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Voltage regulator dropout

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HarveyH42

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I've only used 78xx regulators, so don't know much about other options. My problem is trying use a 5 watt solar panel to replace a 12 volt/350 mA wallwart, which feeds a rechargeable battery pack/inverter. A while back I had a opportunity to purchase 10 Black & Decker Pocket Power (CP120B) for $40. Thought it a pretty good deal, 9.6 volt rechargeable battery pack is worth way more than $4.00. A 20 watt inverter, and charge controller seemed interesting as well. A 5 watt CFL runs about an hour. Also a USB power output.

Back to the problem. Built a basic 7812 regulator to the solar panel, works fine if I get a couple hours of uninterrupted sun. The problem is that the internal charge control falsely reports a complete charge. I put a huge capacitor on the input of the regulator, but not much help. The panel puts out over 20 volts, no load. Haven't had many clear sunny days since starting this.

Was wondering if there is a voltage regulator that has a pin to signal when it drops out, or a simple way to detect? Basically, I'm looking to disconnect the charger when the regulator drops out (12 volts or nothing at all). Maybe there is a regulator with a similar function.

I'm basically looking to charge these things, and keep them charged until needed. Might add a night switch later on. Mostly I need to get the solar charging to work. Want to keep the internal charge controller, prevents over charging/discharging. The case is semi-weather resistant, so kind of want to leave it alone. I do have one the dog was experimenting with, caught him before getting real far into the tear down, still works, but the case is missing a corner...
 
A comparator, zener and potential divider which detects when the output voltage goes below 11.5V.
 
I've only used 78xx regulators, so don't know much about other options.Thought it a pretty good deal, 9.6 volt rechargeable battery pack is worth way more than $4.00.
A 7812 is a poor choice for a solar panel regulator no matter what type of battery it is. Use a low drop out regulator with a supervisory circuit.
 
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I had the 7812 on hand, so was my best and only choice for now. Might have others I could salvage, or even in my big box of unknown. I'm not familiar with low drop out part numbers. Just didn't want to exceed 12 volts. When winter ends, I expect the solar panel to be producing near its max.

I search a little while taking a soldering break (Christmas gift for younger brother), and most of the supervisory circuits seem to be 5 volts or less. Suppose I could divide down the 12 volts, which got me thinking... I have an analog comparator on my microcontroller, and was going to add one eventually, maybe I can make use of it. Haven't messed with that chip feature yet, and only used the ADC a couple of times, need to read more on it.

Just out of curiosity, why is a 7812 such a poor choice? When the sky is clear, the panel is producing over 15 volts with the load.
 
A LDO regulator is better suited because it can operate with smaller input voltages. A 12V LDO regulator typically requires the input to be about 12.7 V; the 7812 requires about 15 V.
What's more, many LDO regulators include an Enable pin that could be very useful for your purpose. You could take advantage of that feature in order ot make a voltage supervisor with few additional parts.
 
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Any part numbers I could look into? Something fairly common, that I might scrounge. Don't really have much else to add to an order, except maybe some more IR LEDs.
 
Hi,

Just to note, you may wish to look into a switching buck regulator instead of
a linear like you are currently using. The difference goes something like this...

With 24v panel output at 1 amp (if that is possible of course) and a 12v
linear regulator, the cells will charge at 1 amp. So the panel puts out
24 watts while the cells use only 12 of that, and the linear regulator burns
up the rest (12 more watts).
With that same panel and a switching buck regulator, the cells will charge
closer to 2 amps (twice the current!). That means when the panel puts out
24 watts the cells get almost 24 watts too.

Of course the efficiency of the buck regulator affects the total output too,
but you get the picture:
For large voltage differentials, linear regulator poor, switching regulator good.


Some of the National Simple Switcher ic's are not that much harder to use
than a linear like 7812, with the addition of a Schottky diode and some
decent input output caps, and maybe a couple resistors to set output
voltage (unless you get a fixed output voltage model and then no resistors).
 
I've got it out in the sun right now, was curious about the load voltage of the panel, with the current setup. Only getting 12.6 volts, but there are scattered clouds. Only got up to around 77 F to day. Really getting tired of the chilly weather :) ...

Did some digging around in the parts bins, found that I have 10 x LM2576T-ADJ. Looks like I have the right part on hand. The enable pin has a 1.4 volt threshold (ON<1.4 volts>OFF). Huge data sheet, need to figure out which parts are the fixed out, and which pertain to the adjustable parts I have on hand. It calls for a fast diode, not sure what I have in that department. Also need a 100 uH inductor, don't have a huge variety, not sure of the values. Take a closer look tomorrow, still have to finish up a Christmas gift. Few more parts, test, and shove it in a box...
 
Your temperature was 77 degrees F today and mine was 28 degrees F.
The sun rose today at 7:37AM and it was dark at 7:00.
The sun set at 4:41PM today and it will be dark at 5:00.
The shortest day is in two weeks.

I never get hurricanes and there are no alligators nor lizzards.
The trees have no leaves but many squirrels are still running around.
It snowed a small amount today, about 1cm.
 
I've only seen alligators a few times, even the small ones are scary. Don't have very many lizards anymore, dog plays a little rough with them. Squirrels dig up the biscuits he buries, so chases them off. Haven't had a hurricane pass through in several years now. Haven't seen snow or a hard freeze in nearly 20 years now... How do you keep the polar bears from knocking over your trash cans?
 
The polar bears in Canada live far away from cities, up North where it gets very cold.
Raccoons knock over trash cans but are not around when it gets cold.
My dog tries to eat all the snow but doesn't make much difference.
 
I've only seen alligators a few times, even the small ones are scary. Don't have very many lizards anymore, dog plays a little rough with them. Squirrels dig up the biscuits he buries, so chases them off. Haven't had a hurricane pass through in several years now. Haven't seen snow or a hard freeze in nearly 20 years now... How do you keep the polar bears from knocking over your trash cans?
Shoot them?

Don't do that, if you shoot them, you'll only make them mad! (name that quote)

Dan
 
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You need Maximum Power Point Tracking to get anything resembling the rated power which is called out as full sun at the equator.

Dan
 
HarveyH42 said:
I've only seen alligators a few times, even the small ones are scary. Don't have very many lizards anymore, dog plays a little rough with them. Squirrels dig up the biscuits he buries, so chases them off. Haven't had a hurricane pass through in several years now. Haven't seen snow or a hard freeze in nearly 20 years now... How do you keep the polar bears from knocking over your trash cans?

Here in the UK we don't have any wolves or bears, we do have some lizards and snakes but only one is poisonous.

The climate varies widely from the mild Isles of Scilly where date palms grow to the cold snow capped mountains of Scotland where nothing but grasses and heather grows.
**broken link removed**

**broken link removed**
 
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You need Maximum Power Point Tracking to get anything resembling the rated power which is called out as full sun at the equator.

Dan

Live close enough to the equator (central Florida), so no real need here for this project. I'll need less than 4 hours of full sun, if I can keep the internal charge controller from cutting off prematurely. Think this LM2576 will work out just fine.
 
Central Florida is farther from the Equator than from my part of Canada.
Cuba is tropical and was very nice because no obnoxious Americans were there.
 
Scilly is on the same latitude as Winnipeg, the gulf stream keeps it warm in winter and cool in summer.

Cuba is run by a communist government that will have you beaten up and thrown in jail for saying the wrong thing. Some parts of London are worse, say the wrong thing in earshot of a gang and you'll be stabbed.
 
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I've got it out in the sun right now, was curious about the load voltage of the panel, with the current setup. Only getting 12.6 volts, but there are scattered clouds. Only got up to around 77 F to day. Really getting tired of the chilly weather :) ...

Did some digging around in the parts bins, found that I have 10 x LM2576T-ADJ. Looks like I have the right part on hand. The enable pin has a 1.4 volt threshold (ON<1.4 volts>OFF). Huge data sheet, need to figure out which parts are the fixed out, and which pertain to the adjustable parts I have on hand. It calls for a fast diode, not sure what I have in that department. Also need a 100 uH inductor, don't have a huge variety, not sure of the values. Take a closer look tomorrow, still have to finish up a Christmas gift. Few more parts, test, and shove it in a box...

Hi Harvey,

Excellent choice :)
You'll have to get some diodes then, like 1N5822 or similar.
They are not expensive. A regular diode wont work.
The 100uH inductor can be as low as 50uH, but i dont think i would
go lower that that, but the current rating also has to match
your application by around 150 percent to be sure. Thus, if you
need 500ma output then get one rated for at least 750ma,
or better yet, 1 amp.
If the inductor saturates, the circuit wont work very well.
Look for output ripple with a resistive load set for your intended
output current.

Since you are charging batteries you can probably get away with
a much lower value output capacitor too.
 
Went and checked the parts. Have the 1N5822, and some 100 uH inductors. Not sure about the rating, about the size of a 1 watt resistor. Didn't dig out the L/C meter (batteries probably dead), but the bands work the same as resistors if I remember correctly, they are silver-brown-black-brown...
 
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