Teardown and operation of an MGE NOVA AVR 600

The MGE Nova AVR 600 is a ten year old model used to power a single computer (360W/600VA). In 2007 MGE was purchased by Schneider and merged with APC UPS series. Since I had both an MGE and an APC UPS before they merged, I have the opportunity to show the differences and similarities on their design and their operation.

To begin with the MGE Nova UPS, the schematic of the power stage is shown in the following figure.

The ac grid is connected to the left and the output plug to the computer is on the right. The battery connection is on the bottom and together with its inverter and rectifier charger. The main component of this UPS, as well as on the majority of the UPSs of this power range is the low frequency (50-60Hz) transformer with multiple windings.

The inductance as well as the ohmic resistance for each winding are:

  • L1: 1mH, 100mΩ
  • L2: 1mH, 100mΩ
  • L3: 700mH, 8.8Ω
  • L4: 25mH, 8.5Ω
  • L5: 6.3mH, 300mΩ.

The main power board is shown in the next image. The ethernet connectors on the left are surge protected via MOVs (TVR 07471 - 470V). On the top, the relays S1-S4 switch the power depending on the grid voltage value. The heat sink on the bottom houses four transistors (for the inverter) and one regulator. The microntroller is housed on the right hand side of the board

Two fuses in series with the 12V battery are connected in parallel (40A each).

The switches S1-S4 are shown in detail, together with the low pass filter Lf, Cf, Rf. Cf consists of two 100nF capacitors connected in parallel.

The inverter transistors are the ST P55NF06 (60V, 50A, 16mΩ). Their connected in parallel in pairs. The LM217 on the right is a linear regulator for the auxiliary circuit.

Whereas on the top all the components are through hole, on the bottom their are many SMD components. The high current traces are soldered in order to have lower resistance.

Finally, the MOVs for the surge protection of the main power are not found on the board but are soldered directly on the plugs (not the neatest solution). The MOV type is the TVR 20471 (470V).

The operations that a UPS must do (and will be shown though oscilloscope graphs below) are: normal operation, operation under undervoltage, operation from battery, charging of battery, surge protection. To begin with, the simplest operation is the surge protection, achieved by Z1, Z2 and Z3. Under normal operation, the input be directly connected to the output. This is achieved by having all switches (S1-S4) on. When there is an undervoltage, the UPS does not supply the voltage via the battery. The battery is only employed when the grid is lost. So, this can be done by using an autotransformer formed by L3 and L4. If S2,S3 switch off, then the voltage is stepped up. T0 start with the oscilloscope waveforms, I first tested the UPS operation when an undervoltage occurs. The UPS as shown in the schematic (and as validated by the measurements as well) has a single auxiliary tap to support operation when in undervoltage. So, by using an external autotransformer I began with a voltage input of 230V and slowly decreased it. At 167V S2 and S3 switched off, activating the auxiliary winding L3. So the voltage was stepped up from 167V to 193V which continuous to seem rather low. This operation is shown in the following figure. Notice that the battery charging current (shown in green) is a higher when the auxiliary tap is activated. The opposite action is observed when the input voltage rises. The tap is deactivated when the input voltage reaches 175V, so there is a hysteresis control.

Now in the previous waveform the battery charging current waveform was shown. To elaborate a bit more, the following figure shows the charging current (negative) in detail. The battery is charged with a pulse switching current when the instantaneous grid voltage (and consequently the corresponding voltage of L5) reaches a specific threshold.

To continue with the undervoltage operation, the UPS switches to battery mode when the grid voltage drops bellow 145V, an extremely low value. This transition is shown in the following figure. Before the transition the battery is being charged (negative current). After the transition, the battery supplies the current so the current is positive.

The output voltage of the UPS is far from a sinusoidal wave. However, the principal loads of UPSs are computer power supplies and so a sinusoidal voltage is not a strict requirement. When the transformer of the UPS is loaded, the waveform starts to seem like a sine wave. The battery current is constant.

During startup in battery mode, there is a peak of the buttery current up to 10A for the first cycle, possible to charge the circuit capacitors. The peak is then decreased to about 5A.

The inverter transistor Drain-Source voltage is zero when it is ON, 12V when no transistor is conducting and ~24V when the other transistor is conducting.

Finally, when the grid is again present, the UPS switches from battery supply to grid supply. The threshold to consider that the grid is present is 150V.

Driving a Hard Disk motor

Hard disks spin via a three-phase brushless motor through a driver operating at 12V. Normally, the driver is operated by the hard drive microcontroller, which ensures the quick acceleration at startup and the constant velocity, typically 5400rpm or 7200rpm. Since the motor operates synchronised to the frequency of the pulses, if this frequency is kept constant, then the drive speed is kept constant as well.

I build a drive using an old hdd using an STM32 microcontroller. The hardware is fairly simple. You need a 12V supply (can be a UPS battery or a PC power supply), a full bridge driver (I used a L298), a pulse generator (STM32F429 discovery) and of course the hard drive.

Now the first thing you need to do is to determine the hard drive connections. The motor has 4 cables (three phases and the neutral). The neutral is not needed. To determine which is which, a multimeter can be used. The three phases need to be connected to the output of the full bridge driver. My bridge had 4 outputs (for a stepper motor), I only needed three.

So, the next thing is to generate the pulses to drive the motor. The motor operates synchronously, so the speed is determined by the frequency of the pulses. Therefore, to accelerate the motor, the frequency must increase gradually. The pulses are simple 50% duty cycle pulses, with 120 degrees phase difference.

In the designed program, the controller has a reference speed of 9000rpm and it gradually accelerates the hard drive to reach the reference speed (the nominal speed of the drive is 5400rpm). There are also touch buttons on the screen which will change the reference speed of the drive. So, two timers were used: One (TIM2) to generate the 120degree phase difference pulses and another (TIM3) to control the acceleration of the drive, depending on its speed. In lower speeds, a rapid acceleration will desynchronise the drive. The same applies to very high speeds. A closed loop can be implemented (by optically measuring the drive speed or by measuring the EMF of the winding that is not used), however the system works sufficiently well even without it.

Building a WiFi power switch – The UNIX client

In previous posts the hardware to build a power switch controlled by WiFi, using the ESP8266 module, as well the microcontroller code were depicted. The switch can be controlled via accessing a web URL:

  • "http://<IP address>/gpio/1" turns the switch ON
  • "http://<IP address>/gpio/0" turns the switch OFF
  • "http://<IP address>/gpio/status" displays the status

The aim of the power switch is to control the exterior lighting, so it needs to switch on and off every day. A UNIX system using the cron scheduler seems the perfect implementation. As the system needs to be autonomous, in case the ESP8266 does not respond, the client needs to make several attempts before giving up. In the meantime, it needs to log every failed attempt (using logger) for future reference. The time between each attempt is doubled each time. The DHCP server of the router is configured to give a permanent IP address to the ESP8266 module. The above requirements are embedded in a bash script. The script is used with two arguments, the first is the ip and the second is the state (0, 1 or status).

max_attempts=10 #maximum attempts to try to connect
timeout=2       #initial timeout in seconds
if [ "$arguments" -ne "2" ] 
then echo "usage: setstate <ip> <state>"
exit 1
while (($attempt &amp;lt; $max_attempts)) do curl -s --max-time 1 $url > /dev/null
    if [[ $exitCode == 0 ]]
    logger -p error "[WARNING] Wireless $ip not responding, retrying in $timeout"
    echo "Failure! Retrying in $timeout.." 1>&2
    sleep $timeout
    attempt=$(( attempt + 1 ))
    timeout=$(( timeout * 2 ))
if [[ $exitCode != 0 ]]
    logger -p error "[ERROR] Wireless $ip not responding"
    echo "Maximum attempts reached trying to connect ($@)" 1>&2
exit $exitCode

The full project for the ESP8266 power switch can be found on my Github page.

Building a WiFi power switch – The Software

Having built, as explained in a previous post a power switch controlled using an ESP8266 wireless module, it is now to develop the program running on the ESP8266. To recap, I am using an ESP8266-01 wireless module which has two GPIO pins (GPIO0 and GPIO2) and two UART pins (Tx and Rx).

  • GPIO2 will be used as an output to control the relay, active low.
  • GPIO0 will be used as an input to manually override the setting and activate the relay, active low.
  • Tx will be be used for debugging, to read the microcontroller events</li>

The ESP8266 is preloaded with some simple AT commands. There are many toolsets to program the module, but the easiest in my opinion is using the Arduino IDE, available on github. It supports all main functions and has a pretty good reference. Each time a new program is loaded on the microcontroller, the Arduino core is loaded as well, so it takes about a minute. To program the module, it needs to enter in bootloader mode, essentially to pull down GPIO0 before power up. To sum up:

Bootloader mode

  • GPIO2: high
  • GPIO0: low
  • RST: high
  • CH_PD: high

Normal mode

  • GPIO2: high
  • GPIO0: high
  • RST: high
  • CH_PD: high

The github page of the Arduino IDE for ESP8266 contains analytical instructions on how to install the toolset.

The developed program is a modification of the "WiFiWebServer" example with the following requirements:

  • Connect automatically on the network
  • Create a webserver that every device (client) can access.
    • By accessing "http://<ESP8266IP>/gpio/0", GPIO2 becomes low and the web server returns "ON"
    • By accessing "http://<ESP8266IP>/gpio/1", GPIO2 becomes high and the web server returns "OFF"
    • By accessing "http://<ESP8266IP>/gpio/status", the web server returns the status ("ON" or "OFF")
  • If GPIO0 is low, the ESP8266 enters manual mode, meaning it sets GPIO2 low, discarding any change from the web
  • If the network connection is lost, the ESP8266 attempts to reconnect
  • If the ESP8266 gets an IP from a DHCP server *.*.0.*, it disconnects and tries to reconnect.

This last behavior programmed is due to the repeater found in the house. If the main router becomes inoperable, then the repeater acts as a DHCP server providing IP address 192.168.0.*. When the router is active again, then the ESP8266 does not change address and it becomes unreachable. So, if this occurs, then the module reconnects only when the main router is present again.

#include "ESP8266WiFi.h"

const char* ssid = "SSID";
const char* password = "PASSWORD";

int val=1;
int flag=0;

// Create an instance of the server
// specify the port to listen on as an argument
WiFiServer server(80);

void setup() {

// Start the server
Serial.println("Server started");

void loop() {
if (WiFi.status() != WL_CONNECTED) {
Serial.println("WiFi lost, trying to reconnect");

if (digitalRead(0) == LOW && flag==0) {
digitalWrite(2, val);

if (digitalRead(0) == HIGH && flag==1) {

// Check if a client has connected
WiFiClient client = server.available();
if (!client) {

// Wait until the client sends some data
Serial.println("new client");

// Read the first line of the request
String req = client.readStringUntil('\r');

// Match the request
if (req.indexOf("/gpio/0") != -1)
val = 1;
else if (req.indexOf("/gpio/1") != -1)
val = 0;
else if (req.indexOf("/gpio/status") != -1)
Serial.println("getting status");
else {
Serial.println("invalid request");

// Set GPIO2 according to the request
digitalWrite(2, val);


// Prepare the response
String s = "HTTP/1.1 200 OK\r\nContent-Type: text/html\r\n\r\n<!DOCTYPE HTML>;\r\n<html>\r\n";
s += (val)?"OFF":"ON";
s += "</html>\n";

// Send the response to the client
Serial.println("Client disonnected");

// The client will actually be disconnected 
// when the function returns and 'client' object is detroyed

void WiFiBegin(){
pinMode(2, OUTPUT);
digitalWrite(2, val);
pinMode(0, INPUT);

// Connect to WiFi network
Serial.print("Connecting to ");

WiFi.begin(ssid, password);

while (WiFi.status() != WL_CONNECTED) {
Serial.println("WiFi connected");

// Print the IP address
IPAddress IPadd = WiFi.localIP(); 
if (IPadd[2] == 0) {
Serial.println("Connected to repeater, disconnecting");

The full project for the ESP8266 power switch can be found on my Github page.



Building a WiFi power switch - The Hardware

I wanted to add some exterior lights using the existing wiring, which would operate during the night. The most common way is to install a programmable timer switch before the lights, which is a hardware or software timer and a relay. However this needs to be installed indoors (unacceptable for my case) or outdoor, on a waterproof enclosure, which however can not be easily programmable.

So, instead, I used some simple components to create a switch using a relay. The switch is connected to the home WiFi, so it can be operated using any other client connected on the same network, or via the internet, if enabled. The circuit is housed in a IP55 junction box, near each light.

The main component of the circuit is the ESP8266 WiFi module, which includes a microcontroller and a TCP/IP stack. The ESP8266 needs a 3.3V supply, which is provided by the ac grid. Finally, a GPIO port operates a high voltage relay which connects the light. The following figure shows the complete circuit schematic.

To provide the 3.3V, a USB power supply is used, together with a 3.3V LDO. The AMS1117 regulator needs to provide at least 9mA to operate correctly, so a resistive load needs to be added, together with bypass capacitors and a protection diode. For the current project, an external regulator board was used, which has all the necessary components. A supplemental 100nF capacitor is connected, since during transmission, the ESP8266 demands up to 250mA.

The ESP8266-01 module is used. The ESP8266 has many variants. Different boards, provide different ports. The ESP8266-01 board has the least amount of ports, two GPIOs and the UART peripheral Tx/Rx. For normal operation, during power-up, the GPIO2 needs to be high. The UART peripheral is not used during operation. So, Tx is not connected and Rx has a pull-up resistor, to not be affected by noise. The R3 resistor pulls GPIO2 to Vcc (although D1 does the same job by the manner it is connected). CH_PD enables the module, and needs to be pulled high. RST resets the module when low, so it is pulled to Vcc via R1. GPIO2 is used as an output to drive the relay, whereas GPIO0 is used an input to override manually the state, as it is (still) quicker than to connect via the mobile phone.

The light is connected via a high voltage relay (10A - 250V). Although for the current prototype an external relay board is used, it can be easily implemented in the same board, so it is analytically depicted in the circuit schematic. An optocoupler is used in the input, which can isolate the ground of the microcontroller with the ground of the relay. However, this is not necessary, as the relay provides isolation as well. LED3 shows that the relay is powered on, whereas LED2 shows if the relay is activated. The diode D4 is necessary, to dissipate the power of the relay coil. If this diode was not present, an overvoltage would occur during switch off, which could damage the nearby components. Connector P2 is a high current, high voltage connector.

The ESP8266 module, together with the regulator board were installed on a single sided veroboard. A connector is used for the ESP8266 module, in order to remove it for programming.

A 5V power supply from an old nokia bluetooth headset is used, which implements a flyback topology. The high voltage circuit (before the flyback transformer) is isolated with hot glue, to ensure that there will be no short circuiting when installed inside the enclosure.

In this photo, the whole circuit is depicted. Power comes from the connector strip shown on the left. The neutral and ground are bridged to the light, whereas the phase passes through the relay. The 5V supply is powered by the phase and the neutral which powers the ESP8266 board through the LDO. The ESP8266 board provides power to the relay board, as well as the signal to switch on the relay.

Finally, the installation is shown. A 8cm x 8cm waterproof (IP55) was used, although with careful wiring, the components could fit in a smaller enclosure. The optional manual override switch was not installed in this light box, as the installation hight makes it inaccessible to the user.