Wednesday, May 29, 2013
Power Supply Monitor Using LM3914
This circuit is use to a few resistors and some LED, a simple expanded-scale voltmeter is easily constructed. Furthermore, it runs from the same single 5V ± 10% supply it monitors and can provide TTL compatible under voltage and over voltage warning signals. The complete circuit is shown in this figure.
This circuit is useful where quick and easy voltage adjustments must be made, such as in the field or on the production line. The circuit’s low cost makes it feasible to incorporate it into the system, where the overvoltage and under voltage warning signals provide an attractive extra. Of course, these techniques can be used to monitor any higher voltages, positive or negative. Calibration procedure is the LM3914 output thresholds have been shifted up by 100 mV and output #10 is or-tied with output #9. Other outputs may be wire or together if 100 mV resolution is not necessary. If desired, the outputs can be color coded by making LED #1 and LED #10 red, LED #2 and LED #9 amber, and the rest of the LEDs green to ease interpretation.
To calibrate, set VCC at 5.41V and adjust R6 until LED #9 and LED #10 are equally illuminated. (A built-in overlap of about 1 mV ensures all LEDs won’t go out at a threshold point.) There’s no need to vary the system supply voltage to perform this adjustment. Instead, disconnect R1 from VCC and connect it to an accurate reference. Then, at 4.5V, adjust R4 until LED #1 just barely turns on. There is a slight interaction caused by the finite resistance (10k, type) of the LM3914’s voltage divider, so that repeating the above procedure once is advised. The LED driver outputs can directly drive a TTL gate, so that the LED #1 and LED #10 outputs may be used for under voltage and overvoltage warning signals. These may be used to initiate a soft shutdown or summon an operator, for example. The 470Ω resistor R8 ensures that the LM3914 output will saturate to provide the proper TTL low level. Pull-up resistor R9 provides the logic high level.
This circuit is useful where quick and easy voltage adjustments must be made, such as in the field or on the production line. The circuit’s low cost makes it feasible to incorporate it into the system, where the overvoltage and under voltage warning signals provide an attractive extra. Of course, these techniques can be used to monitor any higher voltages, positive or negative. Calibration procedure is the LM3914 output thresholds have been shifted up by 100 mV and output #10 is or-tied with output #9. Other outputs may be wire or together if 100 mV resolution is not necessary. If desired, the outputs can be color coded by making LED #1 and LED #10 red, LED #2 and LED #9 amber, and the rest of the LEDs green to ease interpretation.
To calibrate, set VCC at 5.41V and adjust R6 until LED #9 and LED #10 are equally illuminated. (A built-in overlap of about 1 mV ensures all LEDs won’t go out at a threshold point.) There’s no need to vary the system supply voltage to perform this adjustment. Instead, disconnect R1 from VCC and connect it to an accurate reference. Then, at 4.5V, adjust R4 until LED #1 just barely turns on. There is a slight interaction caused by the finite resistance (10k, type) of the LM3914’s voltage divider, so that repeating the above procedure once is advised. The LED driver outputs can directly drive a TTL gate, so that the LED #1 and LED #10 outputs may be used for under voltage and overvoltage warning signals. These may be used to initiate a soft shutdown or summon an operator, for example. The 470Ω resistor R8 ensures that the LM3914 output will saturate to provide the proper TTL low level. Pull-up resistor R9 provides the logic high level.
