Author Topic: Low Cost Power Point Controller, Linear Current Booster  (Read 2458 times)

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OperaHouse

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Low Cost Power Point Controller, Linear Current Booster
« on: January 17, 2017, 10:19:39 AM »
   Low Cost Power Point Controller, Linear Current Booster

This is an update on the simple Power Point controller proposed a few years ago. Using this
300W 40V 8A ebay buck converter, a decent 5A charge controller can be built suitable for small
applications like a garden shed.  The problem with buck converters is they will try to supply
the voltage and current needed by continuing to draw increased current from the panel.  This
makes the panel voltage continue to drop further making it even less efficient than PWM.  What
is needed is a method to prevent the panel voltage from dropping below a specified voltage.

Every buck converter compares the output voltage with a reference to determine how hard to drive
the buck converter.  This converters reference is 1.26V and it is compared to the output voltage
through a voltage divider.  The lower resistor of this voltage divider is about 250 ohm resistor.
Externally supplying a bit more than a half ma to it will successfully shut off the buck converter.
Find the voltage pot and look for the two pins connected together. This is the voltage the XL4016
chip looks at.

A TL431 is used as the panel voltage comparator. These three terminal chips can be found with several
prefixes LM, TL and others. Anything with a 431 is likely one.  These can be found in many old wall
warts and PC power supplies. The reference voltage is 2.5V and in this example the voltage divider
is made with a 36K/2.7K voltage divider with a 5K pot for adjustment.  The 431 has to be powered so
the lowest cathode voltage is a little over 2V.   That is more than the 1.26V that will turn the buck
converter off. The LED and two diodes are used as a Zener to subtract about 3V from the pull up
voltage.  As a bonus, the red LED being on indicates when the converter is shut off.  The circuit
as shown works only with this buck converter due to the 250 ohm input resistance.  The 2K pull up
resistor can supply 10ma to the LED.  That resistor can be increased to 4.7K if a lower brightness
is acceptable.

The power point is a fixed voltage.  It does not change with temperature or seasons. It can set
seasonally or just set  at a nominal voltage of 16-17V just to prevent the converter from dragging
the voltage down too much. Replacing much of the 36K resistors voltage drop with a number of diodes
in series will provide some temperature tracking.

Linear Current Boosters are often used for pumps in fish ponds and operate without a battery. This
same circuit can be used for that purpose if a couple of thousand uF capacitors are added to the input
and output to handle surge currents.  The regulator voltage is set to the pumps rated voltage. A DC
motor can be thought of as a 1V motor with the back EMF supplying subtracting from the supply voltage.
This is the principle a generator works on. That back EMF is proportional to speed. A motor supplied
with 1V and 1A will turn.  Any extra voltage goes into increasing speed. The buck converter will work
as hard as it can, acting as a current transmission, till the panel voltage drops below the set voltage.
This current boosting allows pumps to operate without batteries at much lower light levels than connecting
the motor directly to a PV panel.

A picture is provided of the buck converter since listings can disappear after a while. The price should
be around $4. If you pay more than $3 for a hundred 431 chips, you are paying to much.  These are a handy
chip to have around as they can drive a small relay for low voltage dropout or replace a zener in a number
of applications.  A hundred may seem like a lot, but buying less only saves about a buck. They are limited
to uses less than 36V.  Adding a zener to the cathode will raise the operational voltage.


« Last Edit: January 17, 2017, 04:33:41 PM by OperaHouse »

Warpspeed

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #1 on: January 17, 2017, 03:45:28 PM »
This is an excellent idea, and a great writeup by Opera House.
A great deal of thought has obviously gone into developing this circuit.

I have been using a similar method of control in my own home built solar controller, although my own circuit is very different, the operating principle is exactly the same.

The idea is a simple one, hold the solar panel voltage constant, and pull as much power from it as required to do that.
And at the same time regulate the output voltage to some maximum to suit the desired load.

This satisfies the requirements of both panel peak power point, and the load, and ensures maximum possible power transfer from twilight to full sun.
I am surprised there are not a lot more people doing it this way, its very simple, and very effective.

Highly recommended.

OperaHouse

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #2 on: January 18, 2017, 09:05:27 AM »
If you need more logic options, controlling this buck converter can be done with a UNO.
I used the PWM output of pin #5 because it is at a higher frequency of 960Hz which makes
it easier to filter. This pin feeds a 750 ohm resistor to a 220uF cap which is connected
tp common to average the PWM to a DC signal.  A diode still isolates the buck converters
pot from the output of the UNO.  A PWM count has to be about 90 before it starts reducing
the output of the SMPS. That presents about 1.6V before the diode.  Increasing the PWM
continues to lower the output voltage of the SMPS.  The SMPS 250 ohm resistor sees the
current from both the UNO and from the output voltage divider.

letERblow

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #3 on: January 18, 2017, 01:13:04 PM »
Thanks for sharing this.
Could this controller work for a simple battery charge controller, I was thinking NO but possibly would at one fixed voltage. If not, what type of charge controller would be needed?

Warpspeed

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #4 on: January 18, 2017, 02:40:47 PM »
It can in theory be added to just about any type of  battery controller, but with the proviso that any negative feedback around a control loop must be slow enough to be made stable.
A typical battery charger controller has two separate feedback loops.

The first control loop reduces the output power when the output current exceeds some set maximum (current limit). When in current limit, the output current will be fixed at maximum, and the output voltage will not be controlled, but will be somewhere below maximum.

The second control loop reduces the output power when the output voltage has reached some maximum (voltage regulation).  The output voltage will be fixed at maximum, and the output current is not controlled, but will be somewhere below current limit.

Both the above control loops use output voltage and output current sensing to limit the respective values.
All this is pretty obvious and well known by all of us. Nothing new there....

For MPPT control, we need to add a third control loop.  This control loop works in the same way as the others to reduce the output power of the controller, but by sensing the INPUT voltage to the controller, which is assumed to be the solar panel voltage.

The way this works, output power of the controller is reduced when the solar panel voltage tries to fall below the maximum MPPT solar voltage.  Its actually trying to regulate the voltage coming out of the solar panels so that the panels can never be loaded down below the maximum power panel voltage.

You will find at sunrise the solar panel voltage will rapidly spring  up to the maximum power voltage and sit at that voltage all day, or until the battery is fully charged.  Only when the battery is fully charged can the solar panel voltage increase upward towards the full open circuit voltage.

What actually happens is the controller pulls as much power from the solar panels as is available, and no more.
On a cloudy day it may only be only a few watts, in full sun it could be hundreds of watts.  It puts all of the available solar power into the battery until the battery is fully charged without overloading the panel.


OperaHouse

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #5 on: January 18, 2017, 02:47:51 PM »
Not sure of your question.  This is one of the few small buck converters I can recommend as sturdy enough for battery charging.  At $4, two could be used in parallel with a little modification for higher current.   These do have current limiting but that an be tricky because it is in the ground loop. Lead acid batteries tolerate bulk charging pretty well.  Actively used many charge controllers never get out of bulk mode.  For lithium, there are additional BMS boards I would add.  If just battery maintaining for the winter, a transformer source could be used for power and a switch could change between 13.5V float for example and 14.5V for charge with current limiting.

You have used a UNO so making a more complex charger is possible.   Have you got your mill up and running again.  Yesterday I was looking on youtube at hot water heating videos again.  Nothing worth watching.  It is amazing the low technical quality of these projects.  Your system would add a little class to the water heating videos.  This year I will have to learn how to upload my water heating system.
« Last Edit: January 18, 2017, 02:53:11 PM by OperaHouse »

letERblow

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #6 on: January 19, 2017, 08:38:43 PM »
OperaHouse
After hitting the post button I realized I left out information only available to a mind reader. Along with all your other talents you do quite well at that too. I was thinking this power point control circuit could be the front end to a simple battery charge controller and you guys shed some light on that. I was thinking of using this controller in my green house to run some blowers for moving hot air through the subterranean heat exchange tubes. And exhausting excess heat, 12v system. A winter use would be charging a battery for powering some LED grow lights to extend daylight hours. Need to put some of the free for the shipping PV panels my son acquired to use.

As for my wind turbine, after all you guys help in getting the FET controls working it only lasted 4 months till a high wind incident shorted my stator winding between phases. I was able to find the short and dig out the winding but have not had a chance to get it back in the air. Need to refinish the blades too while it is down.

To many projects, not enough time, one being building the green house. As with most projects it turned out to take a lot longer than anticipated. Three concrete block walls buried 9 feet in the earth with south facing glass wall. It has 5 runs of 4" tube buried beneath the floor and back in the side walls to store some of the summer heat for retrieval in the winter. We have had a run of -15 F temperatures and little sun this past month but it has stayed above freezing.

OperaHouse

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #7 on: January 20, 2017, 12:10:06 PM »
Just what is the open circuit voltage on these panels and what is the current of these motors. How many motors?  I first saw those underground tubes many years ago at a greenhouse in Vermont.  They used them to extend the growing season and prevent frost.  Is that your intent or can the motors just run in the sun shine to reduce over heating.  I'd like to do a dual system that runs the motors with a linear current booster and a smaller battery for lights.  f you have an abundance of panels, a standard switching supply could run the motors.

I'm not married to any one idea. I have strong farming interests and have been wanting to get
involved with some of these greenhouse projects on other boards. They were either too far away
or not up to it technically.  I'm sure you have some good ideas and have the ability to implement
them. Of course I would like to see them done with a NANO for full automation. Temperature sensors
to start exhaust fans when it is hot and circulate underground fans if frost is possible. Operate
these directly off panel voltage only when the sun shines and switch to battery at other times.
Operate misters periodically to water or to cool the plants with evaporation.  Monitor moisture
content of soil.  Circulate hot water under plants for fster germination. I've even grown plants
with ultrasonic sound. That is something you should try, increases germination percentage and
reduces germination time in slow growers.  All this automation stuff is expensive if buying little
pieces of electronics, but almost free with arduino. Did you see my dog watering?  Time to start
collecting stuff.  I've done a few block walls, not fastbut strong.

« Last Edit: January 21, 2017, 09:25:18 AM by OperaHouse »

letERblow

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #8 on: January 21, 2017, 07:59:40 PM »
The panels I have are 5.5 VOC. The one blower I have now is a 110 AC controlled by some of those Chinese $3.00 12v temperature controllers with on and off setpoints, one for high temperature to store heat and one for low to retrieve that heat. I modified another controller with a SPDT relay to control the vent dampers. A third one controls a 12v RV ceiling fan for air circulation. That little bugger is amazing, ECM or stepper motor, has ran about 8 hr a day for over a year now.  I chose these 12v controls with the thought of maybe getting a solar / battery system some day, the panels came along sooner than expected.

I will need batteries for running the heat retrieval blower in winter at night and some LED grow lights. I haven't found an affordable DC blower yet, maybe have to build my own housing for an automobile heater blower. Are the newer ones brushless types? I've started  acquiring and programing an arduino for data logging and controls but haven't go too far yet.

Bruce S

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #9 on: January 23, 2017, 09:45:45 AM »
Try looking for old desktop computers. Most of them have 1 or 2 120mm fans in them, these are great for pushing air through.
AND free is a great price. You could even stack one at inlet the other at outlet. A temperature control could shut them off/on.

What kind of LED grow lights are you looking at? OR building them yourself?

   
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OperaHouse

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #10 on: January 23, 2017, 12:35:02 PM »
I'm still contemplating this.  Are these tubes  a 6 inch U under the slab recirculating with a fan? With a second fan for upper exhaust of hot air.  Ideally I would want to pwm the fans from the raw panel voltage.  If kept at low voltage the battery charging might as well be pwm if panel voltage is low.

DamonHD

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #11 on: January 23, 2017, 01:58:05 PM »
BTW, be careful of mo(u)ld growing in the underfloor tubes if you've not already dealt with it.  It's caused grief to many an 'energy efficient house' builder.

Rgds

Damon

letERblow

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #12 on: January 24, 2017, 12:46:57 PM »
The heat exchange tubes ( 5 ea) are 4". A U , suction from high up on one side wall, out 2 ft into the earth, down under the floor and back wall, up the opposite side and discharge high on the wall. Drawing from the hottest area and discharging cooled air back into the hottest area.

At present, exhaust port is dampered and capped with a turbine attic ventilator, not performing as well as hoped.

This is a pretty small green house, 8 x 12 ft floor space.

I did get a mouse trapped in a tube during construction, not good, good thing plants don't have noses.

I have several 4" muffin fans 12v at 0.46 amp. but hoping to get something with a little higher static pressure.

So many good ideas and options, what to choose?

Bruce S

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #13 on: January 24, 2017, 01:20:46 PM »

Here's hoping you got the mouse flushed out !
Do you have temp probes in those tubes? I'm curious about the heat gain/loss.

Those fans can push/pull a pretty good amount of air.
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OperaHouse

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #14 on: January 24, 2017, 01:42:53 PM »
Those blow up Santa Clause use two  computer fans in series, pushing works better.  Use a couple of them in parallel.  If China does it, it has to be the cheapest option!

letERblow

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Re: Low Cost Power Point Controller, Linear Current Booster
« Reply #15 on: January 25, 2017, 01:50:52 PM »
I'll have to set up a test. Could make an inclined water manometer for static head easy enough. Maybe just a sheet of paper hanging over the exhaust port and measure how far out it get pushed for volume.

No official inlet and outlet differential temperature tests, just hold a thermometer in each one and smile at the difference. Have seen 30 deg F but that depends on the soil temp. During extraction mode I saw around 10 deg rise.

Yes, I got the mouse out, one sniff at a time.