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Old 04-19-14, 12:18 PM   #381
wuck
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Rich,

The FET circuit should work fine, but, the devil is in the details. Here are a few thoughts (without looking at data sheets) . . .

Vin must swing from 0V (off) to >>Vth to ensure the FET remains in the switch mode, not anywhere in the linear mode where the power dissipation can kill it. Sometimes a bipolar transistor is needed to drive the gate if your controller doesn't quite get to 0 volts. Pay close attention to rise and fall times here also, as a slow ramp will cause the FET to dissipate more power than you planned on while switching.

Another thing to watch is the open circuit (No load) voltage from your array, and the cold(ish) resistance of the tank heater. These can push the dissipation up when switching.

Make sure you do the math to check the device power dissipation in the "on" state as shown in your linked article. It's good practice to derate power dissipation specs by up to 50% for reliability purposes. The real key here is junction temperature, which can be derived from the data sheet and your circuit design. A good heat sink can help.

From a simple standpoint, no gate protection is required. However, you may get some inductive coupling from other power/control lines routed nearby, especially if they run next to inductive switched lines - like the compressor, or maybe a switched relay, or even long wire runs. These spikes can most likely be clamped with diodes on the gate, taking advantage of the input resistor (Rin) to provide the impedance to drop that spike when the protection diode conducts.

Maybe some of the above can help figure out why your DC SSR failed?

Pat

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Old 04-19-14, 03:21 PM   #382
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Since I have a dry contact from the controller (closed with calling for heat, open when the tank is hot enough),
I could use it to switch +12V into the Rin..

When the contact opens, Rgs would pull the gate down to Zero volts...

I'll check out the specs on the FETs to see what resistors will be needed to
develop the gate voltage needed for full turn-on..

The PV panel array can't deliver any more than 8Amps. It impossible for
PV to exceed it's amperage rating, since it's limited by sun light.

Only when you get certain solar-cloud-effects, will you get more power..
And that will be on the order of 2 to 20% more..
It would take a supernova to get much more than 20%..

I can short out the PV inside the house and never blow the 10A fuse on the array..
Which is why it's a waste of time to put fuses on solar panels..



Not sure why those DC SSRs suck so bad. I think it might be the DC signal source I'm using. Spikes maybe..

I'll have to think about a way to pull down the DC signal level quickly in case of a grid failure.
Don't want the gate voltage to drift down slowly, after the power supply input dies.. Need to load it down..
I figure the (AC powered) controller contact will fall out fast enough to prevent much over-heating of the SSR, if the AC failed..
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Old 04-10-15, 09:30 PM   #383
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We've had very little solar lately and yesterday I noticed the A7 had been running for about 4 hours..
Since it didn't cut off and the water temp was about 98F, I went downstairs
and found the HX coil (cold output vent) was covered with frost..

It's about 50F in the basement and the humidity has been 90 to 100%..

Last night, I reprogrammed the AC Power timer, to apply power for one hour on,
and one hour off, then repeat.. (Between 6AM & 11PM).

Today the weather is still the same, but the A7 is working much better.
It did two runs this morning and one this afternoon (afternoon hot water use).

There's a rumor we might be seeing the sun pretty soon!
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Old 03-18-16, 09:18 AM   #384
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Quote:
Originally Posted by Xringer View Post
Looks like a bad PEX crimp on the cold water input.
I did a quick inspection today, and noticed a brown stain on top of the tank.
Pulled off the insulation and saw the leak is at the PEX attachment point.
It looks like it's been weeping for at least a month.
The iron pipe is a mess of rust and it will have to be replaced.
Of course the nipple at the leak point will need replacement too.


New leak! Seems to be on the bottom of the iron nipple. Water filled the bottom cup and came upwards under the foam insulation.
Maybe the vibration of A7 is shaking the connection and causing the leak..?.

I'm thinking maybe a new Brass nipple might be better than iron.?.
Will there be a dis-similar metal problem?
Edit: I just went back and looked at all the pictures.. Everything going into the tank top is brass, with one exception.. The iron must go!

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Old 03-18-16, 09:56 AM   #385
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You could also see if there is a threaded male/pex (probably 3/4) that you could thread into the tank and then a pex on the other side of the fitting... I just bought fitting for a laundry tub sink that went directly from female threads to pex...even swiveled. That is if you have enough play in your pex to reach that far...

http://www.supply.com/shop?nid=48868...FZGIaQodcQcHyQ

Something like this..Menards or Home Depot probably carries them. On the cold water side i wouldn't worry about anything..on the hot side i'm not sure if they want pex coming right out of the tank like that.
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Old 03-18-16, 10:30 AM   #386
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I had problems before with that Swiveling pex to 3/4" copper. (It's above somewhere).
Edit: It' on page 33.. http://ecorenovator.org/forum/geothe...heater-33.html

The pipe has to be 6" tall, so the A7 heatpump has some support on that side..
The support bracket clamps onto the 6" pipe.. A 6" brass nipple should be a drop-in install..

One mod I'm thinking of is to Shock-mount the clamp a little..
Maybe wrap some 1/8" rubber sheet around the pipe, under the clamp..
That might cut the vibrations a bit..

Home Depot used to carry them. I had one on my HD wish list.. They no longer carry the 6" version.
I think maybe I'm going to go for this 6" Brass LASCO 17-9501 3/4-Inch by 6-Inch Red Brass Pipe Nipple - Pipe Fittings - Amazon.com

At this point, it's time to try Brass.. I'm just wondering if Teflon tape is the best way to go..
Is there some sealant that works well and the pipe will still be removable later on?
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Last edited by Xringer; 03-18-16 at 11:09 AM..
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Old 03-18-16, 11:11 AM   #387
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I've done quite a bit of plumbing, but i'm no plumber...but i never use teflon. I use that pipe thread compound...Harveys..its cheap and seems to work good. We just did a bunch of plumbing at my mother's house and i used that on a few fittings and had no issues with leaks.

Threaded fittings can be a bear in plumbing systems..If your water is really hard, sometimes slow leaks (like really slow) can seal themselves up over time.
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Old 03-18-16, 12:23 PM   #388
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Thanks for the tip. I'm going to try some compound. http://www.amazon.com/gp/product/B0002YPAFY

I noticed the tape above the leak, seems to have some rust marking.
A very small amount of water was leaking down from the brass fitting.
So it was leaking from both ends..

Maybe this time, it's going to hold.. Until the tank gets old..

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Old 03-18-16, 01:57 PM   #389
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There are a number of compounds.

I've been using a blue paste 'Leak Lock' that was recommended for the refrigerant connections when I did the mini-split and it says it works on anything.

I had trouble getting a leak free seal at the water heater with the A7 connectors myself. The water heaters included nipples were the only thing that really worked.
Both thread size, thread base length, and whether it narrows or not, and then the sealant on the threads made for lots of factors, and it's a lot of time to test them all out.

If the pex crimp is not happy with the vibrations, one of those push to connect pex adapters (Watts ro Sharkbite) might be more forgiving as it has a flexible ring seal on the inside. I've used them all over on my plumbing retrofit and never a leak. Works well for me as and upgrading piece by piece so I can modify pretty easily.
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Old 03-18-16, 05:34 PM   #390
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The T Plus 2 pipe thread sealant has a lot of good reviews. Should do the trick.
No problems with the Pex crimps. I'm using some full-sized tools now.
The leak before was the two piece brass-to-Pex fitting. The one piece fitting is the right one.
http://ecorenovator.org/forum/31155-post323.html
I started of buying the wrong part for this application..

In reading the reviews on the T+2 sealant, I saw a lot of people say the tape failed, but the T+2 worked fine.

The order should be here by Monday 17:30 and the oil burner will be off-line by the Tuesday noon..

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