diy solar

diy solar

Bench power supply to simulate solar panel

Any DC power supply will do to provide power, if that's what you are after
I tried DC power supplies with constant current (CC) and voltage control but i cant get the voltage to go higher than 29V without the current exceeding the transformer's max current. The MPPT controller is sinking current too much that I cant control with the CC varistor.
 
Keep in mind that most controllers are expecting a "voltage source" from a panel, and not a "current source" from a power supply - and may affect your overall design if this was going commercial for safety reasons.

In simplistic terms, the difference is this:

1) For a current-source, such as a battery or power supply - if you drop a wrench on the output terminal for a direct short, the current will max out and the wrench will vaporize. Battery / Supply may let out the magic smoke.

2) For a voltage source, such as a solar panel, if you drop a wrench on the output of the panel terminals, all current flow will simply stop. The cells in the panel don't care - they don't feel shorted or care. :) Power just simply stops.

In the very old days of SCC's - typically the hysteresis ones prior to pwm, some controllers would repetetively short the panel output leads during the later stages of charging. Some scc's would open one side instead of short, but the effect is the same.

Just saying - it may or may not be a consideration. Perhaps you will design to allow both sources in the internal fusing / safety of the SCC. Or just put a sticker on it saying "no power supplies allowed, panels only!" :)
 
Keep in mind that most controllers are expecting a "voltage source" from a panel, and not a "current source" from a power supply - and may affect your overall design if this was going commercial for safety reasons.

In simplistic terms, the difference is this:

1) For a current-source, such as a battery or power supply - if you drop a wrench on the output terminal for a direct short, the current will max out and the wrench will vaporize. Battery / Supply may let out the magic smoke.

2) For a voltage source, such as a solar panel, if you drop a wrench on the output of the panel terminals, all current flow will simply stop. The cells in the panel don't care - they don't feel shorted or care. :) Power just simply stops.

In the very old days of SCC's - typically the hysteresis ones prior to pwm, some controllers would repetetively short the panel output leads during the later stages of charging. Some scc's would open one side instead of short, but the effect is the same.

Just saying - it may or may not be a consideration. Perhaps you will design to allow both sources in the internal fusing / safety of the SCC. Or just put a sticker on it saying "no power supplies allowed, panels only!" :)
If you short out the panel, the current will still flow up to the Is spec of the panel. PV panel is the current limited source.
 
[Previous reply deleted]

Wait - you are right. In fact that's how I test a panel by shorting it and measuring the current!

I've got some research to do.
 
Yeah, it's a weird point to get my head around about the difference.

I guess I'm obsessing over the effects of direct short between the two and how that *may* affect a design decision interior to an SCC.

The little marketing guy in my head came up with this to tell customers:

1) If you direct-short a current source, it will let the blue smoke out. Or vaporize your crescent wrench.

2) If you direct short a voltage source like a solar panel output, you'll get full performance!

An over-simplification obviously, but the details are letting the blue-smoke out of my head. :)
 
If you dead short a solar panel you'll get Isc or whatever the max is with the current solar conditions, BUT it will make very little voltage doing this, and Watts will be amps X volts where volts are just above zero. So.. very little power.

I have run multiple PWM SCCs off of multiple PSUs. It is absolutely workable.
 
If you dead short a solar panel you'll get Isc or whatever the max is with the current solar conditions, BUT it will make very little voltage doing this, and Watts will be amps X volts where volts are just above zero. So.. very little power.

I have run multiple PWM SCCs off of multiple PSUs. It is absolutely workable.
This I can understand, a shorted fully illuminated panel will have Isc flowing through it, there is no or little voltage at its shorted output connectors. So there is no usable power from the panel.
But does'nt each cell in the panel has voltage across it? From what I have read an illuminated and loaded cell will generate 0.46 volts and 3 amps or about 1.38 watts. So, an illuminated shorted panel of 36 individual cells will dissipate 1.38 x 36 or 49 watts as heat at the panel's surface. Is my understanding correct?
 
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Good question, you might have just gone over my head!

So... any time any current flows through any conductor, some voltage drops due to resistance and converts some of the potential energy to heat. This is as true in a solar panel as it is anywhere else. The thing that's different about a solar panel that's pushing current due to energy from the sun, is it's only converting a small fraction of the sun energy hitting it, to current! It's already converting a bunch of energy from the sun into heat in a totally non-electrical way. So while yes a shorted solar panel will electrically self-heat, the amount that it self-heats electrically is probably not exceeding (possibly not even close?) the amount of energy it is converting to heat non-electrically by sitting in the sun in general.

I hope this was a relevant comment.. lol
 
In shorted panels, some cells will get hotter. Not all cells are perfect and the weakest ones will get hotter than the rest. There are many FLIR photos of this happening when panels are operated at Isc below power point. You won't find any statistics on this, but I suspect many used panels that show degradation were operated in positions that caused them to go into diode bypass.
 
So, what do rapid shutdown modules attached to panel connectors do in shutdown mode, do they short the panels, or just disconnect them from the others or both? If they short the panels, it seems that it is a bad idea to leave the modules in this mode.
 
Keep in mind that most controllers are expecting a "voltage source" from a panel, and not a "current source" from a power supply - and may affect your overall design if this was going commercial for safety reasons.

In simplistic terms, the difference is this:

1) For a current-source, such as a battery or power supply - if you drop a wrench on the output terminal for a direct short, the current will max out and the wrench will vaporize. Battery / Supply may let out the magic smoke.

2) For a voltage source, such as a solar panel, if you drop a wrench on the output of the panel terminals, all current flow will simply stop. The cells in the panel don't care - they don't feel shorted or care. :) Power just simply stops.

In the very old days of SCC's - typically the hysteresis ones prior to pwm, some controllers would repetetively short the panel output leads during the later stages of charging. Some scc's would open one side instead of short, but the effect is the same.

Just saying - it may or may not be a consideration. Perhaps you will design to allow both sources in the internal fusing / safety of the SCC. Or just put a sticker on it saying "no power supplies allowed, panels only!" :)
Clearly you don’t understand voltage and current sourceds

PV panels are non linear current sources. It’s the whole reason we have mppt , if they were linear current sources there’s no
Mppt power point.

If your merely powering up the ssc a power supply in CC mode will do fine. A voltage source ie CV will work but the mppt tracking will not.

Again in a constant current source is a source that theoretically outputs any voltage to maintain the designed current.

A voltage source outputs any current needed to hold the designated voltage point

Of course real life sources have practical limits and non zero output resistance in the case of CV and non infinite admittance in the case of CC.

Hence a power supply in CC mode will protect the scc but it will not activate the mppt function if that’s important.
 
Sounds good, but the question is this:

Why do solar panels have an ISC rating and still function while shorted, whereas batteries and power supplies do not have an ISC rating, and simply vaporize your wrench, or blow a fuse?

I mean that's the standard way I test a panel's ISC - you know - at noon in summer trying to expose as best I can, with an ammeter in line. Do that with a battery / power supply, and it's trip time.

This points to something. Can't put my finger on it with words for some reason. :)
 
A PV panel is a series stack of PV cells which has a diode conduction voltage (0.5-0.65v) clamped, illumination based current source. The greater the sun illumination, the greater the generated current. A battery is more of a constant voltage source. A PV panel is more of a constant (dependent on illumination level) current source.

In the real world there is some shunt leakage current and a small amount of series resistance to each cell.

Isc illumination current, for given illumination level, is fairly constant until cell approaches diode clamping voltage. Voc is the diode clamping voltage times the number of series connected cells in panel. When panel external load allows voltage to rise to point where clamping diodes conduct, the panel voltage is clamped to Voc and all the illumination generated current is shunted down the clamping diode.

Vmp is the panel loading point where just a little bit of illumination current is allowed to leak down the cell's inherent clamping diode. This creates the highest voltage without too much illumination current loss, yielding the maximum output power.

PV panel info.png
MPPT voltage vs illuminaiion.png
 
Is there any reason why a real solar panel can’t be used, with a bunch of lamps.
 
Because you would only have light when there was light? ?

As far as the difference in shorting behavior from a battery to a solar panel, I think it basically comes down to batteries being orders of magnitude larger as a current source by weight and surface area than a solar panel. For example, a car battery and a ~300w panel might both weigh 30ish lbs, but while the solar panel has a ton of surface area and creates heat dispersely across that wide area, the battery has very little surface area for its weight but can put about 100 times more current into heating it when short circuited. Figuring most 30lb car batteries will at least do over 800amps into a short. Thats if its heating itself, BUT a car battery is far more likely to heat the thing you short it with as well.

So consider that in simple terms, volts drop against resistance. Voltage also drops proportionally to resistance. This means the voltage drop across a fixed resistance will change if that resistance’s proportion of the total circuit resistance changes. This is what is causing the ‘terminal voltage’ reading to differ so much between a panel and a battery when shorted. A solar panel as a current source has a relatively high resistance. If its own internal resistance is only 1% of the total circuit, you will 'see' 99% of all the volts on the outside terminals. However, if you lower the resistance of the rest of the circuit by so much that the panel’s internal resistance represents 99% of the total resistance, only 1% of its voltage will be present on the outside terminals, because the rest is dropping internally and converting to heat in the panel. Watts of heat on the wrench you used to short the thing would be terminal voltage times amps(Isc), which would be fairy low because the volts are in the low tenths. Anything times zero is zero, and since volts here are almost zero, so are watts of heat in the wrench.

On a battery the internal resistance is extremely low compared to the solar panel, so hooking the same resistance of ‘short circuit’ to it would result in far more voltage being dropped across that resistance, and less inside the battery itself because it represented a smaller proportion of total resistance. While i have shorted solar panels and seen voltage well below 1v, I would guess that a ‘shorted’ car battery (with a wrench for example) would probably be putting out 4-7 volts at the terminals. This is extrapolated based on what ive seen load testing batteries at their rated cranking amps level. Since watts of heat in the wrench would be volts times amps, the car battery would put more heat into the wrench (and less heat into itself) even at the SAME current level as the solar panel. But… in reality it will probably be 100 times more current, or more. So ~10x higher terminal voltage to drop across the wrench, times 100x the current, is an absurd Watt number that can heat the wrench EXTREMELY quickly. Funny thing is, even then there is less heat being generated outside the battery (in the wrench) than there is inside the battery where the majority of volts are dropping. It ‘s only because the thermal mass of a battery is so much higher than the wrench we’re shorting it with, that we notice the wrench heating so much faster. A small wrench might melt all the way to open circuit before the battery even gets hot enough to boil its water.
 
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Because you would only have light when there was light? ?

As far as the difference in shorting behavior from a battery to a solar panel, I think it basically comes down to batteries being orders of magnitude larger as a current source by weight and surface area than a solar panel. For example, a car battery and a ~300w panel might both weigh 30ish lbs, but while the solar panel has a ton of surface area and creates heat dispersely across that wide area, the battery has very little surface area for its weight but can put about 100 times more current into heating it when short circuited. Figuring most 30lb car batteries will at least do over 800amps into a short. Thats if its heating itself, BUT a car battery is far more likely to heat the thing you short it with as well.

No no. A battery is primarily an (imperfect ) voltage source.

A PV panel is primarily an ( imperfect ) current source. It’s nothing to do with power capability. Read up on current versus voltage sources.
So consider that in simple terms, volts drop against resistance. Voltage also drops proportionally to resistance. This means the voltage drop across a fixed resistance will change if that resistance’s proportion of the total circuit resistance changes. This is what is causing the ‘terminal voltage’ reading to differ so much between a panel and a battery when shorted. A solar panel as a current source has a relatively high resistance. If its own internal resistance is only 1% of the total circuit, you will 99% of all the volts on the outside terminals. However, if you lower the resistance of the rest of the circuit by so much that the panel’s internal resistance represents 99% of the total resistance, only 1% of its voltage will be present on the outside terminals, because the rest is dropping internally and converting to heat in the panel.

No. The battery has a variable output resistance. A panel has a variable admittance.

In a battery for a given charge level. The chemistry attempts to maintain a fixed voltage

In a PV panel for a given illumination the process tries to maintain a fixed current.
Watts of heat on the wrench you used to short the thing would be terminal voltage times amps(Isc), which would be fairy low because the volts are in the low tenths. Anything times zero is zero, and since volts here are almost zero, so are watts of heat in the wrench.

On a battery the internal resistance is extremely low compared to the solar panel, so hooking the same resistance of ‘short circuit’ to it would result in far more voltage being dropped across that resistance, and less inside the battery itself because it represented a smaller proportion of total resistance. While i have shorted solar panels and seen voltage well below 1v, I would guess that a ‘shorted’ car battery (with a wrench for example) would probably be putting out 4-7 volts at the terminals. This is extrapolated based on what ive seen load testing batteries at their rated cranking amps level. Since watts of heat in the wrench would be volts times amps, the car battery would put more heat into the wrench (and less heat into itself) even at the SAME current level as the solar panel. But… in reality it will probably be 100 times more current, or more. So ~10x higher terminal voltage to drop across the wrench, times 100x the current, is an absurd Watt number that can heat the wrench EXTREMELY quickly. Funny thing is, even then there is less heat being generated outside the battery (in the wrench) than there is inside the battery where the majority of volts are dropping. It ‘s only because the thermal mass of a battery is so much higher than the wrench we’re shorting it with, that we notice the wrench heating so much faster. The wrench will melt all the way to open circuit before the battery even gets hot enough to boil its water.
Again a 100w battery and. 100w PV panel have the same potential power output. Again you need to understand the Fundamental differences between real world voltage sources and real world current sources. This exposing the characteristics.

PV panels are not batteries.
 
I was intentionally couching the comparison in unusual terms.

I think you are choosing to try to teach to a different level than i am. I dont think its practical to try and differentiate between a current source and a voltage source when either is necessarily a source of both and the audience might appreciate a more practical distinction (such as having volts at all vs having volts in a particular circuit location) before deciding whether or not they want to become an EE (which i still dont, and obviously havent?). I already write long posts so i dont want to open any cans of worms I don't plan to close, i suppose.
 
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Interesting discussion. When I first read of testing solar panels by shorting them I was appalled because I thought of them like any power supply where being shorted is a "bad thing". I still would not test them this way even though I now know that it is acceptable.
 
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