ones to blow will do the same - increase the current available to the rest, and so on.  A cascade effect will rapidly occur where just one LED blowing could cause the rest of the LEDs to blow.

Of course, all this relies on the battery holding out, which at 4.5A and the typical 565mAh of an alkeline PP3, would take (T=mAh/mA = 565/4500) = 0.12555...h, or about 7 and a half minutes.
Logged

Why not visit my eBay shop? http://stores.ebay.co.uk/Majenko-Technologies
Replacement for the Arduino IDE: UECIDE - Proper serial terminal, graphing facilities, plugins, overhauled internals.
Java isn't bad in itself, but it has enabled morons to write programs.

Valencia, Spain
Offline Offline
Faraday Member
**
Karma: 118
Posts: 4554
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Those 9V batteries are capable of putting out a couple of amps, but only for a few minutes before they're completely drained.

I accidentally shorted out a 3xAA battery pack yesterday and it melted the battery box.

Logged

No, I don't answer questions sent in private messages...

UK
Offline Offline
Faraday Member
**
Karma: 92
Posts: 3969
Where is your SSCCE?!?!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Nice.

Talking of that...  Back to the PP3.  4.5A through 2Ω -

P=I²R
= 4.5² * 2
= 20.25 * 2
= 40.5W

That's gonna get warm smiley
Logged

Why not visit my eBay shop? http://stores.ebay.co.uk/Majenko-Technologies
Replacement for the Arduino IDE: UECIDE - Proper serial terminal, graphing facilities, plugins, overhauled internals.
Java isn't bad in itself, but it has enabled morons to write programs.

Offline Offline
Full Member
***
Karma: 2
Posts: 138
W9LZ
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

So I learn best by trial and error. I don't know what I learned but I like it.

You learned that rather than researching and understanding what's going on, that you can simply wait for answers from others, nice!
Logged

Pages: 1 [2]   Go Up
Jump to: