RE: Re: [OTish] Electronics (for HA)

["Keith Doxey" <lists.diyha@xxxxxxx>]

Well unless you have a typo....

both ends are connected to +ve so it wont light!

What voltage is +ve?

You calculate the resitor value like this
(Volt_drop_across_resistor)/(current_required)

Volt drop is Supply-LED_Forward_Voltage (usually about 1.7-2.0V) so for a
5
volt supply assuming 2V for LED @ 2mA

(5-2)/(0.002)=1500 ohms

for a 12 volt supply

(12-2)/(0.002) = 5000 ohms

You can work out the current through a resistor by using
VoltageDropped/Resistance so for the 220R resistor you have specified
you
will be dropping about 3 or 10 volts depending on a 5 or 12 volt supply

3/210  = 0.0142 = 14.2mA

10/210 = 0.0476 = 47.6mA

Given that UK48 is a low current LED there is a distinct possibility
that
you have converted it into a Dark Emitting Anti-Diode aka DEAD.

Dont be too downhearted, I bought my first LED when I was at college in
1976. It cost me 75p and my wages were only ?21 per week. Within 45
minutes
the first experiments had been conducted. Using the LED and series
resitor
and 6V lantern battery - LED lit, reversed the LED - not Lit, without
the
series resistor, it lit but not as brightly as with the LED. tried the
with/without a few times. Notices that the LED leads got warm when used
without the resistor. Also noticed strange acrid smell followed shortly
by
smoke followed by lack of light. LED now DEAD :-((

Until you get the hang of it, electronics can be an expensive hobby
because
you keep buying bits and destroying them.  :-(

Once you get the hang of it, electronics can be an expensive hobby
because
you keep buying bits and building things :-)


Keith

www.diyha.co.uk
www.kat5.tv


-----Original Message-----
From: andy_powell_is [mailto:ukha@xxxxxxx]
Sent: 28 April 2002 17:00
To: ukha_d@xxxxxxx
Subject: [ukha_d] Re: [OTish] Electronics (for HA)



A fine explaination again!

Now I understand. At the risk of getting on your nerves, I've also
got some LEDs that I can't get to work :(

(Maplin part no UK48 5mm 2ma)

My thinking was :

+ve --- 220 Ohm Resistor---LED --- +ve



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