Vary the output current in 7805

[rafiq]

in7805

Hi,

I need to use 7805 regulator. The output going to a connector.
But I need to vary the current from 0.05A to 0.95 A.
If I use a potentiometer in series, Can I vary the current or not?
The Load resistance is 3.6 ohm.

Thanks & Regards,
Rafiq

 

[IanP]

7805 current source

7805 can be used as current regulator ..
Just connect it as shown on the attached picture ..
7805 will try to keep 5V accross R1, so if you vary it the output current I[RL] will also vary ..

Rgds,
IanP

 

[rafiq]

typical output current of 7805 regulator

Hi IanP ,

Thanks for your reply. In 7805 pin 2 is connected to GND, then Vxx is always
will be 5V. Then how I will get the value for Io. How about the o/p voltage?
Will it be always 5V or not? ( I need 5V, but I want to vary the current only).
The Load I have to connect across the output (pin3) and the GND or not?
Actually the resistance of the Load is 3.6 ohms.

Regards,
Rafiq

 

[nguyennam]

quiescent current 7805

IanP gives you the good solution for your idea of sourcing the load of 3.6ohm with currents from xxx to xxxA.

It is funny that you want to adjust different current through a fixed load resistance but want to get load drop voltage constant at 5V??? Do you know the Ohm law?

 

[TekUT]

in 7805

Hi IanP ,

Thanks for your reply. In 7805 pin 2 is connected to GND, then Vxx is always
will be 5V. Then how I will get the value for Io. How about the o/p voltage?
Will it be always 5V or not? ( I need 5V, but I want to vary the current only).
The Load I have to connect across the output (pin3) and the GND or not?
Actually the resistance of the Load is 3.6 ohms.

Regards,
Rafiq

1. The 7805 act to regulate the voltage between output and reference pin, no matter about the reference was to 0 or not this is the reason why this circuit can be used as a current source
2. The minimum current value that you can reach with this type of connection can't be less than the quiescent current of the 7805 regulator Iq
3. output voltage and current must follow the Ohm's law then V=R*I, if you have a fixed resistor value and you like to change the current, then force a current trough the load, you must have a final voltage across the load resistor which is equal to R*I no matter about it. If you like to have a constant voltage with variable current you must have a variable load, or to better say, a electronic load able to change the equivalent resistance when the current is changing to achieve a constant 5 V or a value that you prefer. This can be easily done by using a MOS transistor and a current / voltage sensing circuit and a control circuit which act in order to change the MOS conduction state to keep the voltage across the MOS at your desired value, regarding the current that are flowing trough the MOS itself.