I have Solar panel 20w 18v / 1.1a I used LM317 to reduce the voltage to 14.5v or 13.8 for floating charge.
but it wast the power that come from my solar panel

I want to build new circuit using step down regulator LM2576 or LM2575 (I have it in my hand).
but as I understand step down or switch down regulator also will wast my power that come from the panel, by ignoring a steps, Am I correct?

The idea how can I save that power to reuse it, is there any circuit, what its name, or i am in wrong way?

Thanks in advance.

 

The LM2576 is a switching regulator with over 90% efficiency. Look to the Texas Instruments Simple Switcher website for instructions and math.

http://www.ti.com/ww/en/simple_switcher/index.html?DCMP=simple_switcher&HQS=switcher

 

...as I understand step down or switch down regulator also will wast my power that come from the panel, by ignoring a steps, Am I correct?
/QUOTE]

Not sure what "by ignoring a steps" means, but there will always be losses in every thing one does. It's inevitable.

Getting 90% as #12 would be an excellent efficiency for that application. One member here who designs these things professionally stated a low voltage output solar panel is not worth the effort to use a switcher: the switching losses are about as bad as just using a linear regulator.

FYI: Changing from the simple LM117 to the LM2576 brings the efficiency from 77 to 90% (and that assumes you can actually realize the 90% from the switcher).

 

In Short question: If my input is 20w what should output be? is it (20w-the power that dropped by regulator)?

 

Not sure what "by ignoring a steps" means, but there will always be losses in every thing one does. It's inevitable.

Switcher cut the input into chunks and skip or use the chunks, so if we use it on battery it will be good than 317 that the switcher reg leave some chunks, it is mean saved in the battery.

but if we use the switch regualtor after solar panel, this skipped chucks considered as wast power not saved?

 

If "efficiency" is 90% and power in is 100W.. then power out is 90W
Its 90% efficient.

 

Switcher cut the input into chunks and skip or use the chunks

Not true. A switcher essentially chops DC power to make AC power. AC can change voltage very efficiently with a transformer. The losses are due to the switching losses when converting DC into AC.

 

Not true. A switcher essentially chops DC power to make AC power. AC can change voltage very efficiently with a transformer. The losses are due to the switching losses when converting DC into AC.

Same what i said, chunk or chops, is all chops used to generate output or it is skipped ?

 

All chops are used to pass power. Losses are far less than the linear approach, just 10% versus 23% using Ernie's numbers from #3.

The question is whether that extra power is worth the complexity. Buying more PV surface area is probably a more cost effective solution? Just guessing.

 

so why not put an electrolytic cap on the output of the PV? it can charge between chops and help during chops.

 

The switching regulator is similar to the 317 in that it will try to keep the voltage where you set it. Let's say 13.6 volts. When the voltage goes above 13.6 it will turn off so you don't over charge your battery. Your right, this power is lost, but it is lost because there is no place to store it not because it is wasted as heat.
But, it may not be the best way to do it because when the battery is low it will want more current than the panel can supply, so the panel voltage will drop. The regulator won't like that. Another way to do it is to connect the panel to the battery until it's voltage reaches 13.7 volts then short out the solar panel so the voltage can't go above 13.6 volts. This is called a shunt regulator. When the battery is fully charged the remaining power is wasted (but there is no place to store it anyway). The good thing about this is almost all the panel power is used to charge the battery.
How big is your battery (AH)?

 

Yes @ronv, for now I want to charge 7ah battery, but let us make simple examples

I have load need 12v 1.5a = 18w to run perfect.

Let me say I have 2 kind of sources, 1-battery and 2-solar

1 - Switch regulator will reduce the voltage of battery to 12v and there is no wasting the power, the power kept in the battery.

2 - But when I use a solar panel it will be wasted, why, because the power that skipped are not used and not saved/kept into battery in previous example.

@alfacliff maybe adding a capacitor in the input of regulator, i think it is good idea, but i need to know the good value for it (my PV 18v 1.1a)?

Thanks you all for your answers.

 

In the circuit that found in the manual there is a capacitor 100uf is that good enough 20w?

 

Same what i said, chunk or chops, is all chops used to generate output or it is skipped ?

All chunks are used. The switch rapidly turns fully ON and OFF to regulate the voltage and wastes very little power when ON. All the energy from being ON is stored in an inductor and goes to the output capacitor and load as needed for efficient voltage conversion. It's not wasted in a resistive element like a linear regulator does. When the switch is OFF, the inductor continues to supply energy to the output while energy from the panel is stored in the input capacitor for use when the switch again turns ON .

 

2 - But when I use a solar panel it will be wasted, why, because the power that skipped are not used and not saved/kept into battery in previous example.

That's just not true. You will get more power, end-to-end, by using the buck regulator. I'm not an expert on how they operate, but I believe energy is stored and released from an inductor. The power flow from the PV panel is NOT chopped into on/off segments with the "off" segments lost.

 

I have Solar panel 20w 18v / 1.1a I used LM317 to reduce the voltage to 14.5v or 13.8 for floating charge.
but it wast the power that come from my solar panel

I want to build new circuit using step down regulator LM2576 or LM2575 (I have it in my hand).
but as I understand step down or switch down regulator also will wast my power that come from the panel, by ignoring a steps, Am I correct?

The idea how can I save that power to reuse it, is there any circuit, what its name, or i am in wrong way?

Thanks in advance.

From the 13.8V, I assume you intend charging a lead acid battery - you can boil the battery by over charging, or regulate it - many people just use a shunt regulator which is the least efficient of the lot.

Unless you also want to run other things from the same panel, efficiency isn't of the most earth shattering importance.

A buck regulator is the most efficient solution going, but the panel is still producing power that isn't being used for anything when the regulator turns down its current draw (unless you can turn the sun off).

If you've got slightly deeper pockets; a buck-boost converter will still keep going even if the solar output goes lower than the battery - but AFAK that's a narrow margin of not much energy anyway.

 

Here is a pretty simple circuit. It turns on the panel when the battery voltage is below 12.6 volts and turns it off when the battery voltage is above about 14.7.
The load draws current from the panel if it's voltage is higher than the battery. The diode may already be built into your panel. The only thing you might find that is more efficient would be a maximum power point controller.

 

Here is a pretty simple circuit. It turns on the panel when the battery voltage is below 12.6 volts and turns it off when the battery voltage is above about 14.7.
The load draws current from the panel if it's voltage is higher than the battery. The diode may already be built into your panel. The only thing you might find that is more efficient would be a maximum power point controller.

I've seen a car battery with a label on it stating: "Charge not to exceed 14.4V". There are some minor variations in chemistry with equally minor differences in charge voltage specifications - so its something to bear in mind.

Most maintenance chargers (like the Optimate for example) sustain the battery at 13.6V - but you need to take it higher than that for the main phase of charging.

The OP said 13.8V, which is close enough for most purposes - but it will gas when fully charged, so the electrolyte level needs checking periodically.

 

That all depends on temperature as well, when you're talking about tenths of volts.

 

20W panel typically produces Vmp = 17.5V and Isc = 1.15A.

When you charge your battery at 14.5V, you get 14.5 x 1.15 = 16.7W. You waste 3.3W.

To get more, you need a controller which will maintain panel voltage at 17.5V (typically called MPPT controller). With 90% efficiency, it will produce 18W. You waste 2W.

So, your gain out of new controller will be 1.3W - 7% of capacity. Not that much. Not to mention that it doesn't make any difference when the battery is already full.