Test batteries without a voltmeter, part 2
Harry Gibbens Jr, PowerStream Technology, Orem, UT -- EDN, March 15, 2001
From a high-volume-production point of view of the Design Idea 'Test batteries without a voltmeter' (EDN, Nov 9, 2000, pg 167), it is a time-consuming and laborious task to tweak the large number of potentiometers on each of the comparator input references. An alternative to this onerous adjustment chore is to replace all potentiometers with 1%-tolerance fixed resistors (Figure 1). Before calculating each voltage-reference resistance value, you should use a reasonable selected total resistance value, RTOT, ranging from 100 kΩ to 1 MΩ. You can usually obtain each of the resistance-divider values from off-the-shelf fixed resistors. After calculating all the resistance-divider values (with RTOT=100 k?), if the closest reasonable stock values are unavailable, just increase the value of RTOT. For the example in Figure 1, you calculate the values using a spreadsheet to obtain quick results. This example uses RTOT=182 kOhm. All the calculations use Kirchoff's law:
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Rearranging the terms in the formula, you determine the resistance value, RX:
where VX is each comparator's reference voltage, VREFX; VTOT=VCC; and RTOT is the manually selected value. The first step is to calculate the 'Formulated R' in Table 1, using the rearranged formula. An example follows:
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Next, calculate the first resistance value, R0, by using the formula
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Calculate the rest of the resistor values, R1 through R7, by using the formula
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For example, for R1:
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Finally, use the rearranged formula to find the last resistance value, R8.
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In Table 1, you match the rounded-off calculated resistance value (Calculated R) to the nearest available fixed-resistor value (Fixed R) as shown in the last two columns. Note that R0 consists of two series-connected resistors.
