Inverter Load Sensor Update

[baggo]

The inverter load sensor described in my diary back in September has now been in use for 2 months and has worked faultlessly so far. For anyone who didn't see the diary, this was a circuit designed to automatically switch a cheap inverter on and off as required, rather than leave it switched on all the time. The more expensive inverters usually have this feature built in.

A couple of modifications were done to the original circuit:

1. The 0.33uf capacitors originally used across the cfls' were found to waste quite a bit of power so these were replaced with 0.1uf. This required an increase in the sensitivity of the load sensor circuit to account for the reduced current taken by the smaller capacitors. This was achieved by replacing the wire shunt (shunt 2 in the original circuit) with a 0.22 Ohm wirewound resistor and also increasing the frequency of the 555 oscillator which supplies the AC sensing voltage. Increasing the frequency increased the current taken by the 0.1uf capacitors.
   
2. The separate battery supply originally used for the circuitry was dispensed with and the circuit is now fed from the inverter battery. This caused a bit of head scratching as the circuit kept operating without a load being switched on. This was eventually traced to the fact that I use a PME earthing system (Protective Multiple Earthing - This is a UK system where the AC neutral is permanently bonded to earth. The neutral acts as a secondary earth if the main earth becomes disconnected for any reason.) There was a very slight current flow between the earth and neutral (only a few microamps) which was causing the sensor to operate. This problem was solved by reducing the frequency of the 555 until the circuit operated reliably with the true loads but not any stray cable capacitance etc.
   
   

The only downside to using a common supply for the inverter and the load sensor is that the battery bank is now earthed via the 0.22 Ohm shunt resistor. This is so low however that I cannot see it causing any problems.

[commanda]

Have you thought of using a dc-dc converter for supplying your electronics. This way the batteries would be able to float. I'm talking about the little pcb mount ones that handle a couple of watts.

Amanda

[baggo]

Hi Amanda,

Yes, that is the eventual aim. I haven't bothered yet because I will be getting a new 24 volt battery bank (12 Trojan T105's)in the new year so a lot of the electronics etc. are going to have to be altered anyway to work with the higher voltage. Will sort it all out properly then.

John

[jumpingjackflatch]

I have been following the progress of your project and

it is just what I need. I also have a reasonably cheap

MSW inverter that lacks autostart. Who wants to run the

inverter at 35 or so watts all night just so you'll have

light when you flip the switch on a midnight trip to the

bathroom! Are you considering posting the PCB artwork so

others can attempt to follow in your footsteps?

Randie

[baggo]

Hi Randie,

No problem there. It may take a bit of time though as I'm pretty busy at the moment but keep checking back. Will post a pcb layout, schematic, parts list etc. as soon as I can.

John