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Old 01-20-15, 01:20 PM   #21
Xringer
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The SSR setup is still installed. AFAIK, it's never activated.
For some strange reason, (beyond my comprehension),
the solar power out of the panels has not pushed the water temperature very high since last May.

Once in a while, we see 150F peak, but then the clouds come back and it's over.
Today is actually the best solar day we've had in a while.. 141.4 F right now.
The other day when it was really cold and sunny, the PV power was up around 1,300 watts.. But it didn't last more than a couple of hours.

I'll leave the SSR installed for now. I don't like to fix stuff unless it's broke.


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Old 03-07-15, 02:50 PM   #22
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Default A dead SSR? Oh no!

This weekend is the 1st weekend without snow we've had during the last 7 weeks..
This morning it was really cold (20sF) and the sun came out all morning.

The hot water soared up to 167 and the DC voltage was applied to the SSR.
But, when I checked the PV voltage, it wasn't shorted out..
It was about 68Vdc.. Still heating the heck out of the 40 gallon tank.
Saw 7A on the clamp-on meter..
I'm assuming the SSR got blasted.. Can't see how less than 8A could have killed it..

I'm going to start looking for some large relays now.. And I hate relays..
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Old 03-19-15, 10:28 AM   #23
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Default Re-wire



After thinking about the 'blasted SSR' for a while, I started wondering if I was
being too quick to toss it out..

Looking at a few diagrams of SSRs, I got to wondering if the Zero-Voltage
detect circuit needed power?

Maybe it does need power and maybe since I was using DC, it wasn't
seeing any (DC) power flow via the load..?. (Reversed biased)..

The other question was, had only one side of the triac blown?
Maybe the other side of the triac was still good?

In either case, maybe reversing the DC (from PV) would allow the SSR to work?

So the other night, I loosened the two output screw terminals and reversed the wires..
The next time the tank temperature got too high, it worked!
The 800W array power was dumped..

I measured the heat of the SSR and it was only about 10 degrees above the 50 F room temp..
Current was low, about 3A and the voltage was less than 1V..
I expected more heat, but it was cool running.. So far, so good..

The 500w array input was still running 5.5A, so the tank was still getting BTUs..
So, I need to add-on another SSR, to complete the over-heat control...
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Old 03-19-15, 11:29 AM   #24
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Excellent! I'd been thinking about your "Blasted SSR's" since you posted, but hadn't thought of a polarity issue. A friend of mine once told me "If the circuit you designed isn't working, then it's not the circuit you think it is"
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Old 03-19-15, 11:44 AM   #25
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Yeah, I like using 'black box' building blocks when building a project..
But, sometimes, it's the black box that fails you.

I have opened up a few sealed boxes a few times over the years..
And sometimes, I was surprised at how few parts were inside,
or how cheaply the assembly was made..

I wonder if this AC SSR problem was as simple as me, just not understanding
how it should be connected to run on DC..

I have to admit, I Googled using AC SSRs with DC and found zilcho usable info..
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Old 03-26-15, 09:09 AM   #26
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Dang, it worked a few times and failed again..

I just ordered a golfcart relay.. Solenoid Relay Switch Continuous Duty Golf Cart 12 Volt | eBay

I'm going to see if it can survive the punishment that I seem to shower on all DC switching devices..
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Old 03-31-15, 02:09 PM   #27
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Default Golf kart relay



Using the golf cart relay to disconnect the 800w PV array when the tank hits 70C.

This is the third day of testing. So far it's working.
I'm not sure what the open relay contact gap is.
But golf carts run between 36 and 48 volts.
So, I'm not too confident in contact longevity, with a PV voltage over 100v.

One other problem is the parasitic load of the closed relay is about 800ma.
That's almost 10w wasted. The PSU is on a timer, so it's not on all night.


Oh, one other problem. The 12v relay coil has a lot of back-EMF when disconnected.
While measuring it's current use, the EMF killed the back-EMP protection diode.
The next spike cost me $40.. It killed my little Turnigy DC meter..
http://i46.photobucket.com/albums/f1...testmeters.jpg

Anyways, I'm thinking of going back to the FETs.. Not as a PPT gizmo,
but just as a DC switch, to do relay duty..
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Old 04-08-15, 03:47 PM   #28
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Xringer,
I just browsed the thread so if I'm repeating something someone said I apologize.

TRIACs and SCR need trigger diodes usually bilateral. Once the current is flowing you need to apply a voltage to the gate to shut off the TRIAC. Just removing the gate voltage won't do it.

I use zero crossing detectors. What they do is sense when the AC wave is transitioning from one half cycle to the other and switch at the zero point. This reduces load stress.

I've included a small circuit that I have used to show driving a TRIAC with a zero crossing IC.

MOSFETs can be tricky. 'N' type work best on the low side, driving an 'N' type mosfet on the high side of a circuit requires a charge pump to make sure the MOSFET turns on completely.

There is nothing wrong with relays, sometimes they are my preferred option. Ideally what you want to do is turn the relay on at the zero crossing point (dry) this stresses the relay contacts the least.

I'm not sure why your heater doesn't have a temperature limiting circuit built in, maybe I missed something in one of your posts.

Rob

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Old 04-08-15, 04:42 PM   #29
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The power source is DC, (from solar panels) so there is no zero-crossing.
The built-in 240vac 4.5kw controls were destroyed by 130 VDC @ 7A. (from the PV).

Judging from what I've seen so far, I don't think the golf cart relay is going to last.
Unless this global warming keeps up and the sun never shines again.. (It's snowing here).

I've got a few IRFP250N HEXFETs. It seems like one of these could do the job.
So, they might be next on the block..


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Old 04-08-15, 07:22 PM   #30
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How are you hooking the DC SSR into the circuit? High side or low side?

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