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Old 05-15-14, 04:18 PM   #11
philb
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I looked for the same thing few years back.

The good DC SSR's and mechanical relays with snubbers were more expensive than I wanted to pay.

You could use a cheap square wave inverter and turn that off and on with an SSR.

Check this tread out: Water Heating Controlled by a Classic 150

Maybe you could rig something up with a snap switch attached to the tank and use a SSR to turn the element off and on?

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Old 05-15-14, 04:34 PM   #12
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Here's another story with a timer board coupled to a relay: A New Bright Idea from Ghurd
No hammering relays.

Last edited by philb; 05-15-14 at 04:38 PM..
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Old 05-15-14, 04:59 PM   #13
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I always heat sink them.. But, I've never run so much power that I've seen an AC SSR actually heat up..
I use them on my Sanyo systems running up to 10A at 240vac.. Never seen one get up to 80F..


AC SSRs aren't designed to run hot.. They are either off or on..


Anyways, I'm done with DC SSRs.. I don't have any.. Not going to buy any more.
They aren't cheap like AC SSRs.. I'm never going to use one again,
unless someone gives them to me for free..
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Old 05-15-14, 05:07 PM   #14
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Quote:
Originally Posted by Ormston View Post
True about the heatsink although I would think it,s more likely the inrush current when trying to short out 110v.
The photo of the blown SSR shows a minimum of 24v DC so it could well be oscillating because as soon as the SSR closes the voltage would be well below 24v DC and switch off again.
This constant switching with very high inrush and poor heatsink can't be good.

As the SSR is easily capable of switching the load of the water heater, could you not just switch of the heaters when the tank is at temp?


Since there is very little capacitance in the panels and the feed line,
there isn't really any in-rush current.. As the sun gets strong on the panel,
it goes from a few MA up to a max of about 8Amps.. That's it..

If there was a big in-rush, it would wipe out the semiconductor before
the heat sink would know it was starting to get warm..

~~~

I don't want to switch the load of the heater. I want to short out the panels..

Unless someone tells me the AC SSRs are going melt at 8A..
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Old 05-19-14, 11:33 PM   #15
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Well, I decided to give it a try. We'll see what happens when Spring comes and
we get a few consecutive sunny days.
I rigged the 800w array to short out and left the 500w alone for now.
(I've been using the back-up AC power and it needs the solar a couple times a week).


I've connected to SSR to the lower heating element, across the 800w PV input.
The DC control input to the SSR is connected to a 12Vdc supply via a 2k resistor.
I have also connected a temperature controller's relay to SSR's the DC control.




When the temperature is below the setpoint (currently 70C), the contact is closed.
That shorts out the SSR input. Zero volts.

But when the water tank goes above 70C, the controller contacts will open,
allowing the SSR to turn on...
It should short the PV out and greatly slow down the heating.

In theory..
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Last edited by Xringer; 05-20-14 at 11:35 AM..
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Old 11-21-14, 11:13 AM   #16
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Quote:
Originally Posted by Xringer View Post
Well, I decided to give it a try. We'll see what happens when Spring comes and we get a few consecutive sunny days. When the water tank goes above 70C, the controller contacts will open, allowing the SSR to turn on... It should short the PV out and greatly slow down the heating.

In theory..
So Rich, did your SSR survive the summer? Any T&P valve excitement?

Pat
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Old 11-21-14, 11:43 AM   #17
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Nope, all the global warming seems to have cut into solar power production..
Not getting the the same amount of sun light these days..
The A7 is taking up the slack.
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Old 11-30-14, 08:49 AM   #18
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Interesting, I was using 40A SSRs to control an electric element controlled by a Tekmar 256 (I think) outdoor reset control. The 120g tank was to be heating a bunch of cast iron rads (while the owner decided on a heat pump or a gas boiler). The SSRs blew up on me after some time (months). I had a massive heat sink on them too. Still don't understand why.
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Old 11-30-14, 10:02 AM   #19
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I've never seen one that died of overheating..
But, I've seen a couple at work, that got killed when too much voltage was applied to the DC control input..


If the SSR output is connected to an inductive load, and there is set of hardware contacts in series with the load & SSR, I could see how the main triacs/SCRs could be damaged.
If the contacts make & break with an inductive load, the back EMF voltage can be pretty high..
High enough to arc over the contacts and make a mess of them..
And exceed the voltage rating of the SSR by a lot..
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Old 01-20-15, 11:50 AM   #20
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Default Oversimplifying?

If you CAN re-aim your array to a less-productive patch of sky, then you could easily get rid of a few stray BTUs per second with a high-temperature pump and a patch of uninsulated pipe to put the heat someplace (ground, swimming pool, etc)

In other words, your controller's response to an over-temperature tank should be:

1. Re-aim array to reduce input of BTUs.
2. Activate "heat waster" pump to shed some BTUs.
3. Return to normal activity when "Safely" below overheat Temperature

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