Design of a Constant Current Source
This article will explain the way a simple
transistor based current source is designed, this
will give an idea on how some components can be used
in a practical way to make the circuit do some
function, the objective is not design but to become
familiar with the basic ideas.
We need a fixed current around 20mA for a voltage
variation of 10V to 20V to drive a LED flasher
The transistor should handle 20V * 2 = 40V and
a current of 20mA * 5 = 100mA. We have to overrate
the components for long term reliability and make
the design rugged. Chosen MPSA92 PNP-300V-500mA
which is good for this job. Look at the pin details
of MPSA92 in the bottom view given in the
right of this page in its TO-92 package, it
has a beta of 25 . The Power dissipation of MPSA92
can be upto 650mW , our requirement may be a max of
20V*20mA = 400mW which is just within limits.
Current Source and MOSFET
Now we need a voltage reference a low cost voltage
reference is a LED which has a 1.6V forward drop. As
the circuit is a not an accurate one CFR 5%
resistors are fine.
4-20mA Loop-Powered System
The LED at 40mW will last long, some energy emits as
heat and some as light. 40mW / 1.6V = 25mA. so
let us choose 20mA max LED current as a thumb rule.
In this circuit the LED is used as a reference so
to keep it cool a 2.2K is chosen. (20V - 1.6V) /
2.2K = 8.3mA on the high side and when voltage is
10V the current will be 3.8mA min. .
You should know that the LED forward drop can
change with ambient light as it is photo sensitive
and will vary with temperature.
Look at the circuit in the right, the LED has a
forward drop of 1.6V which is applied across the
resistor R4 and the base-emitter diode. That means
1V across R4 as a diode drop is around 0.6V. The
base-emiter now gets forward biased and a small
base current Ib flows . The Ic or collector
current is 1V / 50E = 20mA. The Ib = Ic / beta,
That means 20mA / 25 = 0.8mA which flows thru R4
The Load Resistor R6 represents the LED flasher
circuit that consumes 20mA, even on short circuit
of R6 the current is limited to 20mA.
When more current flows in R6 the voltage at emitter
falls, the voltage at base is 20V - 1.6V =18.4V, and
the voltage at emitter should be 18.4 + 0.6V = 19V
for bias and Ib to flow. When Ic
increases the Ib reduces to that extent as
only to maintain emitter voltage at 19V, this way Ic
is kept constant, if Ic reduces the voltage at
emitter builds up to rise Ib which in turn builds up
Ic. so we made a current regulator.
a mV Milli-Volt Calibrator Source
The circuit can be improved by using a zener in
place of the LED or better still a temperature
compensated reference like LM336.
The circuit on the right will be more stable, but
still the forward drop on base-emitter junction is
temperature sensitive. The base current will also
introduce an error, so you can get a 8 bit
stability, that means around 255 counts on an A-D
converter. If you need a more stable current
source you should design with FET and opamps.
LM336-2.5 pdf details, It has a 2.5V
drop. A LM336-5.0 pdf version is also available
for 5V. these are from National Semiconductor.
Operating Current of LM336 is 400uA
to 10mA, 20V The max. voltage 20V / 3.3K = 6mA. so
within limits. Then you can compute the rest, wire
it up to see if your design works.
"If all parts are working,
connected in proper polarities and there are no
dry solders and loose connections then any
circuit well designed ought to work."
Solderman Talks 1702 AD
doc00009.html - Created : 21:46 01-Aug-04 -
Updated : 06:22 14-Dec-08