As some may have seen, I have been trying to work out a very simple, low cost, minimal asset way of switching DC power from panels to a water heater.
There are many ways of doing this involving inverters, electronic circuits and other things that for a pelican like me are either too complicated, costly or just don't fit the idea in my head of simple and straightforward.
The big problem of course with DC is when you open a switch ( kill power) the DC arcs exactly like a welder which can burn the contacts of sa switch first go and keep going till it causes enough heat buildup to cause a fire.
Today I put some ideas to the test and got some results.
I have ( against many warnings) used AC switched on the DC side of my arrays to turn off the power from the panels. I have the switched on both outputs from the panels ( pos and Neg) and switched them many ties with 3Kw of load and had no problem. This is not said to be a safe practice and is not advise just background info. I wondered if this may work also for using an AC relay to switch the DC for powering a water heater.
I tested the idea at about 330V @ around 500W through a DPDT mechanical relay. The relay is rated at 10A @ 250V and is an omron Ly type.
As soon as the contacts opened, the arc began and due to the narrow gaps in the contacts, I believe would continue till the whole thing burnt in flames. This test was a distinct Fail. Switching both poles at once did nothing appreciable to stop the arcing.
Next brainwave was not to stop the power as in switch it off, but rather to divert it.
A double pole, double throw relay is like 2 switches in one ( double pole) and instead of just being on or off like a light switch, It's on energised or on de energised. ( Double throw). IE, Be a bit like a stop and go signal, it's either one or the other. Never both, never off, just alternate.
If you want no power, you just don't connect to the de energised side and it's like a normal on off switch... or you use a single throw relay....
I put a 500W halogen light on one side of the contacts ( still switching both Poles) and a Stove element of about 1100W on the other contact.
this meant as soon as the supply power from the solar panels was connected, something ( the element in this case) was on. When the relay was turned on and energised, the contacts in the relay when to the light.
At 330V open Circuit and at maybe 550W peak, This was a SUCCESS.
There was sparking as the contacts in the relay changed but it was not a lot more than I'd expect to see of a Decent AC load being switched. It may still shorten the life of the contacts but from my very basic testing, I think they would still get a decent service life if not the 10M Cycles speced on the data sheet.
My next test will be to try switching to a lower " Idle" Load. I'll try a couple of lamps in series to accommodate for the Higher voltage from the panels and see how that goes. I think I have some 50W halogen lamps up the shed so If I series them I believe I should get 25W across them at their rated voltage.
If this works then I think the next thing would be to divert the DC power like back to an inverter with the heater element under load and when the inverter has kicked in, release the element so the inverter gets full power. The element could kick back in when it wanted if it wanted.
A few other observations were relating to the idea that one could go above an elements voltage rating IF the total power ( wattage) was not exceeded.
This I found to hold true with the power supplied from the panels panels. May not work with other power sources but with the panels, it did.
The 500W lamp pulled the voltage of the panels down depending on how much cloud there was. The sun has been shaded and clear this morning so ideal for this sort of testing.
The voltage has very consistent but obviously the power output was not. When the sun is clear and the panels are producing well, the light would be bright and the voltage under load went as high as 270V. I could watch as a cloud came over and the voltage fell to 100V and the lamp looked like a candle. When I removed the load, the open circuit voltage was back at 330V +- 4V.
I got a similar result with the stove element which I think is about 1100W. Only thing was at peak output of the panels, that pulled the voltage to about 70.
This leads me to believe that switching considerations aside, I could run 500V open circuit ( probably not a great idea) but it would not hurt the element as long as the power ( wattage) was not above 1100W. Like the lamp, the voltage is more or less self regulating up to the power capacity of the resistance load applied.
I am now satisfied that there is a way to switch the sort of DC loads I am interested in with minimal complication and expense.
It may not be efficient to run a dummy load but that is not a necessity, the power could easily be diverted to other loads if need be.
Now for the low wattage globe test.....