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-   -   3-Terminal Regulator question (http://www.diychatroom.com/f18/3-terminal-regulator-question-44829/)

Robertw55 05-19-2009 07:40 PM

3-Terminal Regulator question
 
i have a charger that I use for my PSP that is 300mA and 4.5v.

The PSP is 5v 2A

this charger isn't the real charger for the PSP, it just happens to fit and it works fine (just takes longer).

im making a charger out of small 1 watt solar panels and using a +5v 1A 3T regulator. So would it be ok to use a +5v 1A charger for the 5v 2A? That wont damage my PSP right? charging under the specs?

also, will it damage a 5v 1a 3 terminal regulator putting 2.5 amps into it from the solar panels? Or is it specifically made to reduce amperage as well?

many thanks

Yoyizit 05-19-2009 08:28 PM

Quote:

Originally Posted by Robertw55 (Post 275857)
i have a charger that I use for my PSP that is 300mA and 4.5v.

The PSP is 5v 2A

this charger isn't the real charger for the PSP, it just happens to fit and it works fine (just takes longer).

im making a charger out of small 1 watt solar panels and using a +5v 1A 3T regulator. So would it be ok to use a +5v 1A charger for the 5v 2A? That wont damage my PSP right? charging under the specs?

Charging protocols are different for each battery technology: nicad, nickel-metal-hydride, lithium. I wouldn't use a constant voltage regulator unless you have inside info that you can get away with it.
http://www.maxim-ic.com/appnotes.cfm/an_pk/680


also, will it damage a 5v 1a 3 terminal regulator putting 2.5 amps into it from the solar panels? Or is it specifically made to reduce amperage as well?

many thanks

An IC regulator may protect itself as to current, but you may need a heatsink if the power into the regulator exceeds a watt or so. There's usually a 2v drop through the regulator. Post a datasheet.
(5+2)v x 2.5A = 18w = 2 sq. ft. panel
To get max power from a solar panel is pretty tricky. Check out the equiv. circuit in this link
http://en.wikipedia.org/wiki/Solar_cell

micromind 05-19-2009 08:45 PM

If it's a 7805, it'll cut back current on overtemp.

If connected to a source that can output more than 1 amp, it'll simply allow not more than 1 amp to pass through. The voltage of the solar cell will fall wherever it will at 1 amp of current.

Heat dissipation depends on voltage across the chip (input voltage minus output voltage), and current through it. The maximum input voltage for this chip is 35 volts.

As stated above, these chips need a heat sink. If no heat sink, the current will likely be less than 100 MA.

Charging a battery with less current than specified is usually OK with Ni-Cd, and Nimh types. L-ion is a different story altogether.

Rob

Robertw55 05-19-2009 08:46 PM

sorry, im a newbie and a bit confused.

So feeding the electricity to my 5v 2A PSP from a +5v 1A 3T regulator isn't a good idea? i thought it would be ok since the voltage is the same. also, i thought because the 3T regulator is feeding 1A into the 2A PSP, it would keep it undercharged, but still be ok. i just wanted to confirm it. hopefully this makes sense. :)

micromind 05-19-2009 08:49 PM

I think it'll work just fine. The PSP very likely has a charge controller built in to it, most stuff made in the last 10 years that has a self-contained battery does.

Rob

Robertw55 05-19-2009 09:03 PM

just saw the post from micromind after i posted last.

it is a 7805

when you say the chip needs a heatsink, are you talking about the chip inside the PSP? if so, how would i know if it has one?

so i don't need to worry about the battery being Ni-Cd and Nimh because the battery is 2A and the charger is 1A right?

again sorry if these are stupid questions :)

micromind 05-19-2009 09:26 PM

There are no stupid questions, but there are stupid guesses. 'Course, I certainly wouldn't be speaking from personal experience or anything......

The chip that needs the heatsink is the 7805. The heatsink can be anything that will transfer heat away from the chip. It can be the finned aluminum type (most effective, and cheap), a piece of basic metal, or any number of other things.

The chip can get pretty hot, around 200F or so. The cooler it runs, the more current it can pass. If you use heatsink compound (white greasy stuff), it'll transfer even more heat to the heatsink.

The tab on the 7805 (like most regulator chips) is connected to the DC-. Usually, this is not a problem, as the chassis of most equipment is already at DC-. If it's a problem, isolating mounting hardware is available.

Rob

johnnyboy 05-19-2009 09:33 PM

A power supply maintains a set voltage, upto the rated current. The device (the load) is what dictates how much current is needed.

If your PSP draws, say, 300ma, you can connect any power supply that is more than that with no problems... a load is a load, it cannot be forced to draw more than it needs.

current is kinda like horsepower, your car might be RATED at 300HP, but that doesn't mean when you're backing out of the driveway you're using all 300HP... you just need a car to be RATED at enough horsepower to get the job done....it's there when you need it. How much is being used depends on the LOAD (ie if grandma is driving or the 16yo)

Therefore, you simply build a device that provides the proper voltage, 5VDC, and make sure the whole project is rated UPTO 300ma or whatever your PSP needs... this all is so don't worry about it.

official datasheet for your ship: http://www.national.com/ds/LM/LM7512C.pdf

10V for instance would fry your PSP, so you have to regulate it down to 5V with the LM7805 chip.

Well, the more you regulate it down to, the more heat you will dissipate and that LM7805 will burn the **** outta your fingers, so attach it to some sort of heatsink, the tab on your chip happens to be ground so you can just screw it down to a metal project box and it will absord enough heat to operate fine.

Robertw55 05-19-2009 10:13 PM

i truly appreciate all the information, could you possibly show me a link of what this heatsink would look like so I don't get the wrong thing. and the ground you are referring to is that end tab with the hole cut out of it right? thanks again

micromind 05-19-2009 10:53 PM

The end of the tab with the hole is indeed the ground. Also known (in this case) as DC-.

For the heatsink, go to Mouser.com, or Digikey.com, or any of the electronic supply places, and search for heatsink. There'll be all different sizes, 3" X 3" will be plenty for a 7805. The case size for the 7805 is TO-220.

Robertw55 05-19-2009 11:23 PM

thank you kindly micromind and everyone else.

i'm not seeing a 3" x 3" on mouser.com, I'm seeing numbers like 36.83 mm L x 17.17 mm W x 21.59 mm H and 0 1.75 in L x 0.375 in W x 1.45 in H. does that basically mean this particular site doesn't have the one I need?

I googled and tried to find a 3" x 3" TO-220 but no luck. is there a more specific size I need for the 7805, or do I just need to look harder to find a 3" x 3"? thanks

also, as far as the white greasy compound, would I still use that? if so what is it called and who would I apply it?

Robertw55 05-19-2009 11:35 PM

another thing,

will I damage the 5v 1a regulator putting 2.5 amps into it from the solar panels?

micromind 05-20-2009 08:40 AM

The supply for a regulator chip can be any amperage. It'll use only what it needs.

For a 7805, the supply voltage cannot exceed 35 volts.

Rob

Robertw55 05-20-2009 09:31 AM

thanks, but i'm still trying to find out what size heatsink i need for the 7805. there are so many to choose from and i dont see a 3x3.

Robertw55 05-20-2009 10:27 AM

would this one work?

http://www.radioshack.com/product/in...ductId=2102856


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