Voltmeter + Ammeter LCD panel

Introduction

This multimeter was designed to measure output voltage and current in a PSU, where the current sense shunt resistor is connected in series with load at the negative voltage rail. It needs only one supply voltage that can be acquired from main PSU.

An additional function of the multimeter is that it can control (switch on and off) an electric fan used to cool the main heatsink. The power threshold at which the fan switches on can be adjusted using One Touch Button Setup.

Specifications

• single uC ATMEL ATmega8 used to handle all the multimeter functions.
• voltage range 0-30V.
• voltage measure resolution 10mV.
• current measure resolution 10mA (depended on current sense resistor value).
• single, non isolated voltage supply.
• one side PCB.
• compact construction allowed to use the multimeter as panel meter.
• compatibility with standard LCDs based on HD44780 controller.

Schematic

Bellow you can see the multimeter schematic. There are some components in the parts list marked as “Do Not Assemble”. That components was needed in a previous software edition. Current software version doesn’t need them, so you just don’t add them. Maybe in a future version of the multimeter there will be a simpler PCB with simpler electronic diagram too.

PCB

PCB files are available on the download section below. There are two version of PCB – normal and mirrored. I think, that anyone who makes PCBs will know which one should be used to produce right PCB.

After soldering all the parts on PCB:

• make two cross connection on PCB (see “Layout – bottom side”).
• short L1 pads.
• place U2 element (+12V voltage regulator) on heatsink .
• pay attention on right polarity of D1 and D2 elements. Wider side of silk screen layout, where part number is placed, is CATHODE.

Parts

Programming

Because uC is in TQFP package, we can program it after soldering all components on PCB. After that, programming is quite easy to perform. Programming signals are delivered through LCD connector. To make the programming cable, you can use an old PC HDD cable. Picture of my programming cable is shown here:

Remembered, that during programming, the circuit must be supplied with +5V. Depending on your programmer, supply voltage is provided either by programmer, or from separate power supply unit.

Programming cable connection list

After connecting uC to prog, you should check, if uC is “visible” for prog. When everything is fine, you can upload  code to uC. The code is available in the “Downloads” below .It is assumed that uC is new and works with its internal RC clock at 1MHz. If not, set appropriate fuse bits to achieve above mentioned conditions. In addition Brown-out detector should be turned on by enabling BODEN fuse. Recommended Brown-out Reset Threshold Voltage is 4V.

The next thing to do is to cross LCD soldering pads number 1 and 5. That’s necessary to provide ground for LCD RW signal. After all, connect LCD module with the multimeter PCB. It is recommended to use a detachable connector for further expandability e.g. software upgrading.

Installation in PSU and Regulation

Mount multimeter to PSU according to the diagram below:

Connectors and regulation elements

Photos

Warnings

CAUTION
This circuit itself doesn’t work off the mains and there are not 220 VAC present, but PSU does.
Voltages above 50 V are DANGEROUS and could even be LETHAL.
In order to avoid accidents that could be fatal to you or members of your family please observe the following rules:

DO NOT work if you are tired or in a hurry, double check every thing before connecting your circuit to the mains and be ready to disconnect it if something looks wrong.
DO NOT touch any part of the circuit when it is under power.
DO NOT leave mains leads exposed. All mains leads should be well insulated.
DO NOT change the fuses with others of higher rating or replace them with wire or aluminium foil.
DO NOT work with wet hands.
If you are wearing a chain, necklace or anything that may be hanging and touch an exposed part of the circuit BE CAREFUL.
ALWAYS use a proper mains lead with the correct plug and earth your circuit properly.
If the case of your project is made of metal make sure that it is properly earthen.
If it is possible use a mains transformer with a 1:1 ratio to isolate your circuit from the mains.
When you are testing a circuit that works off the mains wear shoes with rubber soles, stand on dry non conductive floor and keep one hand in your pocket or behind your back.
If you take all the above precautions you are reducing the risks you are taking to a minimum and this way you are protecting yourself and those around you.
A carefully built and well insulated device does not constitute any danger for its user.

BEWARE: ELECTRICITY CAN KILL IF YOU ARE NOT CAREFUL

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