Tom=E1s:

> I have bi-colour LED's on my board that have about 2.2 volts across them
> when there's 25 mA flowing through them.
>
> I'm thinking of using a single 1.5 volt AA battery to power my board.
>
> I've got two questions:
>
> 1) Can I go straight from the 1.5 V battery into the LED's without a
> current-limiting resistor?

No, and for two reasons:

1) Basically an LED conducts no current until a certain threshold voltage i=
s =

reached, which for your LEDs is about 2.2 volts. Thus, a 1.5 V supply will =

not provide enough voltage to start the LED conducting at all.

2) If your voltage source was great enough to get the LED to conduct (say 2 =

x 1.5V AA =3D 3V) then excessive current would flow, becuase the LED would =
try =

to load the source down until the voltage across the LED was reduced to its =

'magic' 2.2V forward drop. Since the only resistance in the circuit would b=
e =

that in the battery, wiring and driver chip way too much current would go =

through the LED.

> 2) What kind of regulator should I use to provide my uC with 5 volts?

You would need a step-up switching regulator in this case.

HOWEVER:

There is no reason to provide an exact 5V to the PIC. It will work fine on =

quite a bit less, and does not even need to be regulated.

A SHORT TUTORIAL ON LED's (and power sources)::

An LED (or any 'forward-biased' regular diode) has a parameter called Vf, o=
r =

'Voltage, forward biased'. This value varies from one type of LED/diode to =

another, and is determined by the particular type of semiconductor in the =

LED/diode. Basically, no matter what current is flowing through the diode, =

the same (actually nearly the same) voltage will appear across it. This is =

in contrast to a resistor, where the voltage across the device is =

proportional to the current through it. As mentioned above, this has two =

consequences:

1) The LED won't conduct at all until you reach the threshold.
2) The LED will (try to) draw whatever current is needed to get the source =

voltage to drop to its threshold voltage.

Now a voltage source (like a power supply or battery) has its own internal =

resistance which determines its 'stiffness'. For example a tiny button cell =

battery has a rather large internal resistance, so when you try to draw =

significant current from it much of its voltage is 'dropped' across the =

internal resistance (good old Ohm's law) and the voltage seen at the batter=
y =

terminals tends to drop quite quickly as the current drawn increases. In =

fact, I have a cheap little keychain LED flashlight which depends on the =

internal resistance of two button cells to limit the LED current (no =

explicit resistor in the circuit). On the other hand a car battery, for =

example, has a very low internal resistance. You would have to draw a very =

large current from it for much of its voltage to be dropped internally.

So, when you hook up your LEDs, your circuit is effectively the following =

(all in series):

Voltage source (battery or power supply) with its internal resistance, call =

this Vsupply and Rsupply
Resistance of wiring (Rwiring)
LED with its forward voltage and its own internal resistance (pretty low) =

(Vf.led and Rled)
Explicit resistance in circuit (Rdrop)
Resistance and forward voltage drop of driver transistor/chip (Vf.driver an=
d =

Rdriver)

Ohm's law says the current in your circuit will be:

(Vsupply - Vf.lef - Vf.driver) / ( Rsupply + Rwiring + Rled + Rdrop + =

Rdriver )

Notice the only term in the denominator that you have explicit control over =

is 'Rdrop'. Normally you would design your circuit so that Rdrop dominates =

the other resistances in the cicuit in order to result in a predictable =

current.

-- Bob Ammerman
RAm Systems






-- =

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