diy solar

diy solar

Solid State Relays

Offgrid97

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Sep 22, 2019
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Have an idea but not sure of the reliability.
With the BMS protecting the low voltage and cell monitoring, there was talk of getting the seperate port style. The down side is the charging amp limits are low and most do not have low temp cut off.
What if we were to run the solar thru a SSR that was controlled by a tempurature controller that opens or closes the relay at a set temp. The issue being it would be closed when above 32 degrees ( or whatever you set it for) and open when below. How long are SSR designed to stay closed? I could see it building a lot of heat when closed.
Thoughts?
 
SSRs use semiconductors to switch the load. A decent one will carry the full rated current 24 x 7 without any problems. High current devices may require heatsinking on the back but the datasheet will tell you all about it.

A cheapie you buy off ebay or where ever, that's another story entirely.
 
SSRs use semiconductors to switch the load. A decent one will carry the full rated current 24 x 7 without any problems. High current devices may require heatsinking on the back but the datasheet will tell you all about it.

A cheapie you buy off ebay or where ever, that's another story entirely.
Totally agree with the cheap stuff. All items can fail but would be more than worth it to spend good money to save battery bank.
I might try this for fall and winter this year.
 
And this is why I made the great decision of monitoring voltage, current, temperature, etc. with a PLC so I can set all these values to what I want them to be instead of cobbling together components that kinda work for my application. In my case the goal is for the power to NOT go out. So when the voltage is low, I start a generator, when the voltage is high I divert the power to our hot tub, there are still failsafes like shut the inverter off if batteries are dead, but in your application you can turn on some heating mechanism if the temp is too low.

I have a SSR that I use for power diversion, it is a 50 amp SSR but regularly sees 25 amps. It's mounted on an aluminum heat sink without a fan and gets hot to the touch after a few hours, but hasn't failed due to heat. If this SSR is not passing power through it, then it does not heat up even if it is in the "ON" state.
 
And this is why I made the great decision of monitoring voltage, current, temperature, etc. with a PLC so I can set all these values to what I want them to be instead of cobbling together components that kinda work for my application. In my case the goal is for the power to NOT go out. So when the voltage is low, I start a generator, when the voltage is high I divert the power to our hot tub, there are still failsafes like shut the inverter off if batteries are dead, but in your application you can turn on some heating mechanism if the temp is too low.

I have a SSR that I use for power diversion, it is a 50 amp SSR but regularly sees 25 amps. It's mounted on an aluminum heat sink without a fan and gets hot to the touch after a few hours, but hasn't failed due to heat. If this SSR is not passing power through it, then it does not heat up even if it is in the "ON" state.
Sounds like your application is way different than mine. Weekend/hunting cabin for me. Cobbling would not be the best word to describe what I/we are doing. Just want a fail safe to protect batteries in cold weather from charging. Bringing them home is the best option but were is the fun in that.
 
I like this idea perhaps one could divert panels to a heat pad through an SSR or 2 so that if sun comes out and the batteries are too cold the heating pad uses the solar power to heat battery box. Once batteries reach acceptable temperature PV switches to SCC as usual.

There are always more than one way to skin a cat. In fact there is the right way , the wrong way , and whatever works for you and your situation.
 
I like this idea perhaps one could divert panels to a heat pad through an SSR or 2 so that if sun comes out and the batteries are too cold the heating pad uses the solar power to heat battery box. Once batteries reach acceptable temperature PV switches to SCC as usual.

There are always more than one way to skin a cat. In fact there is the right way , the wrong way , and whatever works for you and your situation.
I do like that!
 
Does anyone know if this type of solid state relay can be used to interrupt the PV input to a charge controller? Ie. SSR between PV panels and PV input of SCC and controlled from BMS with an I/O signal and pull-up resistor to give +14V for on and 0V for off?


I’m asking because it’s rated as an AC device, but I can’t see why a solid state relay in the on state could only pass AC and not DC, but am I missing something? It doesn’t have to be just this particular device - I realise it’s maybe too cheap to be as good as claimed, but my Q applies to this kind of AC rated SSR)

Come to think of it, with lithium batteries with a flat charge curve up to 90% capacity, why do we bother with charge controllers at all? If a BMS can switch the PV panels directly to the battery via SS relays and switch them off before they overcharge, why not cut out the middleman so to speak?!

Ps (is @gnubie still around? This kind of question is right up his street)
 
Does anyone know if this type of solid state relay can be used to interrupt the PV input to a charge controller? Ie. SSR between PV panels and PV input of SCC and controlled from BMS with an I/O signal and pull-up resistor to give +14V for on and 0V for off?


I’m asking because it’s rated as an AC device, but I can’t see why a solid state relay in the on state could only pass AC and not DC, but am I missing something? It doesn’t have to be just this particular device - I realise it’s maybe too cheap to be as good as claimed, but my Q applies to this kind of AC rated SSR)

Come to think of it, with lithium batteries with a flat charge curve up to 90% capacity, why do we bother with charge controllers at all? If a BMS can switch the PV panels directly to the battery via SS relays and switch them off before they overcharge, why not cut out the middleman so to speak?!

Ps (is @gnubie still around? This kind of question is right up his street)
Most cheap SSRs open at the zero crossings of the AC waveform. Dc doesn't have zero crossings so the cheaper models wouldn't work. Some of the more expensive ones are able to open at any point, and are often rated for DC as well. Zero crossing topology is often cheaper because there is no load to "break" at 0 volts.
 
Does anyone know if this type of solid state relay can be used to interrupt the PV input to a charge controller? Ie. SSR between PV panels and PV input of SCC and controlled from BMS with an I/O signal and pull-up resistor to give +14V for on and 0V for off?


I’m asking because it’s rated as an AC device, but I can’t see why a solid state relay in the on state could only pass AC and not DC, but am I missing something? It doesn’t have to be just this particular device - I realise it’s maybe too cheap to be as good as claimed, but my Q applies to this kind of AC rated SSR)

Come to think of it, with lithium batteries with a flat charge curve up to 90% capacity, why do we bother with charge controllers at all? If a BMS can switch the PV panels directly to the battery via SS relays and switch them off before they overcharge, why not cut out the middleman so to speak?!

Ps (is @gnubie still around? This kind of question is right up his street)
If made with MOSFETs I would think it would work.
If made with SCRs it wouldn't turn off.
 
Ok thanks guys - I wasn’t aware of the distinction between SCR and MOSFET devices, but I know what to look for now.

I see I can buy a 50 pack of mosfets for €15 or so, maybe just putting some on a Veroboard with some extra wire to thicken the copper traces and suitable heatsink could yield a DIY mosfet switch cheaply enough? These FETs can be paralleled, right? so putting 50A through three 49A Mosfets in parallel would keep them all well within spec and cool enough with a heat sink for good measure? I suppose I can’t assume the current splits equally, as the devices may have variations in DC resistance, but adding enough in parallel and ensuring the total load current doesn’t exceed the rating of each device seems safe enough (with heatsinking in addition).

I need to research to see if such devices need any circuitry to bias the FETs to switch them on, or whether just putting some plus volts on the gate is enough. Can it be that simple?

And what about switching my PV panels directly to my LiFePO4 battery through such a switch, under the control of the BMS such that the switch disconnects when any individual cell reaches, say, 3.6V (or lower) while leaving my Renogy 20A MPPT SCC (which would be connected to a different string of panels) to finish the charge? That way I could add a further 50A of charging capacity to my system very cheaply, avoiding having to buy more SCCs.
 
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Can someone give some examples of good quality SSR's? (DC load). I am looking for some for my project - but I don't have any reference for good ones vs cheap ones.
 
Ok thanks guys - I wasn’t aware of the distinction between SCR and MOSFET devices, but I know what to look for now.

I see I can buy a 50 pack of mosfets for €15 or so, maybe just putting some on a Veroboard with some extra wire to thicken the copper traces and suitable heatsink could yield a DIY mosfet switch cheaply enough? These FETs can be paralleled, right? so putting 50A through three 49A Mosfets in parallel would keep them all well within spec and cool enough with a heat sink for good measure? I suppose I can’t assume the current splits equally, as the devices may have variations in DC resistance, but adding enough in parallel and ensuring the total load current doesn’t exceed the rating of each device seems safe enough (with heatsinking in addition).

I need to research to see if such devices need any circuitry to bias the FETs to switch them on, or whether just putting some plus volts on the gate is enough. Can it be that simple?

And what about switching my PV panels directly to my LiFePO4 battery through such a switch, under the control of the BMS such that the switch disconnects when any individual cell reaches, say, 3.6V (or lower) while leaving my Renogy 20A MPPT SCC (which would be connected to a different string of panels) to finish the charge? That way I could add a further 50A of charging capacity to my system very cheaply, avoiding having to buy more SCCs.

Making an AC SSR would be more difficult. That would require four MOSFETs configured like a full-wave rectifier, high and low side switching, and synchronization of the gate with the power. Unlike a diode, MOSFET doesn't have a diode drop of 0.4 to 1.5V, just resistance, so loss can be lower.

For DC switching a single MOSFET can work. It can be either high-side (for positive rail) or low side.

Low side is a bit easier; n-channel MOSFET has lower resistance (think it is that electrons vs. holes thing - when you play musical chairs, do the people move or do the unoccupied chairs move?) You can get a MOSFET good for 100A, but gate can't be more than 20V above source. Resistance between source and drain varies with voltage gate to source, so that has to be high enough, typically 5V to 10V, to get source-drain resistance down around 0.001 ohm. If no current flow during switching, little power dissipation, but if current is flowing there can be considerable dissipation while voltage drop between source and drain is multiple volts. Low-side switching can be a problem if circuits on both sides use "ground" as a reference, for instance for RS-485 or other communication. The shifted voltage can burn out drivers.

High-side switching is most easily done with p-channel, driving gate no more than 20V below source. But p-channel uses holes for carries, is higher resistance. n-channel can be used, but that requires driving a voltage higher than positive supply. This can be done with a charge-pump (which is slow) or a boot circuit (which can't maintain DC, OK for PWM but not on continuously.) Drive ICs are available.

If current is flowing during switching, considerable power is dissipated in the MOSFET during the time (millisecond? Second?) it takes to drive the gate, and to change voltage of load. This can melt the MOSFET. If switching a capacitive load (input to an inverter with many electrolytic capacitors), that looks like driving a short. Must switch gate gradually, let the 1/2 C V^2 energy of capacity be dissipated in MOSFET over a period of time. In this case, if the load (inverter) starts drawing DC current before MOSFET has been driven on hard, it will burn up MOSFET. Either need a delay before load turns on, or a "power good" signal from the drive circuit to enable it.

MOSFETs can be paralleled. Because they have an "on" resistance which is linear, not an exponential curve like a diode or BJT, they share current fairly well.

MOSFETs may look like they would be good analog switches, like for linear regulation, but they can't perform to the specs shown on their data sheet. Power MOSFETS have been optimized for use in switching power supplies. The "Safe operating area" curves of current and voltage vs pulse duty ratio in the data sheets are incorrect, and portions of the MOSFET will heat up and go into thermal runaway. They need to be derated 10x or 100x from what data sheets say if used for analog.

I list all these issues because I've seen them in PCB designs with MOSFETs and power-good/inrush circuits. The analog and safe operating area issue is written up in NASA papers.
 
Making an AC SSR would be more difficult. That would require four MOSFETs configured like a full-wave rectifier, high and low side switching, and synchronization of the gate with the power. Unlike a diode, MOSFET doesn't have a diode drop of 0.4 to 1.5V, just resistance, so loss can be lower.

For DC switching a single MOSFET can work. It can be either high-side (for positive rail) or low side.

Low side is a bit easier; n-channel MOSFET has lower resistance (think it is that electrons vs. holes thing - when you play musical chairs, do the people move or do the unoccupied chairs move?) You can get a MOSFET good for 100A, but gate can't be more than 20V above source. Resistance between source and drain varies with voltage gate to source, so that has to be high enough, typically 5V to 10V, to get source-drain resistance down around 0.001 ohm. If no current flow during switching, little power dissipation, but if current is flowing there can be considerable dissipation while voltage drop between source and drain is multiple volts. Low-side switching can be a problem if circuits on both sides use "ground" as a reference, for instance for RS-485 or other communication. The shifted voltage can burn out drivers.

High-side switching is most easily done with p-channel, driving gate no more than 20V below source. But p-channel uses holes for carries, is higher resistance. n-channel can be used, but that requires driving a voltage higher than positive supply. This can be done with a charge-pump (which is slow) or a boot circuit (which can't maintain DC, OK for PWM but not on continuously.) Drive ICs are available.

If current is flowing during switching, considerable power is dissipated in the MOSFET during the time (millisecond? Second?) it takes to drive the gate, and to change voltage of load. This can melt the MOSFET. If switching a capacitive load (input to an inverter with many electrolytic capacitors), that looks like driving a short. Must switch gate gradually, let the 1/2 C V^2 energy of capacity be dissipated in MOSFET over a period of time. In this case, if the load (inverter) starts drawing DC current before MOSFET has been driven on hard, it will burn up MOSFET. Either need a delay before load turns on, or a "power good" signal from the drive circuit to enable it.

MOSFETs can be paralleled. Because they have an "on" resistance which is linear, not an exponential curve like a diode or BJT, they share current fairly well.

MOSFETs may look like they would be good analog switches, like for linear regulation, but they can't perform to the specs shown on their data sheet. Power MOSFETS have been optimized for use in switching power supplies. The "Safe operating area" curves of current and voltage vs pulse duty ratio in the data sheets are incorrect, and portions of the MOSFET will heat up and go into thermal runaway. They need to be derated 10x or 100x from what data sheets say if used for analog.

I list all these issues because I've seen them in PCB designs with MOSFETs and power-good/inrush circuits. The analog and safe operating area issue is written up in NASA papers.
Thanks @Hedges, that’s a lot of info to take in, but I think some of the issues you cautioned about would not apply in my proposed application of using FETs to interrupt the power flow from PV panels to SCCs. The switching would be one shot, from on to off, not a PWM type switching which occurs repeatedly and rapidly. Therefore the power dissipation within the FET as it switches should not be a drawback I hope.

So would you think it’s viable to construct a DIY FET switch using the mosfets I linked to above, carrying say 50A between three of them in parallel? As I only need to interrupt the circuit from the PV panels to the charge controllers, I would propose to place one of these FET switches local to the PV panel in the positive line to the charge controller. I suppose breaking the negative line would work just as well.
 
Don't see data sheet on that link. Pay attention to "on" resistance, which will determine power dissipation.

Instead of paralleling and worrying about division of current, how about each transistor dedicated to some PV strings?

Here's one rated 150A, 60V (check Digikey for other models)


How do you like those package leads?? we'd normally use 1/0 or 2/0 wire for that kind of current; this thing is the size of a toothpick!

Maybe one of these packages is more reasonable:



You can certainly build one, but test it out. Observe how fast the gate switches (lots of capacitance makes it slow.) Low voltage at first. Low current at first. Also high current, high voltage, and high temperature. There is capacitance gate to drain, so drain swinging in voltage drives charge into gate, affecting switching speed (and energy deposited.)
 
Those package leads are impressively thin - 0.9mm wide according to the data sheet! But only 15mm long, so as long as they don’t actually melt, the DC resistance and volt drop across them should be limited.

Ive been remembering I actually have some analog FETs around here somewhere, from an induction cooktop I was going to fix some day. IGBT devices if I recall correctly (lesbian transistors!). Maybe I’ll try them as I think they’re rated to 100A or more.
 
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