Ground free DVM Module Supply from 5 V

The majority of hand-held digital volt meters 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 how-ever 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 mains powered equipment.

Ground-free DVM Module Supply from 5 V Circuit Diagram :

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 a mains 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 a 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 pro-vide 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 stabilised.

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.

Source :  http://www.ecircuitslab.com/2012/09/ground-free-dvm-module-supply-from-5-v.html

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DC DC Converter From 1 5V To 34V

An interesting DC/DC converter IC is available from Linear Technology. The LT1615 step-up switching voltage regulator can generate an output voltage of up to +34V from a +1.2 to +15V supply, using only a few external components. The tiny 5-pin SOT23 package makes for very compact construction. This IC can for example be used to generate the high voltage needed for an LCD screen, the tuning voltage for a varicap diode and so on. The internal circuit diagram of the LT1615 is shown in Figure 1. It contains a monostable with a pulse time of 400 ns, which determines the off time of the transistor switch.

If the voltage sampled at the feedback input drops below the reference threshold level of 1.23 V, the transistor switches on and the current in the coil starts to increase. This builds up energy in the magnetic field of the coil. When the current through the coil reaches 350 mA, the monostable is triggered and switches the transistor off for the following 400 ns. Since the energy stored in the coil must go somewhere, current continues to flow through the coil, but it decreases linearly. This current charges the output capacitor via the Schottky diode (SS24, 40V/2A). As long as the voltage at FB remains higher than 1.23V, nothing else happens.

As soon as it drops below this level, however, the whole cycle is repeated. The hysteresis at the FB input is 8mV. The output voltage can be calculated using the formula Vout = 1.23V (R1+R2) / R2 The value of R1 can be selected in the megohm range, since the current into the FB input is only a few tens of nano-amperes. When the supply voltage is switched on, or if the output is short-circuited, the IC enters the power-up mode. As long as the voltage at FB is less than 0.6V, the LT1615 output current is limited to 250mA instead of 350mA, and the monostable time is increased to 1.5µs.

These measures reduce the power dissipation in the coil and diode while the output voltage is rising. In order to minimize the noise voltages produced when the coil is switched, the IC must be properly decoupled by capacitors at the input and output. The series resistance of these capacitors should be as low as possible, so that they can short noise voltages to earth. They should be located as close to the IC as possible, and connected directly to the earth plane. The area of the track at the switch output (SW) should be as small as possible. Connecting a 4.7-µF capacitor across the upper feedback capacitor helps to reduce the level of the output ripple voltage.

The selection of the coil inductance is described in detail in the LT1615 data sheet at www.linear-tech.com. Normally, a 4.7µH filter choke is satisfactory for output voltages less than 7V. For higher voltages, a 10-µH choke should be used. In the data sheet, the Coilcraft DO1608-472 (4.7 µF) and DO1608-100 (10 µF) are recommended. The Schottky diode must naturally have a reverse blocking voltage that is significantly greater than the value of the output voltage. The types MBR0530 and SS24 are recommended. The shutdown input (/SHDN) can be used to disable the step-up regulator by applying a voltage that is less than +0.25V.

If the voltage at this pin is +0.9 V or higher, the LT1615 is active. You must bear in mind that even when the IC is disabled, the input voltage still can reach the output via the coil and the diode, reduced only by the forward voltage drop of the diode. The second circuit diagram for the LT1615 (Figure 2) shows how you can make a symmetric power supply using this switching regulator. Here the switch output of the IC is tapped off and rectified using a symmetrical rectifier. The voltage divider at the positive output of the rectifier determines the output voltage.

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