Shunting a
microammeter
Is it possible to make a 50µA
- 50µA centre zero meter to read
milliamps? If so, could you inform
me of the procedure? (D. B., Port
Macquarie, NSW).
The answer is yes and yes. The
procedure for converting a meter
that reads in microamps to one
that reads milliamps or even amps
involves adding a shunt resistor to
the meter movement. In essence, a
shunt resistor is connected in parallel with the meter’s coil to “shunt”
away most of the current from the
delicate coil itself.
To work out what value shunt
you need, you need to know the
basic sensitivity of the meter. Most
50µA meter movements for example, have a resistance of 2kΩ and by
using Ohm’s Law we can work out
that they will have 100mV across
them when 50µA is passing through
the coil. By extension, we say the
meter move
ment’s sensitivity is
20kΩ/volt. 20kΩ/volt is the same
as 2kΩ/100mV.
A further piece of information is
that the 100mV across the meter at
full scale deflection is the “burden
voltage”. You’ll need to know that
when calculating the shunt resistor
for your particular application.
Even if you don’t know the sen-
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water. It was laid out to dry on a flat
surface (a trampoline) but not in the
sun. Shrinkage is to be avoided at all
costs. Care must be taken to make sure
that the plastic shrouded connections
and the heat controllers do not get wet
or that water does not run down the
leads into the controllers.
When the blanket was fully dry,
and it takes quite a while because
of their two-layer construction, the
connections had partly pulled out of
the blanket which was now rucked up
in several places. Judicious pulling of
the blanket this way and that pulled
the connections back into place so
that the anchor plates could be re-attached. The blanket was then left a
further couple of days to dry, just to
make sure that there was no moisture
in the controllers.
92 Silicon Chip
sitivity of a meter movement, you
can easily measure it by connecting
in series with a high value resistor
to a (say) 12V supply. If the resistor
value is 100kΩ, the current passing
through the meter will be close to
12µA. What does the meter read?
12? Good – now reduce the value
of the resistor until you get a full
scale deflection of the pointer.
You can work out the exact
value of the current by measur
ing the voltage across the series
resistor with your multimeter and
then using Ohm’s Law to make the
calculation. You can also use your
multimeter to measure the burden
voltage.
So say you have worked out
that your centre zero meter has a
100mV across it when it is passing
50µA. You can use that information to work out the shunt resistor.
If you want it to read 5mA at full
scale deflection (FSD), you need a
resistor which will pass 5mA (or
to be really precise, 5mA - 50µA
= 4.95mA) with 100mV across it.
Using the equation R = V/1, the
result is 20Ω.
If you wanted 50mA instead of
5mA, the shunt resistor would be
2Ω.
That broadly explains the principle of shunting a meter. If readers
want a more detailed article on this
subject, please write and tell us.
Higher capacity
speed control
I was interested in the train controller featured in the April 1997 issue of
SILICON CHIP. How about a version to
run the high efficiency well-built 24V
DC motors obtainable for almost nix
from photocopiers, etc? They are many
possible uses: coil winders, power
feeds for small milling machines,
lathes, robotics.
Keep up the supply of articles on
interfacing PCs with various hardware.
I have every issue since you started
and I still think you do a great job. (I.
S., Camberwell, Vic).
It is relatively easy to modify the
circuit to make it suitable for 24V
motors but you would have to mount
the transistors on much more substan-
•
tial heatsinks to ensure adequate heat
dissipation.
A better approach would be to build
a modified version of our earlier Rail
power controller, as published in the
April & May 1988 issues of SILICON
CHIP. This switchmode design could
be modified simply by changing the
12V transformer to one with an 18V
secondary and changing the filter
capacitors to 35VW rating.
Alternatively, have a look at the
24V 20A speed controller featured in
this issue.
Notes & Errata
Bridged Amplifier Loudspeaker
Protector, April 1997: a reader has
pointed out that this version of the
loudspeaker protector cannot be used
in some bridged amplifiers in cars.
This applies mainly to lower-powered
bridged amplifiers which do not use
a DC-DC inverter and which have the
loudspeaker outputs floating at half
the DC supply, around +7V.
It also applies to some inverter-driven bridge amplifiers which
have a single DC rail. In these cases,
the amplifier outputs may be floating
at around +25V DC above chassis, for
example.
Therefore, before you consider
building the Loudspeaker Protector
for installation with bridged amplifiers in cars, you should measure
the DC voltage at both sides of the
speaker outputs with respect to chassis. If the outputs are floating at a DC
voltage above chassis (eg, +7V), the
Loudspeaker Protector will not be
suitable as it would be permanently
latched off.
Note also that the parts list specifies a value of 100µF for C1 whereas
it should be 220µF, as on the circuit
diagrams. The additional 100µF capacitor for the built-in version should
be rated at 75VW or 100VW not 63VW,
where the amplifier supply rail is
between 66V and 75V.
Extra Fast Nicad Charger, October
1995: the lengths of the 0.8mm wires
specified for the primary and secondary windings of transformer T1 are
incorrect, although the number of
turns and the turns ratio are correct.
The length of the quadrifilar primary wires should be 1.7 metres
before termination, while the two
secondary wires (bifilar) should be
SC
3.5 metres.
