8 July 2017
In this column, Kristen answers questions about all aspects of electronics, including computer
hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to
the hobbyist. Feel free to participate with your questions, comments, or suggestions. Send all
questions and comments to: Q&A@nutsvolts.com.
n WITH KRISTEN A. McINTYRE
A Simple Current Source
QI am doing some experiments and need a constant current source as a reference. Most of the designs I have seen appear overly complicated to me, and the off-the-shelf units
are too expensive. Is there a simple constant current source
circuit design you can recommend? I need something in
the range of 0-12V and 1-100 mA.
Los Angeles, CA
ABuilding a functional current source is actually quite simple. The major considerations are how much voltage and power needs to be supplied, and how stable it needs to be — particularly
thermally. Let’s take a crack at this first without much
consideration for stability. We’ll assume that we are going
to control the current with a simple resistor that could
also be a potentiometer. Regulating the current under
computer control with an Arduino or some other single
board computer is quite doable, but adds an extra level of
We start with the simple current source
that you’ve probably seen me use before in this
column. It’s based on a bipolar transistor (see
Figure 1). To understand how it basically works,
we will make some simplifying assumptions.
These assumptions are not entirely true, but
allow us to look at the effects which are
dominant as the circuit tries to supply current.
For this discussion, we will assume that: a)
the base current is negligible; b) Vbe is always 0.6V; c) the
transistor can be modeled from collector to emitter as a
current source itself; and d) that β — the current gain — is
With these assumptions in place, you’ll notice that
the base is held at a particular voltage with respect to the
power supply common point using a zener diode, D1.
In this case, it’s a 5V diode. With assumption b) above,
we can see that the emitter will be at 4.4V. Next, we use
assumption a) to say that the emitter current is about the
same as the collector current. That means we can calculate
the emitter current by simply doing a V = IR calculation,
using 4.4V for V; R is the emitter resistor, R1 in the
With the values shown, the emitter current is 100 mA.
Using assumptions c) and d) tells us that the collector will
work to make its current be 100 mA also, if that’s possible.
With that, we have a roughly 100 mA current source.
What are the limitations, though? First, you may notice
that the lowest voltage at which the collector can be and
still function as a current source is somewhere a bit above
4.4V. Below that value, junctions in the transistor might
become forward biased
that we don’t usually use
that way. Current might
begin to flow in ways that
break our assumptions,
and thus the current won’t
be what we expect.
transistor will go into
saturation, keeping it from
responding quickly to
changes that might make
it behave as a current
source again. How can
we fix this to match the
There are a n FIGURE 1. Simple NPN current source. n FIGURE 2. “Upside down” PNP current source.
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