At 09:18 AM 10/19/02 -0700, Dave Tweed wrote:

>A light bulb will drop much more voltage than a LED, and in fact may drop
>more than the available Vunreg - Vpic.
>
>Consider a bulb rated at 50 mA at 12V, fed from Olin's 20V supply, with a
>5V PIC controlling the transistor. If the emitter resistor is set for 50 mA,
>then the transistor will be in current limiting mode, with 12V across the
>bulb and 3.7V across the transistor, which will be dissipating 185 mW.
>
>However, if the unregulated voltage is less than 17V, then the transistor
>WILL be saturated, and dissipating very little power.

This is where the current sink falls to pieces.  The transistor may be
dissipating very little power but the poor PIC and its power supply is
gonna be unhappy.  You see, as soon as the collector current drops because
the transistor is saturating, the base current INCREASES.  The best way to
visualize this is to simply disconnect the collector of the
transistor.  The E-B junction looks like a forward biased diode and the
base current is (Vdd-Vbe) / R .  In other words, this type of current sink
maintains constant EMITTER current.  If the collector current is too low,
the remaining current comes from the base.

This can be alleviated by adding a resistor in series with the base.  As
long as the transistor is operating in the linear region, the base current
is negligible and there is only a small voltage drop on that base
resistor.  If the transistor saturates or its load becomes disconnected,
the base resistor limits the current that the PIC has to supply.

One final note: as soon as you add that series base resistor, it becomes
real easy to add one more resistor so that the base is driven from a
voltage divider that is driven by the PIC.  This has a couple of benefits:
less voltage drop across the emitter resistor which can mean a smaller
(lower power) resistor, and more headroom (less chance of saturating the
transistor).  This is a real advantage when dealing with an un-regulated
supply that has LOTS of ripple.

dwayne

--
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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