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Two PV strings to an MPPT input of a Power Inverter. Diagram. Automatic switching


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We all wish we had the motorized devices that allow the structure of a PV string to automatically follow the position of the sun.

Unfortunately they are very expensive devices and not only with associated logic but also with mechanical parts, which implies not only having the money, but also the right place for the space they require, in addition to having to give them some periodic maintenance.

A very common problem is having a house with a gabled roof in the direction North-South, that is, each water from the roof one to the East and the other to the West.

Given the drop in the global prices of Solar Panels, we wish we could put more strings of Solar Panels than the Inputs we have in our inverters.

The first answer that comes up is to buy a Power Inverter with more MPPT inputs ...

For a fraction of that cost and a bit of “skill” we can use a single MPPT Entry for 2 Solar Strings.

Here is the simplified diagram:



And here are the relays purchased at ALIEXPRESS:




The device indicated in "Cyclical Timer" is any timer that meets 2 conditions:

1) Have an internal clock of 24:00 Hs.

2) Have a Relay output with Normal Open (NO) and Normal Closed (NC) contacts.


As for the Cyclical Timer Power itself, it is irrelevant and that is why it is not in the diagram but it could be Alternating Current or Continuous.

There are many on the market, I believe that even an "Irrigation Programmer" could fulfill the function perfectly with only having output for 2 circuits of solenoid valves.


Okay. Suppose we have the East-West string configuration.

If we look at the graph, the chain that is energized at this moment (Connected to Normal Closed) is the EAST string.

Assuming that we have symmetric power strings installed and that solar noon is, say at 12:30 pm. approximately at a certain time of the year, as we should Program the Timer so that, from 12:30 PM, the timer RELAY contact open (that is, it goes from NC to NO), this will cause the DC source that energized the High Power RELAY with the EAST Chain Connected, it de-energizes and therefore “disconnects” that chain, while immediately the High Power RELAY connected to the WEST Chain is energized, which closes its Circuit and the West chain remains “Connected”.

Now, depending on the possibilities of the cyclical Timer, we can indicate a certain time for it to change the state of its relay again, say 10 or 12 Hours or we can directly program the time we want it to change again, so that in the morning we already have our EAST chain connected again.

You could also set some time between chain changes, say 30 seconds or 1 minute or whatever you want.

Well what could go wrong?

The worst of all misfortunes is that one of the HIGH POWER RELAYs due to factory failure, for example, burns out and remains "stuck" with the Closed Circuit, that is, in an energized state, this will cause that when the Timer energizes the other RELAY and therefore the other String, since both are connected in parallel, the amperage is doubled and "goodbye to the MPPT input of the Current Inverter"

It occurred to me as the simplest solution to put a pair of more High Power RELAYS, one for each string (See RB1 and RB2) as a REDUNDANT device and connected on the POSITIVE of the chains, the rest of the connections in the control signal sector they would be in parallel with the same connection as the other RELAYS placed in the NEGATIVE (see the diagram), in this way if one of the RELAYS of any chain fails, the other one would surely avoid disaster ...

I hope someone serves you. Regards !!!


Edited by Cef
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  • Cef changed the title to Two PV strings to an MPPT input of a Power Inverter. Diagram. Automatic switching

Two comments:

1. These solid state relays need huge heat sinks when running high current.

2. The losses from combining different orientation panels on a single MPPT are surprisingly small - 0 to 2% according to this https://portal.segensolar.co.za/reseller/images/Presentations/2018/Huawei Brochure/V8_23-10-2018.pdf, about 1% according to some other research that I have seen linked on this site.


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24 minutes ago, Calvin said:

1. These solid state relays need huge heat sinks when running high current.


Yes of course. It is a diagram only. All Solid State Relay requires aluminum heatsinks. Not so big, this must be mounted on a board of appropriate dimensions.


Likewise, with all due respect, that loss figure seems too low to me.

There are many possible scenarios, see it another way, let's simplify, suppose you have an Inverter with only one MPPT and that has a limit of 3000 nominal input Watts. Your property has a roof with waters to the East and another to the West and enough space. With this device you could have 3000 Watts to the East and another 3000 watts in Panels with a West view. The TOTAL DAILY Production is going to be VERY HIGHER than if you install 1,500 Watts to the East and 1,500 Watts to the West.

Edited by Cef
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2 hours ago, Speedster said:

You're only able to use one of the strings at a time. Surely you're losing a lot of generation with this method?


Yes Yes of course. Obviously. Sorry if I was not clear in the description. The electrical diagram is clear.

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AliExpress 17,25 U$S Each...🙂
If you don't know how or don't want to do it, any electrician can fit this for you cheaply. In addition to what is indicated in the diagram, you only need the heatsinks, one for each Relay, the wiring and a board where to install everything ...

I put a theoretical inverter with only one MPPT only to simplify the concept and to understand the idea, which apparently and from what I read so far I have not been able to achieve.
To your question in the following post:

I have a Goodwe 5048Es Inverter with 2 Mppt inputs. Its limit is 6,500 Total Watts, both mppt added.
Previously my setting was about 1830 Nominal Watts on each string both facing North.
Currently I am reconfiguring my installation to leave a nominal 3,050 Watt chain facing North, while for the other MPPT input I am installing 2 asymmetric chains. One to the East of 2440 Watts and another to the West of 3200 Watts, which I plan to install with a logic similar to that of this automatic switching device. And I'm not going to use the cyclic Timer either, but my own software development with which I handle a relay board with a more sophisticated control logic where I can decide the change of chains for the one that has better production at a given time of day.

However using a "Cycle Timer", like the one in the diagram, anyone with the necessary knowledge can install it.


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44 minutes ago, Vassen said:

1) Okay I guess in that case it does make sense. But do you not have enough space on the north facing roof. 

2) Although the Alliexpress stuff is normally cheap, I hesitate to use them for critical stuff. If you are building a controller for it, I would add some additional logic in the form of a current /voltage sensor to ensure that the 1 ssr is actually off before switching the other on. The worst problem is it fails short circuit and you have a bit of a big issue. 

Well, it seems that we begin to understand each other 😉
1) Exact! I have no more room for panels to the North and it is something that happens to many people !!! They cannot bring their systems to maximum production due to lack of space for more panels or lack of MPPT inputs even if there is space in other directions !!!
2) The VOC of one of the string that I am connecting in this way is 351.2 Volts, while the other is 391.2 Volts and the currents are less than 10 amperes ... For my system these Relay are found left over in their specifications and with a good margin.
You would be surprised to see the amount of "cheap Chinese components" that many of the Power Inverters that are regularly mentioned in this Forum have ...
Anyway, I have made some other inventions in my house, with this similar type of RELAYs but nominated for Alternating Current and so far I have not had any problems.
On the other hand, if you read the thread carefully you will see that precisely, placing the RB1 and RB2 in the circuit has that objective to prevent a malfunction of any of the high-power RELAYs, if you are interested you can give this diagram to your electrician and he will explain the same, the circuit is safe ...

Edited by Cef
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Your inverter is specced for 500V at 11A per string. You want to have a total of 8690w across the three strings. Is it not an option to leave your north facing string a bit larger (up to 5.5kw) and then to add the balance across the east / west arrays? That way you're under the capacity of each MPPT. It may mean that some of the capacity is wasted (not sure what happens if total PV goes over 6500w) but that should allow you to get the most out of the arrays / inverter combiation you have.


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Very interesting.

I am running 6 panels north and 6 panels west on a single MPPT.  The reason for this is I wanted to make use of the late sunset in cape town, as well as get the early morning power.

I have a Kodak OG5.48 inverter. it has one MPPT 5000w high voltage

My total solar power is 330*12=3960w during the day I often see my pv power around 3300w which is more than what one can expect taking 20% loss into account

My PV generation currently start around 6:45am and it stops around 7:15

I am not an engineer but I believe in trying something. If it does not work then try something else until you get something that works.

I had this setup now for about a week. Before all my panels were facing north and I would loose generation around 4pm when the sun passes to the west and my panels don't get sun.

I will test this for a couple of months and see what happens. 


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