Simple constant-current source

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< Power Electronic ~ Simple constant-current source

eric

Posted: Thu Oct 05, 2006 11:25 pm Post subject: Simple constant-current source

Site Admin Joined: 01 Jan 1970 Posts: 311

Design: T. Giesberts
The simplest version of a constant-
current source, which we
often use for that reason, consists
of only a FET with the
source and gate connected
together. This utilizes the zerogate-
voltage drain current
(IDSS) of the FET. Sometimes a
source resistor is added to allow
the current level to be set more
exactly.
A disadvantage of such a very
simple current source is that the
maximum drain-source voltage
of most standard FETs is no
more than 30 to 40 V. If we look
at bipolar transistors instead,
the range of available voltages
is significantly larger. This is adequate justification for developing
an alternative current source, using bipolar transistors, that is comparably
simple.
In the example shown here, we use a BC547 and a small Darlington
transistor (BC517). For convenience, we have chosen a current
of 1 mA for calculating component values. Transistor T2 controls
the current, and resistor R1 determines the base-emitter voltage
of T2. R1 also provides the base current for T1, but the value

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