Using RF coax cable as DC Solar PV cable

[Xringer]

Those HF kits come with a 20 ft lead in wire and has chargers for 12V batteries.
If you buy two systems, you will be able to charge 2 batteries.

That coax cable sounds like it might be standard RG-6 and the amount of amps it can
carry is going to be limited to short runs.. There is a chart in the manual.
http://manuals.harborfreight.com/man...8999/68751.pdf

The center conductor is Wire Gauge (AWG) 18 and that's okay for low amperage.
But for higher currents, there will be heat losses. Longer cable runs = more losses.



Wire Resistance Calculator & Table

Use the calculator & input the cable length, set for AWG 18 and press Calculate.
Once you have the resistance in Ohms, and know the amperage,
you can find the heat loss in watts, using Amp squared x ohms..


How are you planning to use the output of the panels? Will you be charging two 12 volt batteries?

[jeff5may]

The main thing to confirm with common RG-6 is that it indeed contains a solid copper center conductor. The copper clad steel stuff used by the cable company does fine with low-current RF signals, but acts like phone line (or worse if rusty) with DC current. This usually converts to the #22-24 range for runs around 50 feet. At around 500 mA draw typical for directv or dish network setups, anything more than 50 ft run will eventually cause big problems with the steel core wire. With the solid copper core cable, 150 feet is no problem with a single run. With a dual or quad run, RF signal loss becomes a problem before DC voltage drop. Even then, I have seen 27 or 30 volt power inserters run juice out to 24 volt switches and amps 100 yards away, which then branched out to dishes another 50-100 feet or so. These power inserters are all current limited to 1 Amp max, so voltage drop at only 1 Amp gets close to 3 Volts through a single solid copper core cable around 175-200 feet. This is using grounding blocks and compression fittings which are all overbuilt and watertight.

[Fionn]

Originally Posted by Xringer

Is that labeled as a 220v~ 3,000 amps??
That's not really going to work with a 0.66 megawatt load is it?

What uses that much power??

It's a 32A breaker, the 3000A is the maximum fault current it can handle, obviously this is unlikely to occur in a realistic scenario.

[Xringer]

Oh Yeah, I almost remembered that ecomodded post. Back on 02-15-13, back when I was 67 years old. (I'm 70 now).
It seemed funny at the time. Like someone photo-shopped a breaker as a joke on his electrician buddy.

Since then, I've had to replace the toggle switch a couple of times..
Any amount of resistance across the switch contacts causes heating,
and somehow causes more resistance. Which = more heat, then melting..
I attempted to control the power with soildstate switches and,
a big golf cart relay and all efforts failed. It's still uncontrolled.
The #3 panel that was damaged and disconnected is still installed.
The array is now at 600W (three 200w panels) and still able to deliver
5 to 6 amps into the 13 ohm heating element. 460Wh=1570 BTUh.
And I still have to disconnect the 500w tracking array in the summer.
(Tank will get too hot). Today, there is no sun and snow in the forecast.
So the heatpump will run a bit..

The drawing is out of date..

[jeff5may]

Can't you just put a little capacitor across the switch contacts to keep them from burning up when you open the circuit?

[Xringer]

It seems like the problem is, any arcing causes the contacts to become slightly resistive.
That is going to be the cause of heating. It can also contribute to more arcing.
Relay contacts actually last a while, then fail..

It's funny that I've been posting about the hot water rig.. I did an inspection tonight and found another leak.
About 4" below the last one.. Where the nipple screws into the tank.. What a pain..

[jeff5may]

I hate it when that happens! Leaks suck.

I like mosfets for switching dc power. The newer igbt transistors seem to act like the dc ssr pieces you describe: they work like the ac relays and burn up for no real reason. I suppose they aren't supposed to do continuous duty at their rated amps. I have had good success with irfp240 and bigger transistors.

I don't know if I would be happy driving a mosfet straight off of relay contacts. The only drawback to them is their esd sensitivity. I would be much happier driving them with a logic gate or comparator of some sort. That way, the mosfet is slammed on or off without heating up. Luckily, there are lots of cheap chips available that fit the bill.

From past experience, I remember the N-suffix mosfets run hotter in circuits than the non-N-suffix parts. In high-power SMPS, this was not so much an issue, but they are also used extensively in car power amps as output transistors. The failed N-suffix parts were routinely swapped out for non-N-suffix parts during repairs. Especially in the competition-style amps, after the component values drifted with age, the N-suffix devices would not do well, even after re-biasing and such. I heard lots of theories about why, but was never really confident of what I heard.

[Xringer]

The IRFP250N was the last one that I used to toast an experiment.
Three left in stock, with about a dozen IRFP150N unused so far.

You can use relays to drive esd prone parts, just need to compensate for any relay bounce. Noise suppression. The 250 was driven by a 555, IIRC.
I've also used a function generator to test a 250 under light load.

Maybe I'll get around to some more fet hacking this summer..
Between bike rides!

[berniebenz]

https://ieeexplore.ieee.org/document/6591474

[CrankyDoug]

Originally Posted by jeff5may

I like mosfets for switching dc power. The newer igbt transistors seem to act like the dc ssr pieces you describe: they work like the ac relays and burn up for no real reason. I suppose they aren't supposed to do continuous duty at their rated amps. I have had good success with irfp240 and bigger transistors.

I think the problem with IGBT's is that, while they will turn off under load, they don't do it very well unless you drive the gate down quickly. The 40N120L3 in my PV hybrid water heater has a recommended turn on gate voltage of 20V and a turn off gate voltage of -20V. Apparently it is normal to shut them down by reversing the gate emitter voltage.

If the IGBT operates for any length of time in its bias region it would dissipate more energy than in saturation. The 40N120 has a 100C rating of 40A and 1200V but its power dissipating limit at 100C is only 247W. Perhaps NiHaoMike can chime in and correct me on this.