Solid state relay for PV disconnect

Bobert

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A fet (short for mosfet) is a solid state relay.
Ok that makes sense that the more you put in parallel the less resistance you would have. I had forgotten what they were called I have made a few gadgets out of them 20 years ago when I was working on security/fire alarm systems.
 

Bobert

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I do this with my make sky blue controller and I have heard it happening with others. I have shade conditions where the 60V array will get stuck as low as 17V. I detect anytime the voltage drops below 42 for more than 30 seconds. That condition opens the array minus lead for 5 seconds and the controller resets to the 50's. Most controllers don't do full scans and can get stuck at several peaks in the curve. I just use multiple FET in parallel to get the resistance down. It is a matter of math. two FET in parallel produce 1/4 the heat of just one. Add another two and it is 1/16.
Thanks for the tip I haven’t done anything with fets for 20 years and had forgotten what they were called. I was wondering if you know of any that would work for an array with a potential voltage of 130 open circuit volts? The normal switching voltage would be less than 65 volts at 8 amps but the array itself could theoretically reach just under 93 volts at 24 amps. I could just put 4 of the solid state relays in series but fets would take up a lot less space. Another possibility I thought of is a normally closed relay as the PV is normally on but again the 90 ish volts dc at 24 amps is the problem. I may eventually add another string of 3 solar panels in the mix so If I could come up with something that would handle 32 amps at 93 volts. Where’s a good resource to educate myself on the naming system for the fets? That would make searches much easier. My wife and I homeschool our children I have 4 children and 3 of them are boys and the oldest just turned 12. I want to introduce him to electronics soon so I am starting to search for resources to re-educate myself and them.
 
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efficientPV

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It is hard to find a FET these days that actually meets specifications as many are relabeled. You need to test anything that is purchased. An old headlight or anything which provides a load of at least 3A can be used to determine the on resistance by the voltage across the FET divided by the current. Never use a gate voltage of less than 10V or the minimum resistance won't be obtained. I found a good source of IRFP260 and bought hundreds of them so I would always have a stock. These are rated at 200V with a current of about 50A, the larger size package gives better heat dissipation. Four in parallel would be suitable. As big a heat sink as possible. When they heat up, internal resistance can double over spec.
 

Bobert

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It is hard to find a FET these days that actually meets specifications as many are relabeled. You need to test anything that is purchased. An old headlight or anything which provides a load of at least 3A can be used to determine the on resistance by the voltage across the FET divided by the current. Never use a gate voltage of less than 10V or the minimum resistance won't be obtained. I found a good source of IRFP260 and bought hundreds of them so I would always have a stock. These are rated at 200V with a current of about 50A, the larger size package gives better heat dissipation. Four in parallel would be suitable. As big a heat sink as possible. When they heat up, internal resistance can double over spec.
It looks like the resistance on the IRFP260 is much better than the ssr I was looking at. I don't need the fets to switch completely off or very quickly as they will be off for one minute and on for 23 hours 59 minutes every day.
If I recall correctly years ago I had to use a diode and resistor to get the fets to switch properly ( I seem to remember calling them rectifiers 20 years ago) if that the case with these what would you recommend. I have a few resistors or diodes left from the alarm system days but they were all sized for small voltage small amperage applications. I also seem to remember that if they are destroyed by overheating that they tend to fail in the on position.
 

efficientPV

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It still isn't good enough, several are needed in parallel to reduce heating. Gate always needs to be tied to source with a resistor to turn off, in this caseabout 100K. Source goes to NEG panel, drain to controller NEG. Remember there is an internal diode. From POS panel, use 100K to 220K to + band of 12V zener, NEG to source. This will provide your +12V drive voltage. Relay contacts from +12V to gate. Each gate could have 100 ohm in series with gate. Not necessary, but helps in diagnosis if one FET shorts. Gates usually short.
 

DThames

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I’m not asking this to be “smart” but how is this dangerous? I have a maximum possible output of 24 amps at about 90 volts true enough and I might get some arcing at that level but if I need to disconnect to reset the most I have ever had is 8 amps at 62 volts. My connectors are in a dry location and there is no voltage potential between the solar wires and anything else in that location. To get shocked I would need to disconnect the mc4 connectors from both sides of the array and stick a conductor in each of them and grab on to it. So far I have never even had an audible arc when disconnecting and no visual damage to the pins. Maybe I can get an arc large enough to get burned at 90v 24amps? I’ve never tried that not sure I want to. Am I missing something?
Not sure if this has already been better answered but.....You need a disconnect, breaker, fuse, or something at the input to your charger so you can open the circuit and leave it open. After the circuit is open, you still have voltage concerns when doing any wiring with sun on the panels. But if you disconnect or connect a PV connector under load, it doesn't take many volts before there is a pretty good soft and fuzzy arc that will etch your connectors.
 

Bobert

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Not sure if this has already been better answered but.....You need a disconnect, breaker, fuse, or something at the input to your charger so you can open the circuit and leave it open. After the circuit is open, you still have voltage concerns when doing any wiring with sun on the panels. But if you disconnect or connect a PV connector under load, it doesn't take many volts before there is a pretty good soft and fuzzy arc that will etch your connectors.
Yes I actually have a double pole breaker but I haven’t figured out where to mount it without interfering with the airflow of my AIO inverter. I have no visible markings on any connectors yet but I know it’s only a matter of time so I plant to come up with a solution asap.
 

Bobert

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It still isn't good enough, several are needed in parallel to reduce heating. Gate always needs to be tied to source with a resistor to turn off, in this caseabout 100K. Source goes to NEG panel, drain to controller NEG. Remember there is an internal diode. From POS panel, use 100K to 220K to + band of 12V zener, NEG to source. This will provide your +12V drive voltage. Relay contacts from +12V to gate. Each gate could have 100 ohm in series with gate. Not necessary, but helps in diagnosis if one FET shorts. Gates usually short.
Thanks that’s super helpful. I had considered using 8 fets that would get my losses down to almost nothing. Wondering if there will be any additional factors to consider if I use that many?
 

Bobert

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Thanks that’s super helpful. I had considered using 8 fets that would get my losses down to almost nothing. Wondering if there will be any additional factors to consider if I use that many?
I got the fets working at least enough for testing. I had some trouble with the zener diode circuit at first but I figured out what I did wrong and I’m getting 12 volts now. The fets are switching fine so hopefully I’ll have the solar running through the “switch” tomorrow.
 

efficientPV

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I forgot to tell you to put a minimum of 4.7uF capacitor across the zener. When switched on, all those FET gates will have a lot of capacitance and drag down the gate voltage for a very small amount of time. The capacitor will supply the current in that time to keep the FET in full conduction. It will work as it is for testing.
 

Bobert

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I forgot to tell you to put a minimum of 4.7uF capacitor across the zener. When switched on, all those FET gates will have a lot of capacitance and drag down the gate voltage for a very small amount of time. The capacitor will supply the current in that time to keep the FET in full conduction. It will work as it is for testing.
wiil that be necessary? It takes over a minute before the charge controller starts drawing amperage from the panels once it senses voltage over 60v.
 

Bobert

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I forgot to tell you to put a minimum of 4.7uF capacitor across the zener. When switched on, all those FET gates will have a lot of capacitance and drag down the gate voltage for a very small amount of time. The capacitor will supply the current in that time to keep the FET in full conduction. It will work as it is for testing.
I thought you might like to see the external look of the fet switch setup. I was able to integrate a PV breaker and 2 fuel gauges into the same tower. I will probably add a switch for the water pump and a switch for the fuel gauges into the same tower soon. I had a 4” gap between the cabinet my AIO is mounted in and the strike board for a sliding door
Just enough space for a tower of 4” electrical boxes. It’s one of the few places that I have space left in the camper. I’m thinking of painting the tower to kind of blend into the corner once I get it finished. ( add the switches and neaten up the breaker cover)
 

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efficientPV

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The capacitor in this application is not required in this application, but is suggested. Some other viewers might want to copy this for another application where it would be necessary. FET are pretty easy to use. This is a great project that many should adapt for their needs. You should show schematic and construction. To many are using SSR bricks that do not perform as well and cost much more. You shouldn't have any heating with that heat sink.
 

Bobert

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The capacitor in this application is not required in this application, but is suggested. Some other viewers might want to copy this for another application where it would be necessary. FET are pretty easy to use. This is a great project that many should adapt for their needs. You should show schematic and construction. To many are using SSR bricks that do not perform as well and cost much more. You shouldn't have any heating with that heat sink.
Here is a on the fly diagram of what I did. The diagram shows the timer as a switch I did not include any more of that circuit to keep it simple . The second picture is a bit messy but shows how I put 7 fets in parallel on a 4x4 heat sync with enough room to still fit everything in the box. I hope to add more pics when I take the fets heat sync “module” off to use some electrical silicone to give some vibration resistance to the connections and stiffen things up a bit.
 

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sunshine

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Does anyone have experience with solid state relays on a PV application? What is the expected longevity of this type of relay? I have 9 240 watt panels 3s3p unloaded in the sun they max out close to 120 volts
Back to the original problem:
In the specs-
MPPT Range @ Operating VoltageDC 120~450V


Does this mean the AIO is only operating in PWM mode?

Also the 'stuck at 63v' also matches the specified overcharge protection of 63v.......this kind of correlates with my $15 PwrMr PWMs that stay at 14.4v even though the V ouput may peak at 15.5v if there is a sudden PV increase like after a rain shower....fixed with loads that come on at 14.5v on the battery side of the PWMs. I do have relays on the PV side but the little modules seem to keep resetting from the voltage swings.

Maybe there is a better solution by someone that understands these ALL in Ones better.
 

Bobert

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Back to the original problem:
In the specs-
MPPT Range @ Operating VoltageDC 120~450V


Does this mean the AIO is only operating in PWM mode?

Also the 'stuck at 63v' also matches the specified overcharge protection of 63v.......this kind of correlates with my $15 PwrMr PWMs that stay at 14.4v even though the V ouput may peak at 15.5v if there is a sudden PV increase like after a rain shower....fixed with loads that come on at 14.5v on the battery side of the PWMs. I do have relays on the PV side but the little modules seem to keep resetting from the voltage swings.

Maybe there is a better solution by someone that understands these ALL in Ones better.
My unit is 60 to 150 VDC. I think the problem with my unit is that the range that the MPPT hunts for power point isn’t setup for my array and it’s conditions. If the controller would hunt a wider voltage range every 20 minutes or so once wattage reaches 250watts at 60v it would work fine but those type of settings cannot be changed at least not that I am aware of. My array’s output is apparently not linear if I reset the PV so that tha controller retries to find maximum power point before I have transitioned from early morning sun the maximum power point is actually between 60 and 65 volts bms it’s way to 60 volts you will see that the panels will produce amperage at higher voltage but less than at 60 and with gaps where the amperage is so low that it doesn’t show up. I’ll give an example it’s 8:00 am and the sun has been up for a few hours but is still low on the horizon I’m producing 175watts at 60 volts and I turn off the PV and back on again the MPPT restarts and I see a open circuit of 100 volts gradually the MPPT places a load on the panels any the voltage slowly drop but I notice gaps in amperage at 90 volts I might register 1 amp nothing at 80 and 2or 3 amps at 75 nothing at 70 and gradually increasing amps once it’s below 68 until it hits maximum power point somewhere between 60 and 65volts. Once the mid morning sun comes out the controller never hunts more than a few volts ant there is often 7or 8 volts of next to nothing between 68 volts and maximum power point and an even greater gap in full sun. If I reset the PV at 9:00 in the morning this time of year and it’s overcast or better conditions the controller will find max power point every time
 

sunshine

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My unit is 60 to 150 VDC.
Can you link to this?
My array’s output is apparently not linear if I reset the PV so that tha controller retries to find maximum power point before I have transitioned from early morning sun the maximum power point is actually between 60 and 65 volts bms
I am trying to understand.....shouldn't the mpp V be stable and much higher than this....maybe 115 v for 3 in series?
 

Bobert

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sunshine

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This is the unit I have
" MPPT Voltage Range: 60-115Vdc, "

Maybe someone more knowing can interpret this spec for us ?
My interpretation is this is the range the mppt algorithm sweeps over before settling down to the mpp for the panels .
This-
In full sun my maximum power point is nearly 90v but maximum power point varies depending upon how much sun you have.

goes against my understanding that the panels should be operating at the V close to the mpp if in range....only the i would be lower.

Edit for clarity- a panel with a Vmp of 36v should always be operating at 36v for max output.
 
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