How to make an adjustable constant current source at moderately high current?

electronics
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(Calum Nicoll) #1

I have a 30V 30A DC source and would like to adapt this for TIG welding thin stuff. To do this I need to be able to set the current to a constant value. What’s the best way to accomplish this? It will only be running for a minute at a time then I can let it cool down for a while. I only have experience with using constant current IC’s at say a couple of amps for battery charging and am thinking there is probably a better/clever method when at 30 amps…

I’d also like, further down the line, the ability to vary the current fairly quickly with a microcontroller.

Thoughts?
Thanks very much!


(joeatkin2) #2

The simplest way :

A big resistor

Almost as simple

A pair of 2n3055 will do it if they are on a big lump of metal

You might need more will have to look at the secondary brake down curve .

Or less simp8 a igbt and a swiching supply

You are going to need a way to start it


(joeatkin2) #3

Took a look at the datasheet it will 8 2n3055s

But they are cheap


(Calum Nicoll) #4

By big resistor you mean massive compared to the load? I.e if I’m wanting 30A out and lets say load is varying between 0.5 and 1 ohm I don’t get how this could be accomplished with a big resistor? Unless you mean big enough that it’s resistance is much greater than the load’s? But then it’s going to be needing huge amount of power?

Also don’t entirely follow how to do it with 2n3055’s?

Thanks


(Calum Nicoll) #5

Is this in the right kind of idea? http://www.radio-electronics.com/info/circuits/transistor/active-constant-current-source.php


(joeatkin2) #6

That sort of thing, but you are going to need a tripleington or a compound triple.

The 2n3055 is old and low spec by today’s standards but very cheap and tuff as old boots plus it’s in a to3 case so thermal transfer is fantastic


(joeatkin2) #7

And yes I was talking about in 1r 1kw resistor

Easy enough to make with enough small ones in parallel in a paint tin full of oil


(joeatkin2) #8

If you are planning to make more than 1 the only sain way is a swiching regular, but they can be really difficult to get working but would be better in every other way .


(Calum Nicoll) #9

But a 1 ohm 1kW resistor wouldn’t make current constant? In my mind you’d need at least 20 ohm or so in series with load to be reasonably limiting (unless I’m missing something obvious?) but then you’d be dropping 20kW in your bucket of oil…


(Calum Nicoll) #10

Yeah I agree - there was a limited range that I could find that could switch that kind of power, the LT1339 looked pretty promising but the design tools are a bit lacking compared to other switching regulators and then I saw this…! http://www.analog.com/media/en/technical-documentation/application-notes/an73f.pdf

I like the idea of doing a bunch of transistors, 5W dropped or so in each transistor should be managable…


(joeatkin2) #11

30v x 30a = 900w

Yes a 1r resistor is a very poor aproxamation


(joeatkin2) #12

If you use 10 x 2n3055 's it’s 90w each , do abule with a fan , but still a heater buy any other name.

I think that a switching supply supply , you are going to be quite good at that sort of thing when you get finished if you go that way


(joeatkin2) #13

Remember that you are going to need a way to start it


(Calum Nicoll) #14

By to start it you mean to start it outputting 30A or you mean to start the arc?

As if you mean to start the arc my plan is to make a lift tig start - i.e start at v low constant current when touching the workpiece (low R, low volt) then when torch is lifted and voltage rises to keep constant current as electrode removed from workpiece - then rapidly increase current output further.

The mechanics of how to make a switch regulator have a changable current output are puzzling me though - I suspect there is an easier/better way but ATMO all I can think of is to emulate the sensing resistor inputs to Sense+ and Sense- with a microcontroller while similtaneously measuring the current with the microcontroller and an actual sensing resistor - thus can read the actual current and scale it accordingly to get your chosen output current. But switch regulator at 150khz is not going to be happy if i mess this.

What do you recommend for easy current changing of switch regulator? I strongly suspect there is the equivalent of an electrically controlled variable resistor that I could use as Rsense but I don’t know what the part is called?? Also must handle 30 amps…

Cheers


(Calum Nicoll) #15

Could I just use a mosfet instead of Rsense? And change Rds by just applying different voltages to it.


(Calum Nicoll) #16

Also if you have any ideas on how to get good at switching regulators other than just building them I’m listening - have made 4 circuits with them that worked but have very much just followed the instructions and would be interested to hear your advice on how to learn…


(joeatkin2) #17

Call me


(joeatkin2) #18

No is the simple answer.