Introduction To LED Panel Meter with LED Drivers
The Intersil ICL7107 is a low power, 31/2 digit A/D converters that is used to drive LED displays. Other features included in the IC are seven segment decoders, display LED drivers, a reference, and a clock. It is able to directly drive an instrument size light emitting diode (LED) display.
It features autozero to less than 10µV, zero drift of less than 1µV/Celcius, input bias current of 10pA (Max), and rollover error of less than one count. True differential inputs and reference are useful in all systems, but give the designer an uncommon advantage when measuring load cells, strain gauges and other bridge type transducers.
Voltage is the most frequently measured electrical quantity. In this General Purpose 3 1/2 Digit LED Panel Meter, voltage is being measured to find out the temperature, current, wind speed, resistance or lux etc. After calibrating the meter for its particular purpose the voltage measured will give an accurate digital reading of the analog quantity being measured.
Circuit Description of the 7107 LED_Drivers
Figure below shows the schematic design of the 7107 LED_Drivers.
5V Generation
The 7660 IC is used to do voltage conversion from positive to negative for an input range of 1.5 to 10V, resulting in complementary output voltages of -1.5V to -10V. The -5V DC supply is needed to power up the 7107 IC on top of the +5V DC.
7107 IC with LED Drivers
The heart of the meter is the analog to digital converter built into the 7107 LED Drivers . It uses a dual slope conversion technique. It uses the charging and discharging of an integrating capacitor and having a counter count when the capacitor voltage is above a set value. There are three phases to the process:
Phase 1. Auto Zero.
The autozero capacitor is charged to the integrators offset voltage. This voltage is subtracted from the input signal during phase 2. The integrator thus appears to have zero offset voltage.
Phase 2. Signal Integrate.
The signal input is averaged for 1000 clock pulses.
Phase 3. Reference Integrate.
Input low is internally connected to Common (which may be an offset voltage.) VREF is averaged back to either zero volts or the offset voltage over another 1000 clock pulses. The number of clock pulses counted to return to this value is a digital measure of VIN.
System Timing of 7107 LED Drivers
This is determined by the components connected to pins 38, 39 & 40. The internal oscillator runs at 48kHz.
Decimal Point of 7107 LED Drivers
A jumper selects the decimal point position in the displays that is driven by the LED Drivers.
Analog Section
C1 is the reference capacitor and is unchanged for all ranges measured. IN LO is tied to the analog COMMON pin 32 by the Normal position of the switch except when an Offset voltage is input. The integration capacitor C5 is suitable for all ranges measured but the value of the integration resistor R1 should be increased to 470K for a VREF of 1V.
Auto-Zero Capacitor
This is C4 connected to pin 29. It has some influence on the noise of the system and recovery from overload input. On the 2V scale a 0.047uF capacitor may give better results.
Voltage Measurement
Since the maximum value which can be displayed is 1999, voltmeters with full scale readings of 199.9mV, 1.999V, 19.99V etc. can be made.The user must decide their own need. Then a reference voltage and maybe an input attenuator must be selected. To use the meter to measure 0 - 199.9mV the trimpot is adjusted so that the reference voltage between pins 35 & 36 is 100mV. And to set the meter for 0 - 1.999V, VREF must be set to 1.0V. Measuring higher voltages and nonstandard voltages will be discussed below. The relationship between full scale input voltage and the reference voltage is:
VIN = 2 x VREF
VREF must be in the range 100mV to 1.0V. For a VREF of 1V two components should be changed (R1=470K, C4=0.047uF) to maintain sensitivity and recovery from over-voltage. The 10K trim pot and resistor R3 will allow adjustment for either value, and for intermediate values when required (discussed below.) The following discussion will be using a VREF of 100mV.
Calibration is done by attaching a multimeter to REF HI and REF LO and adjusting the trimpot to read 100 mV. will calibrate the meter to read 0 - 199.9mV.
Voltage Divider
In order to measure voltage greater than 0.2V, an input voltage divider is required. This is the purpose of 4 resistors on the main circuit board. The general relation for full scale sensitivity is now:
VIN (full scale) = 2VREF x RY / ( RX + RY)
For example, a 0 - 20V range (when VREF is 0.1V) can be obtained using a 100:1 voltage divider. This can be done by making RX = 1M and RY = 10K. The decimal point jumper is placed at position '2' so a full scale display of 19.99V is indicated. Similarly, a 0 - 200V range can be obtained with RX = 1M and RY = 1K.
If R = 0.1 ohms then 200mV will be developed when the current through it is 2A. This voltage is applied to themeter which is set up for the 200mV range. (That is, VREF is set to 100mV.) Power disspiation at the maximumreading is IXIXR which is 0.4W, well within the 5W rating of the resistor. To measure a full scale of 200mA then R should be 1.0 ohms in order to generate 200mV input to the meter. Fora 20 mA meter then R = 10 ohms. Note that because of wide tolerances in the shunt resistors it may be necessaryto adjust the reference voltage in order to get the correct reading. So further adjustment of VREF using a knowncurrent may be required.
A very good and detailed writeup and application on 7107 from Intersil can be found in the following link.
