Hello dear Experts ,
I have a project , that I should measure current and make regulation that this current must not exceed 500 mA , can you give some propositions , well my circuit is as you see in attachement below.
Thanks you
Hajar

 

Maximum current is limited by the resistor value. R = E/0.5A.

 

1) what frequency (for 1 GHz it will be extra sophisticated task indeed), what voltage (at 50 kV it ought be shifted to gnd side), what means regulation (with hand looking on ammeter or self -keeping thus mean the constant current source?
2) if that is ordinary 50 Hz, then just small resistor, or piece of steel nail in series with load, or to calibrated `shunt-type current sensor`. Let opamap reads it. If ca 100 kHz converter, then better a ferrite core with one turn of mainline and some 60-100 turns of thin wire. The reading may be organized via optron if will. If RF or microwaves, there exist a Holl sensors readable by operational.
3) the regulation, if mean a non-hand way, has plethora of possibilities. One is transistor with TL431 in between gate and source or base and emitter. Both equally good. Other is second bjt instead of TL431, reading the shunt resistor. Third is pair of diodes instead of it, but then systemic K(u) will be smaller thus the accuracy. The load may be switched as into collector as well at emitter below the sensor.
4) the Wilson current mirror is highly effective, BUT at my case it was helpless as my load must have common negative thus the Wilsson must be organized on pnp what has no way to obtain a 600V qualified bjt~s.
5) The high voltage makes always problem of high thermal power on regulating element. Thus, often better is to apply a MC34063 realizing or step-up or step-down process, thus the thermal sink may be effectively diminished. But the circuit becomes more sophisticated and capricious. How looks the methods, see

Wilsson: https://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-constant-current-source/

Two diodes https://en.wikipedia.org/wiki/Current_source#/media/File:Const_cur_src_112.svg

With TL431 https://electronics.stackexchange.com/questions/107699/tl431-constant-current-source

with second bjt https://en.wikipedia.org/wiki/Current_source#/media/File:Current_Limiter_NPN.PNG

Exists even the very nice circuit exploiting the LM317, but it isnt appropriate for very high voltage aim. https://en.wikipedia.org/wiki/Current_source#/media/File:LM317_1A_ConstCurrent.svg

 

Do you want to charge a capacitor with a contant current of 500mA?
No current source will let you charge to full Evolts.
You will always drop a few volts in the current source circuit.