Current limiting 78xx regulators

[Robert Murphy]

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

 

[mc]

see ..

http://www.otherpower.com/otherpower_lighting_leds.html



Robert Murphy wrote:

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

 

[Phil Allison]

"Robert Murphy" <selectmax@hotmail.com>

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

 

[Phil Allison]

"Robert Murphy" <selectmax@hotmail.com>

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

 

[Alan Campbell]

Robert Murphy wrote:

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





No, that does not work, you can not get 12V and a current limit

simultaneously. In any case the first formula would be correct, but you
would drop about 15V minimum across the regulator. To do it properly,
you need an LM317 adjustable regulator, same circuit, same formula, but
the voltage Vout is only 1.25V, so with a 6.2 ohm resistor you get about
200mA. Put this upstream of a conventional 12V regulator to limit its
input current, and you have something like what I think you want.

In this application (current source) the LM317 does not have a
decoupling capacitor on its output, but it does need one at the input.

Check out Google or National for LM317 (or LM117) applications, there
should be lots of info out there. Another LM317 will also do your
voltage regulator stage, with 2 resistors to set the voltage to what you
want.

The 7812 etc have gone out of fashion, most people stock only LM317 or
similar, because with 2 resistors it gives them any voltage they want
instead of having to stock lots of devices. It can also be adjusted.


BTW the circuit works because the LM317 tries to keep its output 1.25V
above its reference pin, so if you take the output pin to the reference
via a resistor, and take the output to your circuit from the reference,
it tries to keep the voltage across the resistor at 1.25V, so the
current must be constant, apart from some small current through the
reference terminal.

Alan