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.  


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.