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High power adjustable current regulator

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crutschow

Well-Known Member
Most Helpful Member
I'm just wondering if the circuit would perform better with a transistor instead of the regulator and would also be cheaper ?
The regulator has the big advantage of being short-circuit and over temperature protected. The transistor has neither.
 

Hero999

Banned
Transistors also need a lot more drive current.
 

yngndrw

New Member
Ah I see, cheers for explaining that.

Regarding using the el'cheapo voltage reference: What startup and shutdown conditions would that cause ? Would it cause any overshoot at startup and shut down ? I can see how it would work well when everything is in it's stable state but I'm unsure about other states - I also want to be able to use this on laser diodes which are very delicate.

Is there any reason that I'm missing why I couldn't use 10K resistors instead of the 100K resistors ? I have lots of 10K's ...
 

Hero999

Banned
The shut down just protects the regulator from getting damaged and only happens if it overheats.

You could easilly use 10k resistors, it doesn't matter as long as they're all the same value.
 

yngndrw

New Member
Thanks to all of your help, I now have my final circuit. I'll order the parts soon and build it on stripboard.

I may switch the 82K resistor with a 22-turn 100K potentiometer in order to get the maximum out of this. (With the 82K it will be limited to about 1.35A.)

Thanks again for your help guys. :)
 

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Hero999

Banned
There's no reason why you can't do that.
 

jmgelba

New Member
Hi folks,

I'm building something similar to this only using 3x 1084's in parallel. In the last example attachment I see the 1.25v reference. Is this connected to a 1.25v reference IC not in the schematic, or is it just a labeling of one net and they are to be connected together?

Thanks.
 

yngndrw

New Member
It's just a label and they are all connected together.

The generation of the 1.25V reference voltage relies on the fact that the LM317 will always try and keep the difference between its output and its reference input as 1.25V.
 

jmgelba

New Member
Ok, so I can remove this 1.25v reference I have thats stopping this thing from working!

Did you complete yours? Working ok?

I've got 15V in and 9.3V at 4.5A out, with a pot for max current and a pot for 0 - 100% of that max current.
 

yngndrw

New Member
I never got a chance to build mine, ordered the parts just didn't have the time to put it all together. Will do when I get the chance though.
 

Mr RB

Well-Known Member
Well I don't mean to sound cocky but you can do it with one transistor, 1 resistor and 1 pot;

Code:
+12v  ------>|-------,
(unreg)     LED      |
                     |
                     |
+5v  --------,       |
             |       |
             |       |  NPN (or Darl)
   500 ohm   R       C  TIP122 etc
      pot    R<----B    
             R       E
             |       |
             |       |
             |     Rload  4.7 ohms 5w
             |       |    (etc)
             |       |
Gnd ---------*-------*--
This is a constant-current dummy load, adjustable with the pot from 0A to about 1A, depending if you use 3.9 ohm or 4.7 ohm for Rload.

It will drift 1 or 2 percent as the transistor warms up, which is rarely an issue.
 

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Hero999

Banned
I would use a switching regulator for this, it seems silly to buy expensive efficient LEDs, then waste a load of power in a linear regulator.
 
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