DVM issues and isolated power supplies
I just finished a little project that need a DVM (digital volt meter) panel meter. You can buy these cheapies for R60.00. The one I got was a 3 ½ digit, 7-segment LCD with ±0.5% accuracy.
You get two types, one has a common GND connection, the other needs the supply and measurement GNDs to be isolated. I can’t find the common GND variety in South Africa (well RS has them but they are R300.00). If you connect the GNDs together the meter just displays the “-1″ overload indication. What to do?
Bellow is a simple circuit that will isolated the DVMs power supply and will allow you to use it in your project. Here’s a quick run down of how it works.
Click for PDF
The 555 timer outputs a 70kHz square wave, which is rectified by the two BAT85 Schottky diodes. Capacitors C4, C5 provide the isolation between VCC, GND and the DVM supply. C6 is just a smoothing capacitor.
As you can see it’s very easy to build on a bit of vero-board and could be made much smaller with SMDs.
Tags: 555, drivers, DVM, electronics, power supply, project
I have a DVM to measure my car battery voltage ( 12 V DC), however I cannot power the DVM with the same battery, Can you modify the circuit so that I get 9V output isolated to power the DVM with 12 V input from car battery
Further to my comment yesterday for input voltage of 12 V , I also need the output current at about 3 to 5 mA, please suggest cahnges in the circuit.
No real changes to make, it should just work. Most of the DVM’s run fine at 12v. If you specifically need 9V on the output you could always add a Linear Regulator on the input. ie power the whole circuit at 9v. I’d suggest a 78L09 or equivalent.
I need about 3 to 5mA output current, What is the max output current from the circuit?
Can I use 1N4148 in place of BAT85?
Yes it can easly supply 5mA. Yes a 1N4148 will work.
There is no capacitor disgnated as C2, is it not reuired or we need a capacitor as C2 connected to pin 5?
C2 is not needed. You can leave pin 5 floating
can the circuit work with NE555 ( non-CMOS version of 555 IC)?
Yes using a NE555 should be fine.