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Current limiting 78xx regulators

R

Robert Murphy

Jan 1, 1970
0
Hi

I'd like a cheap'n'easy current limited 12V supply (yup, I'm the guy with
the SLAs from a while ago). The NatSemi datasheets show something like this


7812

1 2 3
+ --------------| | |
| \
| / R1
----------------------------- + out

- ---------------------------------------------- - out

But, I've seen in differenet places that Iout = Iq/R1, or that
Iout=Vout/R1+Iq, with Iq being 3mA typical. So if I want a current limit of
say 200mA, this would indicate vastly different values needed for R1 (<<1ohm
and roughly 56ohm respectively). I'm guessing that the second formula is the
correct one to use, but would like to make sure. I've got to admit, too,
that I've got no idea how this configuration works.


Thanks
Rob
 
P

Phil Allison

Jan 1, 1970
0
Robert Murphy said:
I'd like a cheap'n'easy current limited 12V supply ....



** For ( near enough) constant current battery charging you can just use a
5 or 10 watt wire wound resistor after the rectifier bridge.

The charging current can be found with ohms law.






.............. Phil
 
P

Phil Allison

Jan 1, 1970
0
Robert Murphy said:
I'd like a cheap'n'easy current limited 12V supply

The NatSemi datasheets show something like this...

** That is NOT a 12 volt supply - the voltage at the output will simply be
12 volts LESS than at the input.

The max current in the load is set by the series resistor, I = 12
divided by the resistance in ohms.

If you want a 200mA current source then use 5 volt reg with a 25 ohm ( ie
10 + 15) resistor.





............. Phil
 
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