Anyone know of a circuit (or component) to regulate my 300 vdc to 1 mA as the load decreases? I am doing electrolysis and the load decreases as the process continues, due to ion formation.

 

Here is a very simple one. Use 4 or 500 volt transistors.

http://www.merghart.com/p/27/Two-Transistor-laser-current-source

 

Here is a very simple one. Use 4 or 500 volt transistors.

http://www.merghart.com/p/27/Two-Transistor-laser-current-source

OK. Source is 300 vdc, R1 is 10 kΩ, R2 depends on what I want to limit current to, the transistors are like BC547 and BD139 except they will be 500 volts. Did I miss anything? Thanks!

 

You forgot power dissipation. At 300V a 10k ohm resistor will draw 30mA and dissipate 9W so you will want to increase R2's value. Since you only need 1mA, you could increase R2 to about 10meg ohm or so.

 

You forgot power dissipation. At 300V a 10k ohm resistor will draw 30mA and dissipate 9W so you will want to increase R2's value. Since you only need 1mA, you could increase R2 to about 10meg ohm or so.

So to confirm:
R1 stays at 10k ohm.
R2 should be 10 meg ohm.
This limits current to 1 mA with 300 vdc source.
What wattage should R1 and R2 be?

 

I think the resistor ref-#s got swapped: R1 is the larger one. 10 megohms sounds good and will dissipate .009 Watts
R2 sets the current by R2=600/current_in_mA (approx 600 ohms) and dissipates .001*600 = .6 Watts

 

Yes, I meant R1 of course.

Note that the 600 ohm R2 emitter resistor will dissipate .001^2 * 600 = .6mW.

 

Yes, I meant R1 of course.

Note that the 600 ohm R2 emitter resistor will dissipate .001^2 * 600 = .6mW.

Oh gosh..... right back atcha, crutschow.

 

Thanks! I'll let you know how it works.