Thursday, July 18, 2019

Solar Inverter Diversion


Lately I have upgraded by 12v battery system by adding an extra 150W solar panel to the existing setup, totaling my 12v system to 320W (STC). I just have a small limited number of circuits/load on this system and was wasting a lot of power through the Xantrex charge controller configured as a diversion load. On a sunny day, the charge controller was diverting power to a power resistor before noon, wasting power as heat for the whole afternoon.
To make better use of this power, I installed a 12v 1000W grid-tie inverter. The idea is to divert the excess power to this inverter once the batteries are full and use only the charge controller as a fail backup if there is no grid power.




Circuit Diagram
The circuit is fairly simple and I tried to keep it that way! I always follow the KISS principle (Keep it Simple Stupid).

The circuit below is used to switch on/connect the inverter to the panels/batteries once a threshold voltage is reached and then disconnect the inverter after a pre-determined amount of time. I did NOT use a low voltage disconnect and opted for a timely disconnect simply because this will cause the inverter to connect/disconnect too frequently.

Basically, a reference voltage is compared to the 12v battery voltage using a comparator. This will bias a transistor, switching on a relay, connecting the inverter to the grid.



Supply for the circuit is taken from a 24v supply rail (another set of batteries & panels in my case) and this is stepped down to 15v using the 7815 linear voltage regulator. This 15v will power the whole circuit. Capacitor C2 is used as a charge reservoir to maintain a steady 24v supply to the 7815 voltage regulator. Diode D7 (Green) signals that there is a supply voltage on the circuit board.
I have used another voltage regulator 7810 to get a 10v supply. This voltage is divided by resistors R1 & R4 which will give me a 5v signal voltage. This voltage is made steady thanks to another reservoir capacitor C3. Capacitor C6 is used as a charge reservoir to maintain a steady 24v supply to the 7810 voltage regulator
The 12 volt detection is carried through Diode D4, split by potentiometer R2 and smoothed by capacitor C4. Any quick variations in the 12v system are simply filtered out through R2 and C4, thus providing a pretty steady signal to the opamp. This will eliminate any 'quick' variations to the supply rail such as switching on heavy loads or panels cloud effect or even the switching on itself of the grid- tie inverter.
The opamp is an LM324, and I'm using just one opamp out of the four available. The opamp is configured as a differential comparator.

The opamp feeds the base of a bipolar transistor Q1 - TIP122 which in turns powers the relay. Diode D2 protects the transistor from any back emf generated by the relay coil.

Diodes D3 and D8 safeguard the circuit from accidental reverse polarity.

The non-inverting input of the opamp is held at about 5v thanks to the R1/R4 voltage divider. The inverting input is connected to the 12v signal detection. As the battery voltage rises, it slowly starts charging capacitor C4, a 10000uF capacitor. Once the inverting input exceeds the non-inverting input, the opamp output switches on. Orange LED D6 is on. Capacitor C5 starts charging through diode D5 and this will provide a base current to transistor Q1, switching on the relay and connecting the grid-inverter to the grid.

Once the inverter is on, the batteries will start draining heavily and thus the signal voltage will lower down, The non-inverting input will become more then the inverting input and therefore the opamp output switches to low. This will not happen suddenly thanks to the R2/C4 configuration, thus providing some hysteresis. Also, once the opamp output switches to low, transistor Q1 will remain on for some time thanks to the  D5/C5/R5 configuration, providing approximately 5mins of ON time.




The above is a photo of the finished circuit.

Wednesday, July 17, 2019

Product Review - 1000W Grid Tie Inverter



I bought this inverter from ebay from a Chinese supplier for just 120€ including shipping to Malta. Quite cheap considering the output of this inverter.

The inverter is rated at 1000W at 230v output. 
From the outside, the inverter is well built. The input terminals are fused, sturdy and thick enough to handle 10mm cables. The fan comes on occasionally just when the inverter body becomes warm, thus giving plenty of time for the inverter to start the cooling process.  

Specifications:

The big question is how will it perform?

Well, I have been using it for 1 month and the maximum current which is has drawn from the solar panels/batteries is 20amp, therefore outputting about 250W into the grid. The inverter just gets slightly warm. 

Once I'll have more input power, I'll be able to fully judge the inverter performance. Right now the inverter is operating in a 'relaxed' mode.

Thursday, May 16, 2019

12v / 24v Battery Charger


This is a simple circuit of a 12v/24v battery charger. I had originally a 12v battery charger however the transformer burnt. Instead of throwing everything away, I salvaged the charger box and built a new charger, capable of charging also a 24v battery.

I used two identical transformers, 240v Primary (although in actual fact they are a number of tappings on the primary to better control the output voltage). These came from old cheap UPS systems. I used a tapping of one of the transformers (T2) to control the HIGH/LOW voltage.

The charger has got an output voltage selector switch (SW2) together with a HIGH/LOW selector switch (SW1). The relay is used to add the 2nd Transformer output  (T1) in series with the 1st Transformer (T2). Both bridge rectifiers are the block type, 25amp current output. Resistors R1 and R2 have been included across the transformers output to dump any stray voltage. I could measure 50v on the digital multi-meter when in actual fact the output would have been 30v!



Internal close up of the charger wiring.

Transformers used. As mentioned these came from 'old' UPSs
 

I added a heat-sink at the back of the charger to better help in dissipating the heat generated by the rectifiers
 

Close up picture showing the rectifiers...

I also changed the charger cables to a pair of sturdy 10mm cables together with larger crocodile clips 

The original charger box... 


Friday, January 18, 2019

Outback 24V 3Kw Inverter VFX3024e


I finally purchased and installed a proper off-grid inverter capable of handling my ever increasing home energy needs.
I settled for the Outback 3KW Inverter (VFX3024e), the European version outputting 220v at 50Hz.




The inverter has been installed and performing flawlessly for a number of months now. I have never exceeded it's rated output, in fact I doubt I ever exceeded 1KW of load.
The cooling fan output varies with the load demand and the current temperature of the inverter internals and therefore operation is quite. One thing I have to point out however, which is the humming coming out from the device. I installed the inverter in the garage and therefore noise is not an issue however the humming can be annoying if the inverter is installed close to the living area.



Friday, August 25, 2017

24v System Upgrade

Part of my home system overhaul, I also upgraded the 24v panel. 10 years ago when I first built the control panel, my system requirements were much less. 

The photo below is displaying my old control panel. I have 4 amp meters together with their respective fuse holders and switches.
The fuse holders were rated for 5 amp and the switches for 10 amp DC, however during the years, I had issues with both since I had to replace multiple switches and fuse holders because they melted



The photo below is displaying the new control panel. I removed the switches because in 10 years I don't think that I ever needed to switch off a PV string. Also, I removed the panel mount fuse holders, and instead I opted for a heavier duty rail mount fuse holders which have been installed inside the control box. I also replaced all analogue amp meters and analogue voltmeters with digital meters. The only amp meter which is still analogue is the wind turbine amp meter.
I now have 5 amp meters or PV string. Three of the string will feed directly the batteries while the other two strings will export to the grid.
In the middle I installed two power meters. These can measure current (A), voltage (V), power (W) and energy (KWH). One of the power meters is connected on the battery side, i.e. measuring what is going in/out of the batteries while the second power meter is connected on the load side, i.e. measuring what is being consumed (including the inverter load)




Inside the control box, Notice the fuses (left top corner) and their respective 50 amp shunts. The power meters come also with their 100 amp shunts (right bottom corner).




With regards to voltmeter I found this cool device which displays both voltage and percentage charge.

The below are the 50 amp amp digital meters. 
(NOTE - These amp meters require an isolated supply from the current which they are measuring).  


I opted for these 32 amp DC Bussmann fuse holders which I soured from RS Components.

The Power meters which I soured from ebay is displayed below. It's quite cheap when one considers all it's functions.


Thursday, August 17, 2017

Off Grid PV System Upgrade

Finally I found the time to upgrade my off-grid system. As of today it consisted of;
for a total of 1080W STC.

I built a custom frame using 40mm galvanized iron pipe to support the panels.
The structure actually consisted of two parts;
  • One 6 meter long across my roof/property.
  • One 2 meter long across my house water tank.
The 6 meter structure has been installed 1.5 meters high (enough for me to walk under), supported on bricks (filled in with concrete to add more weight) and anchored to the building using chains.

The picture below is showing the new installed panels, photo taken from the side


Picture of the installed panels, photo taken from underneath the panels.



Left sided panels, installed just above the solar water heater panels.



The structure has been installed on filled in brick blocks and anchored to the building.





The new panels have been installed between the grid-tie panels and the 10 year old smaller Sharp Panels installed on poles.


The below is the structure installed on the house roof tank. The roof tank is itself a concrete built tank.


Therefore in total I have installed;
  • 6 x 150W from solartronics for the 24v system for a total of 900W STC.
  • 1 x 150W from solartronics for the 12v system.
Summing up,
12v system = 200W
24v system = 1980W



Tuesday, August 15, 2017

12v System Upgrade

The 12v system has been upgraded. The old system had a too small solar panel and the 12v batteries were very old, always struggling to hold a charge.
It was time to replace the batteries and add more solar power.
1) The batteries have been replaced with a new set of Trojan 6v T-105 Deep-Cycle.
2) An extra panel has also been added to the existing 12v 50W panel. I installed a 12v 150W solar panel from solartronics, giving me a total of 200W STC. 




This increase in charging required me to also upgrade the control equipment. The picture below is showing the new 12v Control panel, and on the left the Xantrex Charge Controller.


The new control panel consists of 2 x 100 Amp Voltage-Current-Power-Energy meters as detailed below.


 and a voltage meter, displaying both the voltage and percentage capacity.
The power meters have been installed, one measuring the charging current while the 2nd meter measuring the load current. Both charge and loads are protected by fuses.


Due to the higher charging current, I installed a Xantrex charge controller to protect against overcharging the Trojan Batteries. Details on the charge controller can be found here



Off-Grid inverter cooling upgrade

I'm current utilizing a PC UPS as my off-grid inverter. The model is BEST FORTRESS UPS 600VA. It produces a clean sine wave and in fact it can power everything (as long as it's within it's power range) including LED lights, halogen lights, fluorescent tubes and even small motors.





It has been in operation for almost 9 years and to be honest I'm really surprised by it's reliability! This UPS was never designed to be in operation/inverter mode for 24/7 with a 50%+ load almost continuously, however it has performed this task for all these years without any hiccups. 
One of the most important upgrades which was required on this UPS to offer all these long years of service was the adding of active cooling. The UPS comes with passive cooling which is just NOT enough!
The cooling system is made up of 6 x 40mm 24v fans. 



I know this from experience, since once the cooling fans stopped working and the UPS went from warm to hot.... in fact the transformer was starting to boil. Thankfully, the protection fuse finally blew, saving the UPS. 

4 fans have been installed on the side (using the existing cooling holes) while another 2 have been installed on top (drilled 2 x 40mm holes), just above the transformer and the power transistors.




The Image below is displaying the power transistors heat-sinks and the power transformer. In between, I placed a thermistor which will record the temperature of both the power transistors heat-sinks and the power transformer.


The image below is displaying the inverter opened without the cover. Instead of the batteries, there is now the fan controlling circuit and a large reservoir capacitor (150,000 uF - 50v). 
The Inverter is taking power from the capacitor which in turn is being fed by 10mm cables. 


The image below is displaying the fan controlling circuit. It's basic function is to switch on the cooling fans when the temperature inside the inverter reaches about 40°C. Details of this circuit can be found here


Inverter cooling circuit


Circuit Description:
The circuit is based around the LM324 quad Opamp configured as differential amplifiers.
QA1a generates a very stable voltage, set to 10v, while QA1b will compare this stable voltage against the voltage difference generated by the thermistor. The thermistor is an NTC 47K. NTC stands for Negative Temperature Coefficient i.e. the resistance decreases when the temperature rises.
Opamp QA1b inverting input (-) is preset to 8v while the non-inverting input (+) is fed from the voltage difference between the 47K preset and the 47K thermistor. When the temperature rises, the thermistor resistance will decrease and therefore the the non-inverting input (+) voltage will rise. Once the non-inverting (+) input is higher than the inverting input (-), the output of the opamp will go high, turning on the transistor Q1 which in turn will turn on the relay RLY1. The relay will in turn switch on the cooling fans.
Capacitors C2, C3, C4 & C6 offer some hysteresis so that the fans will not switch on/off continuously.
U1 is a 7812 voltage regulator. Since the available voltage inside the inverter is 24v and the circuit needs 12v, the regulator is used to step down the voltage, smoothed further by C1.

Parts List:
D1 - 5.6v Zener Diode
D2, D3 - 1N4001
D4 - Red LED
U1 - 7812 voltage regulator
OA1a, OA1b - LM324
R1 - 4.3K
R2 - 5.6K
R3 - 1K
R4 - 5.6K
R5 - 47K Preset
R6 - 1K
R7 - 47K Thermistor (NTC)
R8 - 47K Preset
R9 - 1.2K
C1, C5 - 2200uF
C2, C3, C4, C6 - 1000uF
Q1 - TIP122
RLY1 - Relay

Monday, August 14, 2017

Battery Charger

As part of the house electrical update, I also rebuilt the car battery 12v charger. This has been improved to deliver more current, thus charging the car battery faster.


Circuit Description:
The circuit is built around the voltage regulator U1 - 7812. This regulator has been wired to deliver a higher voltage than 12v, enough to charge a car battery. Output from the regulator is limited to just 1 amp, definately not enough to charge a car battery in a reasonable amount of time. The current has been amplified by using transistors Q1, Q2 and Q3 wired in parallel. The output from the regulator is used as a base voltage/current for the power transistors. Each transistor can carry 4 amp (peaking at 15 amp), thus 12 amp can be easily delivered from this power supply. Obviously, these power transistors will need adequate cooling and therefore I have used a large heatsink. Each Transistor is fed through an 0.1 ohm 9W resistor. This is used to balance the input current. Also, the emitter output from each power transistor is about 30 cm long. This is also used as an emitter resistor to help out in current balancing as well.
BR1 is a 35 amp full wave rectifier. I just needed one diode and opted to use this rectifier since I didn't have a single +20 amp diode readily available.

Parts List:
SW1 - 15amp Toggle switch
F1 - 10amp
C1 - 100nF
C2 - 10uF
C3 - 1000uF
C4, C5, C6 - 4700uF 25v
U1 - 7812 voltage regulator
R1, R2, R3 - 9W 0.1 ohm wire-wound resistors
R4 - 4.7K
R5 - 1K Preset
BR1 - 35amp full rectifier
Q1, Q2, Q3 - 2N3055
Q4 - BC559

This photo below is showing the battery charger connections



The installed battery charger. I opted to install the power transistors outside of the connection box to improve heat dissipation.