78XX series regulator as current regulator question

[bigal_scorpio]

Hi to all,

Just a small query about the 78XX regulators used as a current regulator.

In the drawing below from the datasheet there is a parameter I don't understand "IQ" that is from pin 2 which is Gnd normally when using as a voltage regulator.

So what is this value and how is it found. BTW I know it means Quiescent Current and can be found in the datasheet to be between 4.1 and 6.2mA at 25c dependent on which 78XX, but if it always between these values why is it not shown as such in the diagram and if it needs to be calculated then how?

Sorry if this seems obvious to you but I have been looking blankly at this for half an hour and can only think it must be my own "IQ" thats at fault hehe..am I having one of my stupid days as the wife likes saying to me?

Thanks Al

 

[crutschow]

You just add the nominal Iq from the data sheet for the actual device you will using, to Vxx/R1, to get the total output current. Normally Iq is much less than typical output currents so it can be ignored.

For a constant current circuit with minimal voltage drop, use an LM317 in a constant current circuit as shown in the LM317 data sheet.

 

[bigal_scorpio]

Cheers mate, just wanted it cleared up.

I was planning on using the 78XX series as I have a good few of them and no 317s.

Is there much difference in using either one, if there is I will get some 317s.

Thanks Al

 

[mvs sarma]

RL is the output load and not at the point you defined as output. Actual current output depends on R1, as far as input voltage can support.

 

[colin55]

The only difference is 78XX goes down to 5v etc and 317 goes down to 1.2v Some 317's handle 1.5A

 

[mvs sarma]

78T05 or 78T12 deliver almost 2 amps load.

 

[bigal_scorpio]

RL is the output load and not at the point you defined as output. Actual current output depends on R1, as far as input voltage can support.

Hi Sarma,

I didn't define anything, that is a shot from the actual 78XX datasheet. Is it wrong? If so where should the point marked as output go?

Thanks Al

 

[mvs sarma]

I saw that you adopted the element from Fairchild datasheets. BUT in a current regulator, there is no output voltage. The output is connected to RL and grounded for returning to common negative. I attached a schematic as shown by ON semicondutors (formerly Motorola.)

 

[Mr RB]

That's right, RL IS the load.

It's the same 1-resistor constant current regulator as the LM317 version but the LM7805 will have a minimum voltage overhead (waste) of about 6v where the LM317 will only have an overhead of about 3v.

I have a big bag of LM7805 and have used them for LED constant current drivers, simple things like to drive a 1W LED (3v 330mA) from a 12v battery.

 

[mvs sarma]

In either case the input/ output differential added to drop across the Rsense is the loss part of it.
as such the difference only in Rsenese-- for LM317 it is 1.2V and for LM78 xx it is XX volts
if one can use LDO regulator( of course for lower load currents) it can reduce the i/o differential to a large extent.

perhaps it is better to use two transistors and manage the circuit shown below.

 

[bigal_scorpio]

Hi Guys,

Thanks for clearing that up, I will be deleting my fairchild DS in favour of the On semi one!

Thanks Al

 

[Boncuk]

I simulated the LM7805 as constant current source.

As the current varies with input voltage I think it would be better to use an LM317T. For currents of 1mA to 100mA you might chose an LM317L.

Boncuk