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Xringer · 05-14-14, 10:08 PM

I had two PV arrays connected to the heating elements in my 40 gal water heater.

Recently, we had some really cool but sunny days and it caused the tank to get
a lot hotter than we needed. Moving into the unsafe temperature range.

So, I disconnected the 500W array and now we have a cooler tank..
On bad solar days, the temperature might not get up to 110F..

So, I tried to build a simple controller using a DC SSR.. And failed..

Now, I'm looking for another path.. Maybe Dump-load like the windpower guys use..?.
If the wind is really blowing and the battery bank is at full charge,
they use a very large relay to switch the windpower into a high wattage resistive load..
The excess wind power is now heating the air..
(Sometimes they don't waste it, they heat up a hot water tank).

~~

I once experimented with controlling DC current using an AC SSR..

The AC SSR uses a device called a Triac to switch the AC line into a load.
The Triac is kinda like two SCRs tied together, back to back.
Depending on which way the AC is flowing on that cycle, one of the SCRs does the work (conducting the current).

What I found out about the SSRs that I was using in my lab project, was:
They would conduct DC current when turned on by the control input,
but they would not turn off when the control input was shut off..

So, thinking about my requirements, I realized that once my tank was up over 160F, I should somehow shut off the PV heating..
I don't want to use a mechanical relay, since they arc with high DC voltage..
(I've already fried the standard hotwater heater (230VAC) controls).

But, if I used an AC SSR, I could only turn it ON, and never OFF..

So, if I connected SSR terminals across the PV +&- power lines, turning ON the SSR would short out the panels..
Up to almost 8 amps would flow through the SSR and it would latch ON..
And could not be turned off.. (using the control terminals).

Would that be so bad? Why would I need to turn the heat back on?
If I took a 3 hour shower?

Then it dawned on me.. When the sun when down, the PV DC would
disappear, and the AC SSR would turn off..
The next day, (if the control terminals weren't powered), the SSR Triac would be OFF..
Only when the water got too hot, would the SSR be turned on, for the rest of the day..

I figure that a 20A 240vac SSR should be able to handle 7 or 8 Amps of DC current from the panels.
Since the Triac voltage drop is low (it will have very low resistance when ON),
it shouldn't heat up very much.. My guess would be just a few watts..

What do you guys think? Will it work? Or will it smoke the SSR?

115 VDC at 7 amps into the 13 ohm heater is typical operation..
So, short it out with an SSR and get 0.8 volts at 7 amps (5.6w)?
That should be a piece of cake..

Maybe?

If it worked, the input power to the 13 ohm water heater elements
would see less than a volt.. And all the excess power would be
dissipated as heat in the wiring of the panels and feed lines..

05-14-14, 10:08 PM	#1
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	SSR? a Dump Load for PV array I had two PV arrays connected to the heating elements in my 40 gal water heater. Recently, we had some really cool but sunny days and it caused the tank to get a lot hotter than we needed. Moving into the unsafe temperature range. So, I disconnected the 500W array and now we have a cooler tank.. On bad solar days, the temperature might not get up to 110F.. So, I tried to build a simple controller using a DC SSR.. And failed.. Now, I'm looking for another path.. Maybe Dump-load like the windpower guys use..?. If the wind is really blowing and the battery bank is at full charge, they use a very large relay to switch the windpower into a high wattage resistive load.. The excess wind power is now heating the air.. (Sometimes they don't waste it, they heat up a hot water tank). ~~ I once experimented with controlling DC current using an AC SSR.. The AC SSR uses a device called a Triac to switch the AC line into a load. The Triac is kinda like two SCRs tied together, back to back. Depending on which way the AC is flowing on that cycle, one of the SCRs does the work (conducting the current). What I found out about the SSRs that I was using in my lab project, was: They would conduct DC current when turned on by the control input, but they would not turn off when the control input was shut off.. So, thinking about my requirements, I realized that once my tank was up over 160F, I should somehow shut off the PV heating.. I don't want to use a mechanical relay, since they arc with high DC voltage.. (I've already fried the standard hotwater heater (230VAC) controls). But, if I used an AC SSR, I could only turn it ON, and never OFF.. So, if I connected SSR terminals across the PV +&- power lines, turning ON the SSR would short out the panels.. Up to almost 8 amps would flow through the SSR and it would latch ON.. And could not be turned off.. (using the control terminals). Would that be so bad? Why would I need to turn the heat back on? If I took a 3 hour shower? Then it dawned on me.. When the sun when down, the PV DC would disappear, and the AC SSR would turn off.. The next day, (if the control terminals weren't powered), the SSR Triac would be OFF.. Only when the water got too hot, would the SSR be turned on, for the rest of the day.. I figure that a 20A 240vac SSR should be able to handle 7 or 8 Amps of DC current from the panels. Since the Triac voltage drop is low (it will have very low resistance when ON), it shouldn't heat up very much.. My guess would be just a few watts.. What do you guys think? Will it work? Or will it smoke the SSR? 115 VDC at 7 amps into the 13 ohm heater is typical operation.. So, short it out with an SSR and get 0.8 volts at 7 amps (5.6w)? That should be a piece of cake.. Maybe? If it worked, the input power to the 13 ohm water heater elements would see less than a volt.. And all the excess power would be dissipated as heat in the wiring of the panels and feed lines.. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..