This circuit can be used as a low cost SRAM and Microcontroller-Microprocessor Battery Backup. All the diodes are 1N4148, The diodes prevent battery discharge back to power source. D8 gives a one way path to charge Battery thru R13 which limits current. D4 ensures a one way path of supply to chip when power is present. D5 is backup supply on power failure.
The chip a real time clock, RAM or Processor can be put to standby or sleep on power failure. If it is not a smart chip then make sure on power failure all outputs of chip are high impedance or floating. do not use any pullups or resistor dividers to Vbat, which is the supply to chip. There should be no leakage path from Vbat, decoupling cap of chip must be plastic.
Microcontroller in Process Control
If you want to use this circuit for short term retention or for CMOS logic chips then you can use a 4700uF Cap in place of battery. This works for many hours but the cap has big footprint on PCB. For long duration use more battery AH Ampere-Hour. Vcc is 5V DC regulated.
The Vbat and Vcc can be monitored with comparator like LM339, this circuit can generate the reset or low battery signals. The power on reset and power down reset can corrupt data on brown outs or black outs or even spikes and EMI. So back up data on flash. For Rapid writing and reading SRAM is better and if write-read cycles are high SRAM is best. But if you need to store values and refer to them like a look-up table flash is better.
Serial Interface a 80C31 to ICL7135
The power fluctuations can hang the chip, so a watchdog chip may be required. The conventional way was the to monitor the keyboard-display scan on a i/o port. If the pulses are coming at the rate you programmed the cpu is alive and kicking and doing its job. If the CPU is taking a nap, then the pulses stop coming and it needs to be reset.
Measurement Of Temperature – When power transistors are used, they may tend to over heat. Likewise resistors may also overheat in the event of faults or short-circuits. The knowledge of their temperatures may be advantageous. In addition, measurement of temperature constitutes a basic necessity in day-to-day life.
Measuring the temperature of a body, depends upon the establishment of thermo-dynamic equilibrium between the body and the device used to sense the temperature. In practice, this condition is rarely attained since it is difficult to establish complete instantaneous equilibrium. Hence great care must be exercised in choosing a method suited to the problem so that satisfactory conditions for temperature measurements are obtained. Temperature sensors possess thermal characteristics dependent largely on their size and shape and the materials from which they are made. These characteristics affect precise measurements. The introduction of a temperature sensor into a body tends to modify the temperature conditions at that point. In most cases the sensor is connected to a recording instrument by means of an intermediate system, along which the signal is carried. The intermediate system and the recorder may be subject to temperature and other changes. Hence compensating devices become a necessity to reduce or eliminate errors.
The measurement of temperature in our instrument depends on the fact that the forward voltage drop of a silicon diode changes by about – 2 millivolts per degree centigrade. Thus, by measuring the change in forward voltage of silicon diode kept in a temperature probe, the voltage drop can be converted into temperature.
Since this involves the measurement of millivolt level accurately a precision voltage source is needed. This can be conveniently obtained from the 3 pin + 5v voltage regulator. This voltage is tapped using a preset VR6 whose output is used for adjusting the ice bath temperature reading to zero degree. This tapped voltage is fed to the diode in the temperature probe and the other end of the diode is returned to a negative supply of -8v. The negative supply uses a (-8v regulated output from IC 7808 voltage regulator) which has the least variation with temperature. Now, the voltage at the probe point is connected to the input of DPM via function selector switch ST.
The temperature probe can be made by a length of shielded audio cable connected to any type of mini plug and fitted onto the front panel socket SSG/T. The free end of the cable is soldered to the diode. The diode is kept just at the tip of the cable. A miniature glass diode like 1N4148 is preferred. The soldering makes a good fixture at the end of the cable. The meter can thus measure temperatures from 0°C to 150°C continuously and upto 200°C momentarily since above that the cable starts melting.
Epoxy Resin and a used Metal Pen Refill can be used to make a sensor to insulate the cable. The diode must be thermally and electrically isulated from metal tube.
(above text may have ocr and concept errors)
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