Richard Dunipace
EDN
Low-cost DVM (digital-voltmeter) modules are economical and can significantly reduce design time for instrumentation. Yet, these modules also involve a significant number of design challenges. For example, their inputs are not isolated from the power supply, so you must add an isolated power supply. This task can both consume critical design time and add to system costs. Additionally, many uses for the modules require one- to four-cell-battery operation, and the modules require approximately 9 V, translating to operation from 0.7 to 6 V if you use new batteries until they are fully discharged. This wide input range also means that you should regulate the power-supply output.
DVM modules also have low parts count, and you can implement them using off-the-shelf components. Optionally, the modules can operate with input voltages as low as 0.25 V if you replace the silicon transistors with germanium devices. However, germanium transistors are relatively expensive, so use them only in applications requiring low-input-voltage operation.
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| Figure 1. | This isolated flyback supply powers a digital-voltmeter module from 0.7 to 15 V. |
The power-supply design in Figure 1 is a blocking oscillator that operates as a flyback converter with fixed on-time and variable off-time. The variable off-time regulates how often the transformer charges and delivers power to the load. The blocking oscillator consists of NPN transistor Q2, transformer T1, and capacitor C2. The conductance of PNP transistor Q1 controls the off-time of the oscillator in conjunction with C2. The output of the transformer conducts to the energy-storage capacitor, C3, through diode D2 during transformer flyback. The error amplifier and optocoupler, IC1, monitors the voltage across C3. When the voltage at resistive divider R4-R5 exceeds 2.5 V, the optocoupler conducts more and reduces the conduction of transistor Q1, increasing the time required for the next power cycle.
