Understanding Hand-held Digital Voltmeters
Hand-held digital voltmeters are predominantly equipped with an LCD screen and are typically powered by a nine-volt battery. Internally, these devices often feature an ICL7106 chip or a compatible variant, responsible for measuring the input and driving the LCD display. This IC is widely used and can be found in various laboratory and DIY equipment, providing a straightforward solution for measuring current/voltage and displaying the results. While this seems convenient, there’s a crucial aspect of this device that requires careful consideration.
The Challenge of Floating Supplies
The ICL7106 chip necessitates floating supplies, meaning that the power supply connections (both positive and negative) must not have a direct link to any of the two measuring input terminals. In battery-powered devices, this poses no issue. However, when integrating the ICL7106 into AC line powered equipment, this requirement demands meticulous planning and consideration to ensure the device functions correctly and safely.
Using Floating Supplies in Equipment
The most straightforward albeit expensive approach is to employ two distinct power supplies within the equipment. While a battery can function as an isolated supply, its usage in an AC-powered device might appear incongruous and inconvenient.
Understanding ‘Floating Supplies’
In the context here, ‘floating supplies’ refer to the possibility of having two separate DC voltage levels. Achieving this level of isolation involves the use of capacitors to maintain the separation between the two DC supplies. A circuit published in the July/August 2003 edition of Elektor (circuit number 75) utilized an NE555 IC for this purpose. However, this design required a supply voltage exceeding 10 V. If the DVM module is integrated into equipment operating on a 5 V supply (as is often the case), the circuit becomes impractical. To address this, the author modified the original design by employing a hex Schmitt trigger inverter type 74HC14N instead of the NE555. One of the inverters generates a square wave of approximately 75 kHz. The remaining five inverters are connected in parallel to enhance the output drive current for the voltage multiplier stage.
DC Isolation and Voltage Multiplier
DC isolation is achieved through capacitors C2 and C3. The voltage multiplier setup involves capacitors and diodes. This configuration generates an output of around 8.5 V with a load current of 1 mA, sufficient to power the DVM chip. Additionally, the 5 V supply for the circuit must be stabilized.
Selecting Input Voltage Divider Resistors
The values of the input voltage divider resistors R2 and R3 are independent of the chip’s power supply and should be chosen based on the desired measurement range. Careful consideration of these components ensures accurate voltage measurements within the specified range.