[no subject]

In general for LEDs calculating the resistor goes like this

V = I * R     [This is Ohms Law]

Where
V is the voltage across the resistor
I is the current through the resistor
R is the Resistance of the resistor


So, to find the voltage through the resistor we need to know the voltage
across the LED.  This comes from the LED manufacturers data and is usually
called Vf or Fowward voltage drop

V(resistor) = V(supply) - V(led forward voltage drop)


So our equation becomes

V(supply) - V(led)  =  R  *  I

Where I is the current required throught the LED.  The LED brightness is
determined by the current flowing through it, up to a maximum again given
by
the manufacturer, usually If


So to determine the resistor required calculate

Resistance in Ohms =  (5 - Vf) / If


-----+-----  Vsupply


[R]

[LED]


-----+-----  Gnd




The extra Diode, D3 should help prevent the batteries being used when
plugged into the +5 charging supply.  This works because the batteries only
supply around 4.8v (minus the D2 diode forward voltage drop) and the supply
is +5v (minus the D3 diode drop)

Nicads can supply an awful lot of current if theres is a short circuit so a
fuse is a good idea!


As regards constant charging of the batteries,  again you need to look at
the manufacturers data sheets to find out the charging current.  I'd make
sure that the charging current is a little lower than the recomended slow
charge current.
Simple nicad battery chargers (14 hr types are generally just use a current
limiting resistor)  If you need to charge more quickly then you must limit
the charging time in some way, and it's better to go for an integerated
battery charging IC.

Phew!

Hope this helps

Dave

-----Original Message-----
From: Mark A Wright [mailto:mawright@xxxxxxx]
Sent: 14 July 2004 13:09
To: ukha_d@xxxxxxx
Subject: [ukha_d] OT: Electronics advice needed to make a combined
emergency charger / USB supply lead for my PDA


I'm trying to make a combined emergency charger / USB charger for my PDA.



I've got all the hardware apart from the electronic components. What I
really need is some help in designing the circuit.



With my limited knowledge of electronics I've come up with this...



-[R1]-[LED]-[D1]-+------------[D2]--+

[R2]