# Current limiting 78xx regulators

Discussion in 'Hobby Electronics' started by Robert Murphy, May 20, 2004.

1. ### Robert MurphyGuest

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

3. ### Phil AllisonGuest

** 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

4. ### Phil AllisonGuest

** 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