electronics question

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janeh

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A big order from Rapid today
Well, I got this "bread board" to muck around with.

All I did was stick 2 AA batteries in two holes to power an led - just to figure out what holes did what-all simple so far.

The 2 AA's were in a little holder thing (in series - so 3Volts) with 2 flying leads.

Anyway, end of test, I just left it all disconnected.

20 mins later, went to clear up and batteries were almost red hot. But no sign of a short circuit, so what gives??!?

Took them out of holder and they cooled down. Now each battery reads 1.21v each - so were they shorted and I didn't realise it do you reckon?

Oh, and can I use the batteries again, or is it safer to chuck them.
 
You have a short somewhere, I suggest you look again. Apart from the batteries now being flatteries no reason not to use them provided they are not bulging or leaking. At the cost of AA batteries however, why take the chance! /forums/images/graemlins/smile.gif
 
what colour LED?

a red one has much less forward voltage drop than other colours....

so... for a white/green etc, probably no need for a resistor.... for a red one, then as Beadle says about 200 ohm should do it.... should draw about 20mA...

Its a weird coincidence as I ordered a load of LEDs, transistors, light sensitive resistors and vero board etc from Rapid yesterday too!
 
In the word of the old saying:-

Its volts what jolts but its mils what kills.

In this case your batteries.

The resistor limits the current drawn - when the LED is connected it has a very low resistance and hence will fry, and possibly die if you don't limit it with a resistor.

Just out of idle curiosity - what were you trying to do?
 
Work out first the current the led needs and it's forward voltage drop. A typical LED will drop 1.5V @ 20mA but it depends on the LED. The big ones will drop up to 5V.

Then using ohms law the resistance you need to take up the difference in voltage (ie the drop across the diode) is:

R = V/I = 1.5/.02 = 75 ohms

The power rating of the resistor is P = IV = 0.02 x 1.5 = 30mw. Ie a puny one will do

Et voila!

Dude
 
2 possibilities for the hot batteries (if not the test strip down as suggested.
1 a short in the board set up
2 you have set the LED to heaven withan internal short circuit.

An LED does not act like a resistor. In a resistor (including incandescant lights and heating elements) the current increases in a linear manner with voltage applied. (not always strictly correct but essentially so)

An LED will pass no current until the voltage exceeds the natural breakdown voltage (around 1.5 volts) when this voltage is exceeded the current will increase at a huge rate. Once the rated current for the LED is exceeded it will be destroyed by excess heat. (after glowing brightly)
LEDs can be run at many times their rated current if it is pulsed with high current but for a short period ie average current is less than rating.

So the simple way to operate an LED is via a current limiting resistor.
So if you have 3 volts in battery the LED will drop 1.4 volts then the resistor needs to drop 1.6 volts at the rated current eg 20ma R=E/I
so R is 1.6 volts divided by 20ma (1000/20) is 80 ohms.

Now you will be quick to see that at this rate we are wasting half the power or nearly as much in the resistor as the LED.
So if you run the lED on 2 volts battery you need a resistor of 20 ohms.
The problem here is you will see that if the 2v battery drops to 1.9volts then you will get only 5ma through the LED so not much light while if the voltage rises to 2.1 volts you will get 40 ma through the LED a 100% overload.
So among other things the higher the supply voltage as related to the LED volt drop the more easy it is to stabilise the current over a range of voltages/ (This can be important if you are running off a boat battery which varies from 11.5 volts to 14 volts.)

Many LED devices now will have a switching electronic regulator which can sense and limit current flow with quite a small volt drop and even run the LED off a single 1.5 volt cell ie equal to or less than the LED voltage.

You can not simply connect a 1.4v LED to a 1.4 volt battery as the current through the LED will be anywhere between not enough and too much depending on the condition and size of the battery.

Hence an LED will not be rated at eg 12v unless it has a built in resistor or electronic current control.

Another description might be like getting a small water flow from a restriction on a garden hoes. More pressure more water flow.
The LED is more like filling a bucket with water. None overflows until the bucket is full once the bucket overflows the only restriction to water flowing out of the bucket is the rate of flow into it. If you are filling it with a fire hose then overflow rate can be huge. If you fill from a restricted flow garden hose then overflow can be restricted. (hope that makes sense)

Incidentally this non linear current flow is exactly the same as that describing diodes in a charging circuit. There is always this voiltage drop which once exceeded there is almost no limit to current flow.

good luck olewill
 
Thanks for all these replies, I've pm'd William to say thanks for his detailed response.

I was trying to make an led strip - I now know that I need to input a constant current circuit and so forth.

Yes it was a short - found it - took some digging - stupid battery case.

Just messing around and learning as I go- great things LEDs

Tra la
 
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