3-Terminal Regulator question

3081 Views 16 Replies 4 Participants Last post by micromind, May 21, 2009

Robertw55

Discussion Starter · May 20, 2009 (Edited)

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

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micromind

· #3 · May 20, 2009

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

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micromind

· #5 · May 20, 2009

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

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micromind

· #7 · May 20, 2009

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

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micromind

· #10 · May 20, 2009

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.

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micromind

· #13 · May 20, 2009

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

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micromind

· #17 · May 21, 2009

Here's one that'll work. Go to www.mouser.com, and type 567-641-K in the Part Number search box. The one it brings up is a bit larger than needed (4.125"X3"), and it shows mounting holes drilled for a TO-3 device. The manufacturers specs state that this model has no holes. You'll need to drill one the same size as the hole in the mounting tab of the 7805. (The 641-A model has the TO-3 holes .)

567-120-SA is the heat transfer grease. All you do is smear a little dab on the back of the 7805 before you mount it to the heatsink. Enough to gush out the sides of the chip when it's fastened down. The excess is easily wiped away, or can be left on. It makes more heat transfer from the chip to the heatsink. If you don't use it, the chip will still work, but a reduced capacity.

Rob

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