31by2 digit Voltmeter with LED Display
The objective here is to build a 3l/2-digit digital voltmeter with a standard instrument sized LED display. It may be mentioned here that the IC type number ICL7107 is almost universally used for digital voltmeter application and the cirduit shown in Fig. 18.1 is basically what you would discover inside any digital panel meter with a 3l/2-digit LED display. The said voltmeter has beep designed to operate from AC mains to generate regulated +5VDC and -5DVC for the circuit.
Digital panel meters with an LED display usually operate from AC mains due to significant current drive requirement of LED displays. There is a separate power supply section (Fig. 18.2) that generates regulated +5VDC and -VDC It is also worthwhile mentioning here that this circuit can also be used for the digital display of any electrical or non-electrical quantity that can be converted into a proportional DC voltage. Digital pH-meters, Pressure meters, Digital Thermometers etc. are all built around the basic circuit shown in Fig. 18.1. The digital voltmeter circuit shown in Fig. 18.1 along with the power supply section of Fig. 18.2 can be used to measure DC voltages over a range of 0 to 1000 VDC It also had a provision of measuring the AC mains voltage RMS value.
IC lCL7107 from INTERSIL is the heart of the system. Without going into the internal circuit details of this IC, it would suffice to mention here that the said IC is basically a dual slope integrating A/D converter with its own on-chip oscillator (that serves as the clock), reference, decoder and driver and it is capable of driving directly an instrument ‘sized LED display of the common- anode type. The display reading depends upon the analog input voltage and the reference voltage.
The reference voltage is so adjusted that the display reads the analog input directly. In the circuit shown in Fig. 18.1, switch S2 can be used to select DC or AC measurement. When the switch is on AC position, what is actually fed to the analog input terminals of the 1C7107 is the rectified (full wave) AC input after the voltage divider arrangement constituted by resistor (R5) and potentiometer (P2). We shall discuss the adjustment of (P2) under the heading of CALIBRATION. Similarly, when the AC/DC select switch (S2) is on DC, what gels applied to the analog input terminals of the 1C7107 is the DC input divided by the voltage divider arrangement constituted by (R6) and (P3). Adjustment of (P3) will be discussed when we discuss the calibration procedure. The power supply section (Fig. 18.2) is a conventional AC/DC power supply using a step down center tapped mains transformer (T-1), full wave rectifier circuits (D1 and D2 for positive output, D3 and D4 for negative output) and capacitor filters (C1 for positive output and C2 for negative output). Regulation is achieved using three terminal regulators (VR-1) for posltive output and VR-2 for negative output). C3′ to C6 are decoupling capacitors.
The calibration procedure is as follows:
1. Select the AC/DC switch (S2) to be on DC position, feed 100 VDC from a high voltage power supply at DC IN terminals. Adjust potentiometer (P3) to get 2VDC at point-! of the SPDT switch (S2). If 1000VDC is not available, a lower DC voltage can also be used and in that case, the divided voltage should also be proportionately reduced. For instance, for 100 VDC input, the divided voltage will be 200mV. In other words, (P3) should be so adjusted that (R6) and (P3) give a voltage division by a factor of 500.
2. Now, as a second step, adjust potentiometer (P1) so that the meter directly reads the DC input voltage.
3. Change the AC/DC select switch to AC position. Feed AC mains voltage at the AC IN terminals. Adjust potentiometer (P2) so that display directly reads RMS value of the AC input. Before feeding the AC mains, the RMS value of the AC mains should be checked independently with another voltmeter or multimeter with AC voltage measuring capability. – 4. The voltmeter is now calibrated and ready for else.
5. The calibration of this voltmeter should be checked at regular intervals as it may get disturbed due to inherent change in the resistance values of voltage divider resistors.
You may be required to readjust different potentiometers to restore calibration.
Figs 18.3 and 18.4 respectively show the PCB layout and the Components layout.
Resistors and Capacitors
R2 : IM, 1/4W
R3 :22K 1/4W
R4 :100K, 1/4W
R5 :1M, 15000V (You can also use three 330K 1/2W resistors in series)
R6 :2.2M 1W 1500V resistor
P1 :1 K Preset
P2, P3 :10K Preset
C1, C2 :100µF 16V (Electrolytic)
C3 to C4 :0.1 µF(Ceramic disc)
C7 :0.2 µF (polyester)
C8 :0.47 µF (Polyester)
C9 :0.01 µF(Ceramic disc)
C10 :0.1 µF (Ceramic disc)
C11 :100 µF (Polyester)
Semiconductor Devices and ICS
D1 to D8 : 1N4007 or equivalent
VR-1 :7805 (Three terminal regulator)
VR-2 :7905 (Three terminal regulator)
IC-1 : ICL 7107
(DL-I to DL-4) : Common anode display type no. LTS-542
S1 : Mains ON/OFF switch
S2 : SPDT switch, 230V, 1A F1 : Fuse, IA rating with fuse holder
T-1 : Mains transformer (Primary: 230VAC Secondary: 7.5-0-7.5,
Solder metal, multistrand wires, suitable mounting cabinet.
Pin Connection Diagram of Display
Fig. 18.5 shows the pin connection diagram of seven segment display type no. LTS-542.