Load Dump Circuit

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Contents

Description

Load Dump Overview.png

Purpose

This circuit is designed to divert exccess power when the battries are at 28V from solar panels into heating domestic hot water cylinders or just wasting the power if the water is already hot. This will improve the charging of the batteries by providing a more constant charging voltage on bright days. The circuit will be powered from a 240V DC supply this will be rectified from a small inverter that is used to power the lights in the house using my Soft Start Circuit, the maximum power is likely to be less than 500W and will be fused accordingly.

The hot water will also be heated by the Combined Heat And Power generator, in the winter the generator will be running more to light the house and to make up for the reduced charge provided by the solar panels. This should mean that we have enough hot water all year round.

Operation

This circuit uses an Arduino to measure the battery voltage, when the voltage exceeds a predefined limit(28V for 24V batteries) it increases the duty cycle of one of its PWM outputs. The more that the battries go over 28V the higher the duty cycle of the PWM, if the voltage falls below 28V the duty cycle is reduced. The PWM signal is then used to control a HGTP7N60A4 IGBT, the IGBT is used to switch high voltage DC prower provided by the Soft Start Circuit. The low voltage(5V, 12V, & 24V) control circuits are isolated from the high voltage power circuits using Opto Isolators, the power for controling the IGBT comes from the 24V circuit via an isolated DC-DC converter.

There are three IGBTs each of these powers a differet circuit and each is connected to a different PWM pin on the Arduino, this allows load to be diverted to different circuits depending on where the load is required.

Design

This circuit uses my High Voltage PWM circuit as the basis for its design, it has 3 high voltage high current outputs. The first output will divert current to a 3KW heater in a hot water tank, the second output will be used to heat water for a hot water tap, and the third will simply waste the power to an electric heater.

Voltage divider used to measure 24V with an Arduino
Measuring Battery Voltage

An Arduino is used to measure the battery voltage using a simple voltage divider, this measurment is used to control the Load Dump Circuit as well as the Generator voltage regulator. The ground is common accross all of the low voltage circuits (5V, 12V, & 24V) and the netral of the mains, it is not the same as the negative on the high voltage DC circuit!

The maximum voltage expected on the 24V circuit should be less that 32V, so all equiptment is designed to operate in the range of 20V to 32V. This voltage divider is designed to give a maximum output voltage of 4.85V when the battries are at 32V, this is well within the 5V opperating voltage of the Arduino.

The charging circuit aims to maintain a battery voltage of 28V.

High Voltage DC
This circuit switched a high voltage DC supply, the power comes from the inverter that powers the lights and small ellectrical appliances in the house. The power is rectified and then filtered, it is important that there is not too much noise introduced on to the AC line as this would cause the lights to flicker.

Isolation
The low voltage circuits are kept isolated from the high voltage switching by 3 SFH618A Opto Isolators and the power used to control the IGBTs comes from a IU2415SA DC-DC Converter. This keeps the low voltage circuits safe, and prevents high voltages from forming on any of the Arduino pins.

IGBTs & Drivers



Circuit Diagram

Load Dump Circuit.png

Testing

IGBT Gate Signal

Oscilloscope connected to gate pin on IGBT

Well I dug the oscilloscope out of the garage, for the first time in what must be 5 years and much to my supprise it worked. I wanted to take a reading from the gate pin of the IGBT, to make sure that it has a good strong charging voltage at switch on. The picture to the right shows the oscilloscope probe connected between the gate pin and the gate resistor of the circuit to be tested. Below is a reading is a reading taken from the gate pin of the IGBT, all looks well it is a nice sharp square wave. I was a little worried about my selection of gate resistor, as these are wire wound resistors and their inductance may be a bit high.

Relay driver circuit.
Soft Start Relay

The Soft Start Circuit has a relay in it, this relay is turned after the capacitors have had chance to charge. The relay is powered from the 12V supply, a BC337 NPN Transistor is used to interface between the Arduino and the 12V supply. Testing the relay is simple, turn the digital pin on and listen for the relay to click as it switches. The turn on delay for the relay is set in the program, when I have a finished program I will upload it. It would be a good idea for the Arduino to sense that the capacitors have charge before switching the relay, but for now we will just have to plug stuff in in the correct order.

Mains Voltage
All testing so far has been done using the low voltage supplies that power the circuit, I'm not keen on the idea of connecting the oscilloscope up to a circuit that is floating on mains voltage. So for the further tests the oscilloscope was disconnected. The first test was to make sure that the Soft Start Circuit could charge the capacitors, this was simple connect the circuit up to a mains supply and measure the voltage across one of the capacitors. Well it works but it is a little to soft for my liking, it takes approximaty a minute to reach a high enough voltage to turn the relay on. Turning the relay on after the voltage has reached 200V should be fine, Its just to stop arcing in the plug or the relay while the capacitors charge, and as this circuit is on all the time having to wait a minute is not a big deal.

Measurement taken from the IGBT gate

Parts List

Part Part NÂș Description
3x 1N4004 Diode 1A 400V -
3x 21DQ04 Diode 2A 40V

Progress

Photo Gallery

Load Dump Circuit Photos

9th Dec 2012

Progress 9 Dec 2012.png

The circuit hooked up for testing on the 9th Dec 2012, most of the testing could be done with only the 5V, 12V, and 24V attached the mains was only connected as a final test. The circuit appears to Work fine, however the heater element does have quite a loud wine and I'm a little worried that it may damage the element. Its probably producing allot RF interferance as well. I posted on the Arduino forum asking for any ideas on filting the PWM output from the circuit:
http://arduino.cc/forum/index.php/topic,136762.0.html
So this week end I am going to try a few of the suggesgions, either increasing the frequency and filtering the output or reducing the frequency to 1 or 2Hz.

1st Dec 2012

Load Dump Circuit Dec 2012.png

Current progress as of the 3rd December 2012, the main Load Dump Circuit is build and connected to the Soft Start Circuit. The next job is to connect both of these up to the Arduino, the board on the left is going to hold the Arduino, 5V power supply, and connections for all the other circuits.


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