current and voltage limited source

[Hamilton]

hi, new here, i study in physic but i feel i know nothing of practical electronics.

I have been reading electronics for some time now and i can't find a solution to my problem.

i want to create a steady (almost) current source that have a limit in voltage below 20V that can give up to 20 amp. the voltage limit should be very fast responding as i don't want voltage pick (never) for security reason (don't want to get killed because my supply can instantaneously supply kV because it is a constant current source.

The current should be made variable with a simple analogical Potentiometer.

I found lot of drawing of constant current source but they all have very dissipating solution where you dissipate everything except what you want. It would be much better if the power dissipated is low and the source adjust itself so most of the power that enter in it get use for productive work.

The said productive work would be electrolysis etching of metal plate and other electrolysis.

The source should also behave smoothly in the 0 Ohm load limit by keeping the max current limit running or by turning off fast enough so nothing is broken/ killed


thx everyone!

 

[bychon]

A voltage limited current supply is so easy that I was doing that with one chip, 35 years ago. The problem in your request is to have low losses. That complicates the circuit...a lot. If you can be satisfied with the wasted power, the chip to look at is the LM723.

In other matters, you are mistaken about the kilovolts. A constant current supply can only deliver the volts it has available. It does not magically make kilovolts.

 

[Hero999]

In other matters, you are mistaken about the kilovolts. A constant current supply can only deliver the volts it has available. It does not magically make kilovolts.

You're right of course but he's probably studied real constant current sources i.e. Van de graaff generators which do give kV not solid state constant voltage to constant current converters.

Yes, you can't expect low losses from a linear regulator but they can be minimised by using a centre tapped transformer and taking the power from the tap when the output voltage drops below a certain level.

 

[Hamilton]

can you guys help me find literature or already made drawing i can study and modify to my need? or make it all and explain it to me (if you have freaking high motivation),

I sure want to learn from this as i will be making a Jacob ladder power supply and dot soldering power supply in the near future. And finally, if all go well, a CO2 or copper vapor discharge laser. But for now i'm just too uninstructed too make a good, simple power supply

thx for all the help.

The reason i talk about very high voltage is simply mathematically, as a perfect current supply always supply one set current and from the simple V=RI law we can find that for a big R, to keep I constant V must get very high also. Simple linear equation solution habit without thinking enough.

 

[bychon]

Go to Mouser Electronics - Electronic Component Distributor or www.nationalsemiconductor and look up LM723. That chip has lots of example circuits provided in many places. You can study that chip and find the ideas you need to recombine and arrive at your goal.

 

[Hero999]

The reason i talk about very high voltage is simply mathematically, as a perfect current supply always supply one set current and from the simple V=RI law we can find that for a big R, to keep I constant V must get very high also. Simple linear equation solution habit without thinking enough.

An electronic constant current source won't do that.

It's connected to a constant voltage source, an error amplifier is used to keep the current through the load the same using a sense resistor, if the load is disconnected, the maximum voltage is equal to the power supply.

Here's an example of such a circuit. The error amplfier, keeps the voltage across the sense resistor equal to the voltage across the zener and because I = V/R the current remains constant.
**broken link removed**

To be honest, I think you should go with something with a lower current output. 20A is too high for a beginner.

The suggestion of using an LM723 was a good one. Many circuits can be found using your favourite search engine.

Another option is to build a smaller 2A power supply with the L200VC, then when you've got it working make a 20A PSU.

**broken link removed**

 

[Hamilton]

you may be right, i should make a smaller version, since i need at least 6 A, i will create a 10 A variable regulator witch, hopefully, is still in the newb range ;-). or a fixed 6 A regulator.

here i found some information on a 10 A regulator witch is fixed i think



sadly there is no simple equation for the current at the end given. so i can't figure out what should i change to get 6 A or a variable current one. i presume it is the Amp that force the current to be fixed.

what do you think ?

 

[Hero999]

THat will work as long as you're aware that it has a minimum load requirement of 100mA, which means if the load connected to it doesn't draw at least 100mA, the output voltage will rise beyond the set level, although not to sever kV.