E84 Lab 1: Design of a Multimeter

The schematic diagram of a multimeter is shown in the figure below:

• Pick up the hardware of the multimeter kit from the stock room, including all the components (resistors, potentiometer, meter head, rotary switch, batteries, etc.).

• Study the diagram above carefully to understand how the meter measures the voltages, currents and resistances at each of the positions of the rotary switch. The rectangular shape to the left of the meter head (a circle with an arrow in it) represents two parallel diodes with opposite polarity for the protection of the meter head. You don't need to worry about this.

• The part marked by 10KBVR is a potentiometer, a resistor of 10 but with an additional terminal which is a piece of metal that can slide along the resistor, so that the resistance between the 3rd terminal and any of the other two is variable between 0 and 10 . See here for more detailed explanation. This potentiometer is needed to calibrate the Ohmmeter. Specifically, before taking the measurement of a resistance, the potentiometer needs to be adjusted so that the needle points to zero on the resistance scale when the two leads of the multimeter touch each other for zero resistance.

• Understand how the rotary switch works. It is represented by the 18 circles on top of the diagram and the two layers of rectangular shapes on the bottom. At each of the 18 positions, the corresponding circle on top is connected to both of the rectangles, while all remaining circles are not connected to anything. If you still have difficulty understanding this, it should be most helpful if you take a look at the physical parts and the printed circuit board (PCB) in the kit.

• Find the resistance for each of the 13 resistors with their values erased.

• Turn in your design on paper with all the resistance values before the first deadline.

• Assemble the multimeter in the lab, verify every single position to see if the meter works as expected.

Hints

Following these steps:

• Study the meter head assembly circuitry and find out the needed voltage across the meter in series with the resistor for the current through the meter to be , needed for a full scale display. Then find the currents through the two parallel branches with the resistances of and . Next find the total current needed by this meter assembly composed of all three parallel branches for a full-scale meter display. This current can be used through out the calculation in the following steps.

• Find the resistances needed to measure the currents and voltages at all scales.

• Find out what values are needed for the upper and lower resistances of the potentiometer during the calibration of the ohmmeter, i.e. when the two leads of the meter are directly connected (short circuit with zero resistance) for a full-scale display.

• Find the resistances needed to measure an unknown resistor value at all four scales. Note that the middle value of the resistance measurement is 20, 200, 20k, or 200k for each of the scales, and the corresponding current through the meter assembly should be half of the current needed for a full scale display.

How to find the polarity of a diode? (not necessarily trivial!)

When the voltage at the anode (labeled by a triangle) of a diode is higher than that of the cathode, the diode has a very low resistance (close to a short circuit, called forward biased). If the polarity is reversed, the resistance of the diode becomes very high (close to an open circuit, called reverse biased). The polarity of the diode can therefore be found by checking its resistance using a multimeter.

However, we need to know which lead of the multimeter (when used as an ohmmeter) is positive and which is negative. In most analog multimeters, such as the one you are building, the positive lead (marked by +) is connected to the negative end of the internal battery, while the other lead (marked by -, or COM for common) is connected to positive end of the battery. So if the measured resistance of the diode is low, the end connected to the COM lead of the multimeter is the anode (triangle).

However, the polarities of digital multimeters may be the opposite, i.e., the lead marked by + may be connected to the positive end of the internal battery. In this case, the polarity of the diode can be determined in the opposite way compared to the method above using an analog multimeter.

Moreover, some digital multimeters have a particular position for diode measurement.