7805

[ParkingLotLust]

Just a quick question: will a 7805 supplied by a 12v power brick, pushing around 200mA need a heatsink, or will it stay cool? How about with something like... 7.5v?

 

[ArtemisGoldfish]

I power my PIC with a 7805, regulating from a 9V wall wart (which my multimeter says outputs 13V) and outputs around 600mA. Completely cool to the touch.

 

[ParkingLotLust]

The multimeter says its 13v because its an unregulated supply. But thats what I needed to hear. Thanks!

 

[Nigel Goodwin]

It's all down to the current drawn, you can't do much without a heatsink - in your case you're looking at dissipating 1.4W - you need a heatsink!.

 

[Hero999]

Does anyone actuallty know the thermal resistance of the TO220 without a heat sink?

I haven't seen any infromation on it.

 

[eng1]

Does anyone actuallty know the thermal resistance of the TO220 without a heat sink?

I haven't seen any infromation on it.

The datasheet says that it is about 65°C/W (junction-to-ambient). This value can be slightly different, depending on the manufacturer.

 

[audioguru]

A 78xx regulator in a TO-220 package without a heatsink will have its chip at a temperature very close to causing shutdown if it dissipates about 1.5W.

The one from National Semi says its max dissipation when it has a perfect heatsink is only 19W.

 

[ParkingLotLust]

Mine's in a TO-92 (I think - the size of a 2N3904 transistor) package. And the most it would be running is around 250mA (to charge an MP3 player).

 

[eng1]

Is it an 78L05? Don't use it! It can't dissipate 1.4 W.

 

[Roff]

Is it an 78L05? Don't use it! It can't dissipate 1.4 W.

It also can't pass 250mA.

 

[eng1]

It also can't pass 250mA.

Yes.. 100 mA max, with a maximum power dissipation of about 0.7 W.
He needs to use a 7805. And a small heat sink would be a good idea.

 

[Hero999]

Whilst I agree, 100mA is guaranteed and 200mA is more typical. I'm just being picky what I mean is don't bank on it being able to supply 200mA but don't also bank on it limiting the current to 100mA.

 

[audioguru]

The little 78L05 will get too hot then it will shut down. After it cools then it will start working again then it will shut down again. The thermal stress will probably break it.

 

[eng1]

Whilst I agree, 100mA is guaranteed and 200mA is more typical.

A limit of 100 mA is pretty realistic for typical applications.
The junction temperature, that is related to the power, does matter.
If the dropout voltage is around 7 V, the current should be kept below that value. The thermal resistance of the TO-92 is about 230°C/W.

 

[Hero999]

I agree but I think you've missed my point, which was don't also bank on it limiting the current to 100mA to protect the transformer and rectifiers powering it; most units will limit at 200mA or even more.

 

[desperadogear]

i need to run an lcd with backlight and a microcontroller and a medium sized seven segment display with a common 5v regulator. Total current consumption could be possibly around 600 mA . what regulator would be the best choice in terms of cost and performance...??

 

[desperadogear]

retrieved this from an old post

if u add up two 7805s and the resistors, u will be better off with a single regulator that can handle more current

i got the solution. see 7805.jpg this should work fine if im not mistaken

 

[eng1]

The 7805 can handle 600 mA without an external transistor, if you use a proper heatsink.

 

[desperadogear]

but whenever i use switch the backlight of lcd(16x2) ON the circuit becomes very dull and the alarm seems to cry weakly..
and in another circuit where i use a 128 x 64 lcd the circuit refuses to work at all when i swithc the backlight on...

 

[audioguru]

but whenever i use switch the backlight of lcd(16x2) ON the circuit becomes very dull and the alarm seems to cry weakly..
and in another circuit where i use a 128 x 64 lcd the circuit refuses to work at all when i swithc the backlight on...

Then the backlight is overloading the power supply.