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getting the maximum current of the voltage regulator

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Panji

New Member
if i have a circuit like this:

------|-------|7805|----|-----------
input _ _
ac line _ C1 _ C2 output

------|------------------|------------

how to get the maximum current (by a calculation of course) ?
 

kizzap

Member
It all depends on what the current that you are putting into the regulator and what current the regulator is designed to run at. for example you can get a regulator that only supplies 100mA or you can get a regulator that suplies 1A.

Hope that helps you

Kieran
 

hamfiles

New Member
The lines moved when you posted your diagram, but it looks like the basic regulated 5 volts using the 7805.

The LM7805/MC7805 can only safely supply 1 amp of current. At 5 volts, 5 volts x 1 amp = 1 watt of power. So you cannot connect anything that draws over 1 watt of power, or over 1 amp of current.

To provide more current with this chip you will need to connect a pass transistor. Fairchild has some examples of connecting pass transistors w/ 7805, on page 23 of the pdf.

http://www.electro-tech-online.com/custompdfs/2003/07/LM7805.pdf

The RASON radio club has a circuit that provides 10 amps of current with the 7805.

http://www.rason.org/Projects/vreg/vreg.htm

If you use the RASON circuit above, remove/omit R1 and R2 and do not use them. With R1 and R2 added, the voltage will be 13.8 volts.

Again, the only way to make this chip provide more than 1 amp of current is to add a power transistor or two.
 

Sebi

Active Member
Hey, hamfiles 5V x 1A = 5W total dissipation. :shock: Maybe mistyped. But with very good cooling the 7805 can deliver upto 1.5A, because it have only thermal protection.
 

hamfiles

New Member
Oops. Yeah, I'm kinda tired right now. You can draw 5 watts, if you run it at the default 5 volts.

National semiconductor does say the LM 7805 can supply over 1 amp, but they don't say how much more. Fairchild says their equivalent chip, the MC 7805, can supply up to 1 Amp. Probably 1.5 amps is its highest limit.

You are right Sebi, the 7805 has thermal overload protection. I guess you could use it at 1.5 amps, with a large heat sink. Panji, you could also use the LM 350, which provides 3 Amps of current.
 

bogdanfirst

New Member
o, i think that you are tired too Sebi. you said that the total disipation is 5W. i think that you meant that the power available from the regulator is 5W for a 1A current and 7.5 for a 1.5A current. i have used many 78xx regualtors at 1.5A, but with good heatsinks. in some cases i preffer to use the LM317 regulator, because they are about the same price and the LM317 only requires 2 more resistors.
and i think that the dissipation for the 78xx regualtors can be as much as 30W but i am not sure.....ill check the datasheets
 
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