July 2017 9
couple of different solutions. One
would be to lower this circuit below
the 0V point (where the triangular ground symbol is). That
would work fine, but might be inconvenient. The other
would be to turn the circuit upside down using a PNP
transistor, and raise the supply voltage to compensate for
the 4.4V lost by the base voltage (see Figure 2).
Another thing we might do is reduce the voltage of
the zener diode. This last technique has the drawback
of reducing thermal stability. Bipolar transistors have the
characteristic that Vbe drops considerably with increases in
device temperature. FETs can also be used in a similar way
with fewer components (see Figure 3) using their property
of voltage control at the gate, but FET Vgs thresholds are
also affected by temperature.
Thermal compensation can best be done by using
another transistor (Q2, Figure 4) to put in some negative
feedback. This negative feedback will try to keep the
current constant based on Q2’s Vbe. This requires that
Q2 be thermally isolated from Q1. There is an additional
benefit that the required voltage to
get Q1 to act as a current source is
reduced (it will work down to a little
above 2 x Vbe).
Note that you can easily invert
this to use PNP transistors. It doesn’t,
however, compensate for all thermal
variations — particularly ambient ones.
The best way to compensate for almost
all of the thermal variations — short
of matched devices — is to use large
amounts of negative feedback by
employing an op-amp with a precision
voltage reference (Figure 5). I have
used this technique on precision
measuring equipment I’ve designed.
The op-amp power supplies should
be between the 0V and 18V points.
A Block Heater
QI am a diesel head who drives a 2002 F250. I noticed
that when I plug in the
engine block heater, there
is no indicator light to
tell me that the cord is
actually attached to 120
VAC. Do you have a circuit
for rigging up some kind of
AThis should be a rather straightforward task, I’m thinking. There are a number of LED indicator lights that you can buy that already have the proper current-limiting resistors. This puts
everything in one package and keeps you from having to
insulate everything quite so carefully with those small LED
leads and a resistor. A search on the Internet for “120V
LED” shows me many different lights of different colors
that range around $3 to $5. You can just get a small panel
mount LED indicator. I would not recommend routing the
AC mains far from the heater since that might become a
hazard if there was ever a problem with the insulation.
The LED indicator will likely flicker a bit from it only
lighting on half of the 60 Hz sine wave, but that should be
fine for most people. I personally have a high flicker fusion
rate, so I tend to notice that. Anyway, carefully solder the
LED indicator into the heater and mount it as you please,
QUESTIONS and ANSWERS
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n FIGURE 3. JFET current source.
n FIGURE 4. Thermally stabilized NPN current source.
n FIGURE 5. Op-amp based current source with stabilized voltage source.