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This is a project I have been thinking about since quite a few years, and finally had working last year on my home grid with a 600W HF inverter.
My electricity needs: - most used: lighting and PC , little tools , fridge (most energy hungry appliance is already DC, with cold energy storage -ice- which last 4 days, and is controlled so as to limit battery wear to 5% discharge depth). - sometimes washing machine, or bigger tools. - no heating devices (done with butane, solar hot water to come)
In my anterior flat, only 36m2, my frugal PV install had only a 180W panel and a 12V 100Ah battery. Living in a very sunny and tropical location, this was actually sufficient for one person. All most used appliances were choosen 12V DC. I bought an inverter just to hook on demand some greater AC charges like the washing machine (no water heating): on sunny days, battery charge was'nt even affected during the washing task.
Now I am living in a bigger house, whith 2 panels summing 340W. The house grid now beeing too large to pass 12V DC without excessive resistive losses, I decided to use it with AC, which is also much more universal. But just having the inverter idling most of the time is a big energy waste, especially with little installations and bigger inverters.
So the idea was to look for a load detection system with the least posible current draw, who would start the inverter every time an appliance needs AC, and of course to stop it when AC is no more needed.
Some high ranges inverters have already such a feature, which can be named eco, standby, search or power save mode. All I've seen (Victron, Steca/Studer, Outback, Atersa) are LF inverters, but some have the minimum detected current too high to detect little leds lamps. They are always based on starting periodically the inverter for a few cycles and looking if some current builds up. This is the best way to detect a real AC charge, but not the least energy consuming. My cheap Solartronics pure sine 600W HF inverter had no standby mode and I searched for how to add a load detection circuit. They are some quite simple ideas on the web, based on putting battery DC on the local grid, detecting current if something gets started, and then starting the inverter. - This will NOT work if some LF magnetic charge is constantly connected, as the DC impedance will be low. - It will NOT detect any already connected (and charged) capacitor - BUT it will detect when a resistive, magnetic or capacitive charge is connected. This means most of the actual appliances (motors, adapters, lamp drivers) will be detected on connecting, so this could be usable. On the other hand, power draw during standby mode can be reduced to milliwatts.
I started trying with opamps, but my wanted specs were'nt easy to achieve in simple ways, and I shifted to using a microcontroller. Features I wanted to have: - detecting down to 3W Led lamp - circuit powered from battery - galvanically isolated from AC - standby consumption less than 0.5W - adaptable to most (HF) inverters - working down to 10.5V DC
I ended whith a circuit using: - 1:1000 current transformer - 2 low power relays : one is used as inverter on/off switch, the other connects battery to AC out - LM324 , very common low supply current quadruple opamp, supply voltage up to 32V 3 opamps to amplify and rectify current transformer output 1 opamp generating the microcontroller 5V supply - 8pin ATtiny13 microcontroller drives the 2 relay transistors drives a led giving some info detects AC presence without contact reads AC line current status from LM324 programmed to realize the correct relay and led driving sequence, as well as various checks - other components are 2 transistors for the relays, various diodes (signal, leds, zener), resistors and capacitors
In standby mode, supply current was as low as 7mA (using only 0.1W with 12V battery)
This is the stripboard which was successfully tested and then mounted inside the 600W inverter (current transformer not shown). It has been working correctly a few months, without showing major drawbacks. All my usual needs were fullfilled: - lamps (all 230Vac leds) - adapters to charge a netbook, a phone, etc.. - PC - old transformer based battery charger - motors (drill, fan, old -low power- vacuum cleaner, washing machine)
In all cases, the inverter starts immediately, although its 230Vac output only appears after 1 or 2 secs. When AC current stops, the inverter stays on 8 more seconds before shutting down. A yellow Led shows some info: waiting to start, starting, current detected, errors,...
Adding another security feature: do not start if there is already AC on the output (in case you forget to disconnect the inverter from your home grid before connecting to the power grid) seemed to be working as long as the firmware was already booted. When I tested it booting, the inverter started switching against the grid, and ... smoke. Not been able to repair it yet.
As I am now upgrading to 24V (still waiting for the second 12V battery), a 300W EG8010 based pure sine HF inverter will be used, which is enough to cover most needs. The next load detection circuit will also been simplified, getting rid of the LM324 (no more need to amplify the current with 24V DC), changing AC detection from capacitive to opto, and implementing some hardware security features. Edited 2024-02-01 02:11 by less_is_more
Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 458
Posted: 08:30pm 02 Feb 2024
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I wrote a response but somehow it disappeared. It sounds like you are having fun and that your idea is working. I would suggest that you don't use this on a grid connected installation. Grid inverters have an anti islanding function that prevents them working if there is no grid power. This is to protect workers who may be working on the power lines. If your auto start decided that there was no grid and started the inverter , it could feed back up the line and kill someone. You would end up in a lot of trouble. Just keep the circuit for stand alone systems Pete
Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1024
Posted: 12:21am 03 Feb 2024
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Yes quite true, some are lower consumption than others, works with older water pumps and fridges however I still had issues using it with modern electronic gear, the inverter would send out pulse to check if a load is present this was not enough to get the automatic pump controller to work as it needed time for the electronics to activate the relay to switch the pump on, all it would do is pulse an led on and off, this was using an old selectronic, if the pulse was for 1 second it might have started the pump. Also I cannot have the new fridge running with inverter standby
For your setup it seems to work well, you certainly don't use much power and your idea should work with your 24v EG8010 based pure sine HF inverter.Cheers Aaron Off The Grid
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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... smoke. 
you certainly don't use much power and your idea should work with your 24v EG8010 based pure sine HF inverter.