October 2015 81
two inch diameter form, then loosen
the turns slightly, and spread them
out so the length of the coil is one
inch. With this information, you can
calculate the other resonate circuits.
Ned Stevens
Saint George, UT
[#8152 - August 2015]
Voltage Mod
I am trying to build a small micro
ampere meter project I found on the
Internet (Figure 1). The design calls for
a +9V and -9V supply. Is it possible to
modify this circuit to use a single 9V
battery instead of two?
That’s a very old circuit and it
has at least one glaring flaw: The
value of R3 is way too high. I suspect
it should be 3.3K instead. Also, D3
and D4 seem unnecessary because
there can never be more than about
2 mA of meter current. R1 protects
D1 and D2, but microamp circuit
levels wouldn’t threaten them. The
input bias current of a 741 op-amp
through a 10K resistor produces up
to 5 mV offset that’s not temperature-compensated. It’s nullable, of course,
but the null can drift.
While it’s pretty easy to make a
DC voltage inverter with circuits like
the one in Figure 1, I believe there’s a
better solution. Single-supply op-amps
are available with offset null pins,
completely eliminating the negative
power supply! See Figure 2 for my
(untested) circuit suggestion. TLE2021
chips are available in plastic DIP
packaging from the usual suspects,
such as Digi-Key and Mouser.
The current sampling resistor
should be selected for full-scale
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n FIGURE 1
