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how to build a voltage controlled switch

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terramir

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Hmmm I got to figure out something, how to build a voltage controlled switch the usual voltage output of my solar panels is around 17-18.5 but under low light conditions "evening/ morning" it falls below 16V now I will be using an lm317T to create a stablized 16V because the cells I'm charging can safely handle 16V, but no more. the catch is that while the LM317T can handle voltages of 16.3 and still create a stable 16V output it cannot handle any lower than that I think. so my problem is I need to ensure that this circuit turns the power off when off when the source "my solar panels" go below 16.3V. BTW do you think that 16.3 V is a safe voltage for an LM317T on a 16V output?
Let me know
terramir
 
It sounds like what you need is a Buck-Boost switching converter. Its a DC-DC converter that can take voltages and step them down or step them up depending on whats needed. This will let you run far below the 16V you need. There is another type called a SEPIC (if I remember correctly) converter which does the same thing. There ate a lot of DC-DC controllers that will have app notes on their data sheets, just take a bit of time on digikey to find one that is appropreate.

Hope this helps.

Brent
 
ahh your right but not there :(

disregard below your right I tried the LM317 fluxuates depending on the input voltage Hmmmm so seems like I will need a buck boost problem is all the one at digikey can't handle an output of 16 volts with a max of 3 A output / input is what I need anyone know where I can find a buck boost that can handle 16V max about 3 amps highly effu\icent and will still work even with about 30-50 mA
terramir





Ahhh can't do that because I know when the voltage is below 16V the current won't be suffcient either. I need something that can detect the voltage and if it's below 16 volts triggers an off switch.
because if the input is below 16V the current will be below 50mA, which is not worth it in charging. I might actually divert then the rest of the current to another charging bank. Where it would be worth it.
anyway buck boost I might need for another circuit if you could give me some more details it would be appricitated.
terramir
 
Solar panels are inherently diodes by construction. They do not strongly conduct current out of your battery, but they do have large leakage currents which create trouble for your battery over hours and hours.

Typically a barrier diode is used to block the leakage.

Watch for the voltage dropout. Common regs drop 3v, you need an LDO- Low DropOut. Also look at the leakage current inherent to the regulator's output side, the LM317 uses a resistor network on this side and they can only be made so large. You might do better with an op amp as a comparator driving a P Channel MOSFET, but be aware that it has an inherent body diode so it won't prevent leakage back into the panel even with Vgs==0.
 
come again?
I didn't really understand half of what you were saying.
typical voltage of my solar panels is between 16.5 and 18V. the battery bank I have can handle 16V period not anymore. so the voltage needs to be stablized. right now I have a different battery bank on there that will easily drag the voltage down to it's level till it's full. there cheap NIMH's 90 of them 9X 10 in series in parrallel and I see to it this bank is never full. untill I have my next panel up which won't happen till I have all the circuitry down I get a max short amp of about 1.5A at full sunshine these 3 panels are hanging on the inside of my windows so the current is not that strong. two panels are 36 cells well one is 34 + two in series and one 32+ 4 and 4 half in series the half in parrallel to each of the four cells to compensate for a little less sun in the other window. the other has 32 cells solo this drags the combined parrallel voltage to 17.6-9 V in parrallel. if I final finish the other 4 cell patch panel it will go up to about 18.6V open voltage. I need to regulate the voltage to 16Volts flat for my new bank LI-ion cells 4 volts per cell is good enough for back-up applications and it will enable me to drain and charge my gel cells of my gel cell charging circuit off the stablized li-ion if necessary until I can learn how to program pics.
my ultimate goal is to have about 15 pic's do all this in an automated fashion. check the voltages and ampage of the panel as well as the time of day and decide which battery bank(gel-cell 2x5Ah 12V nominal, the NIMH bank total 10.6Ah 12.4 nominal 14.4 max and li-ion bank 12.6Ah 16V max to be safe 14.4 nominal) to charge. also once I have the pic(s) in place I should be able to get rid of the diodes that are dragging the voltage down. I should have a 17.5-19.2 during the daytime.
a buck boost would be a great thing considering the voltage drops during cover and the short ampage drops to 0.5 during cloud cover but .5 amps will give me around 300 ma charging current that I could use. and after 2 pm if I had a house I would be producing about 5-8 amps right now because I could turn them into the sun, but then I would have alot more batteries ;)
SO again my dilemma I need to stablize the output with a buck boost to 16 Volts anyone know a circuit ready to be put together that can achive this? since my next panel will be on the outside of my window hanging at an angle that will probably give me around 2-2.5 short amps(total 4-4.2) at prime time 10:00 am to 1:30 pm I need to start getting this regulated.
help
terramir
 
A quick look at solar panel V-I curves shows that their output current for a given illumination level is almost independent of voltage (below the knee). This means that, as long as your batteries can handle the peak current, you don't need a voltage limiter. The solar panel is like a constant current battery charger, except the "constant" current is a function of illumination, but not of voltage (except out past the knee, where the current falls off). See the graph below. It is not from your panel, but the general shape of the curves should be the same. Even if your panel's curves are flat out to, say, 40 volts, the current will be the same if you connect a 16 volt battery as it would be if you connected a 40 volt battery.
Do you have a datasheet for your panels, with V-I curves? If so, can you post them here?

I should point out that I am not an expert on solar panels, but I am on electronics in general. So I could be wrong. I was once before. :D
 

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If you look in digikey under DC-DC controllers you should be able to find an IC that you can design your own DC-DC converter around. Most of the datasheets will have circuit diagrams for the common configurations. Just find one with a buck-boost schematica and you should be able to get your own DC-DC converter up and running.

I think Ron H is right about solar panels. They are just big photodiodes in series. They should work like a light dependant current source. Your best bet might be to take advantage of this property and use the PIC to detect input curent and limit it if it exceeds the battery max and stop charging when the batteries are full. This would increace the efficiency of the charging. The PIC would just provide protection. DC-DC converters can also be configured to provide a constant current output if you are still interested in those.

Brent
 
my panels are self-built I wish I had more specs.
but I can tell you the voltage is pretty constant until the light basically goes away. as long as it is still decently bright the voltage doesn't crash it's the current that goes up and down. The voltage needs to be regulated because li-ion's are crazy animals. they will draw current in as much as is avalible till they reach peak or availble voltage. then they switch them selves and start dropping off in current draw till the current reaches about 3% of "C" as long as you keep the voltage constant you should be fine unless you have a defective cell.
that's why comercial chargers also sense time voltage and current. they deliver a constant current charge till they reach peak voltage and then they switch over to a constant voltage till the current drops below a certain point. I tested this with two cells but with the safe voltage of 4.00 per cell I charged them till they reached 4.0 V per cell I "had two connected to 8V" then I monitored the current it started dropping from 450 mA to 40mA that's when I knew they were full. I am losing a little wattage this way becuase if I charged them to 4.2 volts or 4.1V depending on the type ( I still don't know) they would behave in the same manner but the discharge voltage would also start with 4.2 or 4.1V respectivly W= V*A so I'm losing about
hmm if you look at typical li-ion discharge curves the drop at the start from 4.2 to 4 volts is in the 5-10% of charge range so it probably be about less than 25 mW that I'll be losing but I'll be extending the life cycles of these cells dramtically.
I need a constant voltage more than I need a constant current , but the avlible current should be more than 400 mA when it comes to topping off the cells and monitoring termination. since my entire pack will have a C= 12600 mAh but my charging current will never approach even near 1C it's more like 0.3 0.4 C on a really good day with the panel I haven't installed yet. The thing is I will have to monitor the current of the panels as well as the voltage of the cells inorder for the li-ion cells to charge safely.
further more I will have to divert the current to my other banks either the NIMH or the gel cell banks if the current is below 7% of C and the cell voltage has reached peak 4V percell because the termination would be unsafe.
so I need to learn how to speak PIC and learn how to create switches that can divert the power to where it's needed. further more I need to learn how to monitor current/ voltages with PIC's but before I do learn all this I need to keep it simple and just go with safety and manual monitoring till I can create the "charging regulation from pic hell"
and this is all because I choose the hard way instead of just buying one simple huge deep cycle battery.
if I add it all up I have spent more money so far this way than I would have just buying that and a 29.95 charger form fry's.
well that's life. I'll have to deal with it. one circuit and one pic at a time.
it's just all a learning curve. what attracted me to buying li-ion cells is the amount of deep cycles it can survive compared to other battery types.
with all the learning I have been doing I would make a different decision today, namely squash the li-ion's all together. but I invested my money so I'll be darned if I don't use them
hope you guys can help
terramir
 
Problem is, I see no way to reliably determine if the NIMH is full and the current needs to be shutoff. They're susceptible to damage from overcharging. And you can't just apply a fixed voltage and allow them to charge to it.

The 2 methods I know of in practical use are detecting a fairly small and temporary drop in voltage at the time the charging peaks. It won't work if you don't have enough charging current at the time, and the cells don't have a decent amount of discharge in them when the cycle begins.

Charging temp increase also needs a fairly strong charging current. Thus no detection in half sun.

Neither is appropriate for continuously floating the battery, rather, it's limited to a periodic charging cycle.

I'm not familiar with Li-Ion charging cycles but there may be similar problems.

Simplicity of charging one reason why lead acid is used so often. They're also extremely cheap and reasonably charge efficient. Why the Li-Ion? The voltage is really hard to use since it's far from 12V.
 
well I was attracted to the low self discharge rates, and the longer deep cycle rates. if you drain a lead acid below 70% of C and you keep that up it won't last but like 300 to 400 cycles. NIMH cells well they can last about a 1000 cycles but if you deep discharge them forget it you'll get about 500 out of them. li-ion cells can last several thousand cycles if the conditions are right.
# 1 don't charge them with a higher than 4.05 volt charging voltage
#2 li-ion cells are virtually 95%-99% charge efficent :p that's the coolest part :D

#3 I got these for a buck a cell I hope they won't die on me to quickly I don't know how they were stored most of the time :S
well 16 volts is the max voltage usually it will be around 14.8-14.4 which is good for most applications.
as for the NIMH as far as I looked at most charging curves the cell voltage goes even higher than 1.44 volts per cell almost to 1.6 volts for a short peak at one C it's flatter when the charging current is less, but I see if I terminate there I will just lose capacity. honestly I rather lose capacity that risk overcharging them.
well I will have to do some more research on those if I want to use the full capacity.
any more hints would be helpful
terramir
 
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