The majority of hand-held digital voltmeters use an LCD screen and are powered from a nine-volt battery. Inside is most probably an ICL7106 chip (or something compatible). This takes care of measuring the input and driving the LCD. This IC is very popular and can be found in other laboratory and homebrew equipment where it offers a simple solution for both measuring current/voltage and driving the display. So far so good, there is, however, one feature of this device which needs careful consideration. The power supply to the chip (both the positive and negative connection) must not have any direct connection to either of the two measuring input terminals. It requires floating supplies. This is not a problem for battery powered equipment but needs more thought when the ICL7106 is fitted into AC line powered equipment.
The simplest, most expensive solution is to use two independent power supplies in the equipment. A battery could also be used as an isolated supply but in an AC powered device it would seem a bit out of place and inconvenient.
In this case the term ‘floating supplies’ means that it is possible to have two separate DC levels. This level of isolation can be achieved with capacitors to separate the two DC supplies. Back in 2003, we published a circuit in the July/ August edition of Elektor (circuit number 75) which used an NE555 IC. Unfortunately, this design required a supply voltage upwards of 10 V. If the DVM module is fitted to equipment which only uses a 5 V supply (as is often the case) the circuit will not be of much use. The author has solved the problem by modifying the original circuit, using a hex Schmitt trigger inverter type 74HC14N instead of the NE555. One of the inverters generates a square wave of about 75 KHz. The remaining five inverters are wired in parallel to provide more output drive current for the voltage multiplier stage.
DC isolation is provided by capacitors C2 and C3. A classic voltage multiplier configuration is made up of capacitors and diodes. The circuit generates an output of around 8.5 V at a load current of 1 mA. This is sufficient to power the DVM chip. The 5 V supply for the circuit must be stabilized.
The values of the input voltage divider resistors R2 and R3 are independent of the chip’s power supply and must be selected according to the desired measurement range.