current source circuit with clamped voltage 5v

[leongch]

hi... i would need a constant current source circuit with clamped voltage 5v... any1 can provide me the info? thx

 

[Tohu]

Hi,
for what type of load grounded or not, reactive or active? What range of curent, and fixed current or floath, and if the range is floath what acuracy in %. Do you look for high eficient schem or any one is suitable for you.

 

[VVV]

Here is an example of a sourcing current source, clamped at 5V.
The output current is 20mA, fixed. To adjust it, you can change R1:

Iout=Vref/R1, where Vref=2.5V

The output is clamped by the precision clamp IC1B, D1. For the clamp to work, the current should be maintained below 20mA. For higher currents, you would need another clamp circuit.

The components are not critical, but R1 should be an RN55 or RN60 type resistor, for better temperature stability.
You can also use a better reference, if you wish better temperature stability.
The MOSFET can be pretty much any P-ch MOSFET, as long as the power dissipation is not exceeded (with the output shorted the MOSFET will dissipate 9.5V*20mA=0.19W).

 

[leongch]

how does the circuits works?Can brief a bit? and how to get the Vref? thx

 

[Miguel Gaspar]

IC1B clamps the voltage to 5V, when voltage at output tends to go above 5 V, D1 activates and then it converts in a voltage follower.

IC1A is a current source.

Vref you put a resistor and a zener diode, anode to ground, cathode to the resistor, the other lead of the resistor to the voltage source. You can take Vref from the cathode.