What is the... (requlation)

[beakie]

...best way of reducing the current WITHOUT affecting the voltage?


Cheers

 

[beakie]

Is there some sort of 78xx component for current instead of voltage?

 

[Reloadron]

A Google of LM317 Current Regulator will bring up applications where the old LM317 adjustable voltage regulator is used a s a current regulator to limit current. Additionally the data sheet for the LM317 shows current limiter applications. That should serve as a start.

Ron

 

[KMoffett]

You need to give us more info on what you are trying to do...the big picture. "Constant current" regulators change the output voltage to maintain a fixed current output across any load.

Ken

 

[beakie]

I have a 20W/13.8V/3amp bench power supply.

I am building a board to convert it to 5V or 9V but with much less amps.

I have used 2 voltage regulators to do the voltage regulation, I was wondering if there was a simple equivalent for current? Something that would reduce the current WITHOUT affecting the output voltage?

 

[beakie]

A Google of LM317 Current Regulator will bring up applications where the old LM317 adjustable voltage regulator is used a s a current regulator to limit current. Additionally the data sheet for the LM317 shows current limiter applications. That should serve as a start.

Ron

Just been to Maplin and bought one to do some tests tonight. Cheers

 

[audioguru]

Current does not simply blast out of a voltage regulator at full strength. Only a dead short draws as much current as is available.
The load draws only as much current as it needs. So you probably do not need a current regulator.

Your 7809 and 7805 voltage regulator ICs have good regulation up to 1A but limit the max current to about 2.2A. They limit the dead short current to about 750mA for the 5V regulator and to about 425mA for the 9V regulator.

 

[beakie]

Current does not simply blast out of a voltage regulator at full strength. Only a dead short draws as much current as is available.
The load draws only as much current as it needs. So you probably do not need a current regulator.

Your 7809 and 7805 voltage regulator ICs have good regulation up to 1A but limit the max current to about 2.2A. They limit the dead short current to about 750mA for the 5V regulator and to about 425mA for the 9V regulator.

OK, I will give it some tests tonight! Thank you. It's a real benefit having you regulars available to answer questions like this. I don't know anyone that does this stuff and the internet can often confuse instead of help... As ever, you're posts are appreciated.


Thanks!

 

[Mr RB]

Sor simple PSU apps like this I use two regulators in series, the first LM317 needs just 1 resistor to work as a current limiter, and the second (voltage) regulator can be a simple 7805.

You will need about 5 volts overhead for the LM317 plus 3 volts overhead for the 7805, so to get 5v out you need > 13v input at all times.

 

[beakie]

Sor simple PSU apps like this I use two regulators in series, the first LM317 needs just 1 resistor to work as a current limiter, and the second (voltage) regulator can be a simple 7805.

You will need about 5 volts overhead for the LM317 plus 3 volts overhead for the 7805, so to get 5v out you need > 13v input at all times.

OK, so if I want both 9V and 5V I would need to put an LM317 before a 7805/7809 (on switch).

I wouldn't get away with just adding the LM317 into this circuit because I would need more overhead?

 

[audioguru]

The 7809 regulator needs an input of more than 11V for good regulation, maybe 11.5V.
The LM317 current regulator needs more than 2V, maybe 2.5V for good regulation plus 1.25V for the regulation resistor. So the input must be typically 15.25V or more for lesser spec'd devices.

Why do you need current regulation from an LM317? A load draws only as much current as it needs. The 7809 and 7805 limit the current.

 

[beakie]

A load draws only as much current as it needs.

I am having trouble absorbing this :-/

Can you give me a basic explanation perhaps? If I had an LED attached to a voltage regulator (lets say this one... **broken link removed**), what current would it draw???


Cheers

 

[Nigel Goodwin]

I am having trouble absorbing this :-/

If you buy a bottle of whisky do you drink it all and fall down , hopefully not, you drink at much or as little as you want.

Same thing really - a circuit only takes what it's designed to take.

Can you give me a basic explanation perhaps? If I had an LED attached to a voltage regulator (lets say this one... **broken link removed**), what current would it draw???

It would destroy itself - you MUST feed an LED from a current source, a simple resistor is all that's needed - then apply ohms law.

 

[beakie]

If you buy a bottle of whisky do you drink it all and fall down , hopefully not, you drink at much or as little as you want.

I drink the first 3/4 of a bottle with friends and then put the last bit into my "secret dregs" collection for drinking alone. It means I have up to 10 different whiskeys at hand any time I fancy it. It's a good technique for having multiple bottles available at once whilst still allowing me to blow most (if not all) of a bottle with a mate

It would destroy itself - you MUST feed an LED from a current source, a simple resistor is all that's needed - then apply ohms law.

Because I know the resistance and voltage... and therefore I can tell the current. I get ya THANKS!

 

[audioguru]

Don't buy No-Name-Brand LEDs that do not have a detailed datasheet.
They don't reveal the angle of the beam for those cheap LEDs. It is probably focussed into a very narrow beam to make the brightness number high but then it makes only a bright dot of light which is useless. Its quality is probably poor so it might not last very long.

 

[beakie]

Don't buy No-Name-Brand LEDs that do not have a detailed datasheet.
They don't reveal the angle of the beam for those cheap LEDs. It is probably focussed into a very narrow beam to make the brightness number high but then it makes only a bright dot of light which is useless. Its quality is probably poor so it might not last very long.

It was only a hypothetical LED.

I need it to light my hypothetical xmas tree in my hypothetical house

 

[#12]

Dear Beakie, I just found "Ohm's Law for Beginners". Go to the top of this page and click on "theory". The third item on the page breaks this current thing down to very simple terms...if that's what you need.

 

[beakie]

Dear Beakie, I just found "Ohm's Law for Beginners". Go to the top of this page and click on "theory". The third item on the page breaks this current thing down to very simple terms...if that's what you need.

I understand ohms law... I just wondered about voltage regulators. Thanks though...

 

[MikeMl]

I understand ohms law... I just wondered about voltage regulators. Thanks though...

If you understood Ohms Law, you would not have even started this thread. A bit harsh perhaps, but true.

 

[beakie]

If you understood Ohms Law, you would not have even started this thread. A bit harsh perhaps, but true.

I guess there are different levels of understanding... and perhaps I am not at the level of understanding that maybe I needed to be to understand voltage regulators.

On a basic level... I think I do/did understand ohms law, even if not the consequences of it