Diy mppt? - Page 13

mcn14 · 02-20-15, 03:19 PM

Hi,I really like this circuit and have built the circuit, however I am having a few issues.

I have a 100w solar panel which has a sweet spot of 17.5V. The 2K pot on pin 3 is arranged in such away that the comparator switches when the capacitor charges up to 17.5V - hence the fet switches on and the load draws power from the capacitor.

The issue I have is when the comparator does switch the solar panel operating voltage drops and the capacitor in parallel to the solar panel does not charge up.

The voltage drop causes the reference voltage at pin 3 to be lower than 6.2V hence the comparator switches at a lower voltage than 17.5V. I dont understand why the solar panel voltage drops when i connect a load?

Xringer · 02-20-15, 08:34 PM

100w /17.5v = 5.7a of possible current flow.

17.5v /5.7a =3 ohms.. What is the resistance of your load?
I think it needs to be 3 ohms or less..

The 12v wallwart power supply I used had caps on the output.
So, it's 12v output was stable, even when the PV voltage level dropped.

You could try adding 100 uF between pin 8 and ground,
and also add a 10 uF between pin 3 and ground. (across the zener diode).

That might stabilize the IC power and the reference.

It's been years since I blew up my circuit.

So, I hope my suggestions help..

mcn14 · 02-21-15, 07:49 AM

Quote:

Originally Posted by Xringer

100w /17.5v = 5.7a of possible current flow.

17.5v /5.7a =3 ohms.. What is the resistance of your load?
I think it needs to be 3 ohms or less..

The 12v wallwart power supply I used had caps on the output.
So, it's 12v output was stable, even when the PV voltage level dropped.

You could try adding 100 uF between pin 8 and ground,
and also add a 10 uF between pin 3 and ground. (across the zener diode).

That might stabilize the IC power and the reference.

It's been years since I blew up my circuit.

So, I hope my suggestions help..

Thank for your reply, I can see its been a while you've last looked into this

I will try your advice.

My load is a 6- 15V range DC motor, so i think it has a really low internal resistance.

I thought the purpose of this circuit was so that the solar panel voltage never goes below a certain voltage, so its nor normal for my solar panel voltage to be dragged down?

Xringer · 02-21-15, 09:53 AM

That motor should be a good load.
IIRC, the circuit was designed to run a pump motor.

The idea is for the PV to charge up the large C1 input cap.
The more amps your motor requires, the larger C1 should be..

When C1 has stored a lot of power, the switch comes on
and fires a big pulse of power into the motor, causing it to start turning.

Then the PV voltage drops, & that causes the switch to turn off.
Instantly, the PV voltage climbs and charges up C1 again..

And the cycle is repeated, rapidly, sending many large power pulses per second to the motor..

When the sun is really bright and the motor is at high RPMs,
the switch might stay on, all the time..
Especially if the motor (and it's mechanical load) needs less than 100w..

Make sure that D1 can handle the back-EMF from the motor coil..
Otherwise voltage spikes from the motor can pop the fet.

Anyways, during mediocre sun condx, the pluses can keep the
momentum of the motor going.. (if it's mechanical load isn't too heavy).

Like the fly-wheel effect you get when riding a bike. On flat ground,
you don't need to work all the time. Sometimes, you can coast.

I think the main goal the 'inventor' had was to be able to Start
the motor, early in the morning, before the sun was even bright.
Otherwise, his water pump wouldn't come on, until the sun was full on the PV..

If the FET is working correctly, the motor should only see high voltage..
Low voltage should be blocked from the motor.
The motor must wait, until C1 has stored enough power (amps and volts) to give it a good jolt..

mcn14 · 02-21-15, 12:54 PM

Thanks for all information,

I did some testing this afternoon and I think the circuit is working now.

The PV voltage never falls below a certain voltage (which is adjusted by the pot).

Is it normal for the PV voltage to increase and decrease when you adjust the potentiometer?

When calculating the resistor and pot values for the restive divider going into Pin 2 should I use the VOC voltage or the MPP voltage (17.6V)?

Also is it correct that the capacitor can only be charged to the maximum VOC of the solar panel, for example on a sunny day I would have my comparator to switch when the capacitor gets charged to 16.5V, however if it was a cloudy day and the maximum voltage out of the PV was 17V would that mean the capacitor would only be able to charge up to 17V- hence meaning that I would need to re adjust my pot in order for the comparator to switch?

Thanks,
Cem

Xringer · 02-21-15, 01:16 PM

Yeah, the PV voltage will drop as more current is used and go back up as less current is used.
That's normal. If you hook up an oscilloscope, you can see the pulses. It will look like PWM..

The voltage divider should be selected using the VOC (or max DC volts).
Then, you can set the pot for best operation.

The input cap can be allowed to charge all the way up to VOC, but that's not what you
want for normal use with a motor.
Measure the PV voltage and adjust the pot so the PV is at it's MPP or sweet spot.
And it will work great during most of the day.
But, if you want to have more motor power in weaker sun, just tweak the pot
during weak sun condx for best more operation.
There may not be much effect during bright sun operation.
If there is, pick a happen medium..

Here's one for $46 shipped..
New Tigo Energy Solar Panel Maximizer mm ES50 300 Watt Module MC4 to MC4 Arrays | eBay

If I was running a pump off one panel, this seems like something I would try..

mcn14 · 02-21-15, 01:34 PM

I will test it tomorrow with your suggestions.

If I plug the DC motor straight to the solar panel without this circuit, the motor does not even start. It draws a lot of current and the Panel voltage suddenly drops.

Xringer · 02-21-15, 03:43 PM

As I said above, the main goal the 'inventor' had was to be able to Start the motor, early in the morning..
I guess he didn't like his motor getting hot, and not even turning over..

If your motor won't start when the sun is bright on the panel, directly connected,
but will run with the board inline, that might mean it's marginal, for working in poor sun condx.
You might need more PV, or a lower powered motor.
Please post how it works out.. Add some pictures if you can..

mcn14 · 03-02-15, 07:49 AM

If you look at the IV characteristics of a solar panel (like the one shown below) you can see that as the voltage of the solar panel tends towards the VOC the current starts dropping towards 0.

for this particular solar panel the maximum power point occurs at around 33V.

With this maximize circuit there's nothing forcing the panel to work at a particular voltage. in other words there's nothing to stop the panel from operating above the sweet spot.

If i set my resistors in such a way that the comparator switches at 17.6V, the solar panel voltage does not stay at 17.6V, the circuit just prevents the solar panel voltage from falling below 17.6V (providing that the VOC is greater than 17.6V).

Am I missing something here?

The image is posted in the post below because this forum only lets you post images after 5 posts and i only have 4 at the moment

mcn14 · 03-02-15, 07:49 AM

[IMG]

[/IMG]

02-20-15, 08:34 PM	#122
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	100w /17.5v = 5.7a of possible current flow. 17.5v /5.7a =3 ohms.. What is the resistance of your load? I think it needs to be 3 ohms or less.. The 12v wallwart power supply I used had caps on the output. So, it's 12v output was stable, even when the PV voltage level dropped. You could try adding 100 uF between pin 8 and ground, and also add a 10 uF between pin 3 and ground. (across the zener diode). That might stabilize the IC power and the reference. It's been years since I blew up my circuit. So, I hope my suggestions help.. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..

02-21-15, 09:53 AM	#124
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	That motor should be a good load. IIRC, the circuit was designed to run a pump motor. The idea is for the PV to charge up the large C1 input cap. The more amps your motor requires, the larger C1 should be.. When C1 has stored a lot of power, the switch comes on and fires a big pulse of power into the motor, causing it to start turning. Then the PV voltage drops, & that causes the switch to turn off. Instantly, the PV voltage climbs and charges up C1 again.. And the cycle is repeated, rapidly, sending many large power pulses per second to the motor.. When the sun is really bright and the motor is at high RPMs, the switch might stay on, all the time.. Especially if the motor (and it's mechanical load) needs less than 100w.. Make sure that D1 can handle the back-EMF from the motor coil.. Otherwise voltage spikes from the motor can pop the fet. Anyways, during mediocre sun condx, the pluses can keep the momentum of the motor going.. (if it's mechanical load isn't too heavy). Like the fly-wheel effect you get when riding a bike. On flat ground, you don't need to work all the time. Sometimes, you can coast. I think the main goal the 'inventor' had was to be able to Start the motor, early in the morning, before the sun was even bright. Otherwise, his water pump wouldn't come on, until the sun was full on the PV.. If the FET is working correctly, the motor should only see high voltage.. Low voltage should be blocked from the motor. The motor must wait, until C1 has stored enough power (amps and volts) to give it a good jolt.. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..

02-21-15, 01:16 PM	#126
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	Yeah, the PV voltage will drop as more current is used and go back up as less current is used. That's normal. If you hook up an oscilloscope, you can see the pulses. It will look like PWM.. The voltage divider should be selected using the VOC (or max DC volts). Then, you can set the pot for best operation. The input cap can be allowed to charge all the way up to VOC, but that's not what you want for normal use with a motor. Measure the PV voltage and adjust the pot so the PV is at it's MPP or sweet spot. And it will work great during most of the day. But, if you want to have more motor power in weaker sun, just tweak the pot during weak sun condx for best more operation. There may not be much effect during bright sun operation. If there is, pick a happen medium.. Here's one for $46 shipped.. New Tigo Energy Solar Panel Maximizer mm ES50 300 Watt Module MC4 to MC4 Arrays \| eBay If I was running a pump off one panel, this seems like something I would try.. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..

02-21-15, 03:43 PM	#128
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	As I said above, the main goal the 'inventor' had was to be able to Start the motor, early in the morning.. I guess he didn't like his motor getting hot, and not even turning over.. If your motor won't start when the sun is bright on the panel, directly connected, but will run with the board inline, that might mean it's marginal, for working in poor sun condx. You might need more PV, or a lower powered motor. Please post how it works out.. Add some pictures if you can.. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..

03-02-15, 07:49 AM	#130
mcn14 Lurking Renovator Join Date: Feb 2015 Location: london Posts: 12 Thanks: 1 Thanked 0 Times in 0 Posts	[IMG][/IMG]

02-20-15, 03:19 PM	#121
mcn14 Lurking Renovator Join Date: Feb 2015 Location: london Posts: 12 Thanks: 1 Thanked 0 Times in 0 Posts	Hi,I really like this circuit and have built the circuit, however I am having a few issues. I have a 100w solar panel which has a sweet spot of 17.5V. The 2K pot on pin 3 is arranged in such away that the comparator switches when the capacitor charges up to 17.5V - hence the fet switches on and the load draws power from the capacitor. The issue I have is when the comparator does switch the solar panel operating voltage drops and the capacitor in parallel to the solar panel does not charge up. The voltage drop causes the reference voltage at pin 3 to be lower than 6.2V hence the comparator switches at a lower voltage than 17.5V. I dont understand why the solar panel voltage drops when i connect a load?

02-21-15, 12:54 PM	#125
mcn14 Lurking Renovator Join Date: Feb 2015 Location: london Posts: 12 Thanks: 1 Thanked 0 Times in 0 Posts	Thanks for all information, I did some testing this afternoon and I think the circuit is working now. The PV voltage never falls below a certain voltage (which is adjusted by the pot). Is it normal for the PV voltage to increase and decrease when you adjust the potentiometer? When calculating the resistor and pot values for the restive divider going into Pin 2 should I use the VOC voltage or the MPP voltage (17.6V)? Also is it correct that the capacitor can only be charged to the maximum VOC of the solar panel, for example on a sunny day I would have my comparator to switch when the capacitor gets charged to 16.5V, however if it was a cloudy day and the maximum voltage out of the PV was 17V would that mean the capacitor would only be able to charge up to 17V- hence meaning that I would need to re adjust my pot in order for the comparator to switch? Thanks, Cem

02-21-15, 01:34 PM	#127
mcn14 Lurking Renovator Join Date: Feb 2015 Location: london Posts: 12 Thanks: 1 Thanked 0 Times in 0 Posts	I will test it tomorrow with your suggestions. If I plug the DC motor straight to the solar panel without this circuit, the motor does not even start. It draws a lot of current and the Panel voltage suddenly drops.

03-02-15, 07:49 AM	#129
mcn14 Lurking Renovator Join Date: Feb 2015 Location: london Posts: 12 Thanks: 1 Thanked 0 Times in 0 Posts	If you look at the IV characteristics of a solar panel (like the one shown below) you can see that as the voltage of the solar panel tends towards the VOC the current starts dropping towards 0. for this particular solar panel the maximum power point occurs at around 33V. With this maximize circuit there's nothing forcing the panel to work at a particular voltage. in other words there's nothing to stop the panel from operating above the sweet spot. If i set my resistors in such a way that the comparator switches at 17.6V, the solar panel voltage does not stay at 17.6V, the circuit just prevents the solar panel voltage from falling below 17.6V (providing that the VOC is greater than 17.6V). Am I missing something here? The image is posted in the post below because this forum only lets you post images after 5 posts and i only have 4 at the moment