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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- • 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.