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Supply Voltage +-15 V to +-24 V Goal:To have an adjustable single low voltage output (-0.5 V to 0.5 V ) from dual supply voltage.

What I want to do is to have dual voltage output which will be connected to potentiometer for having small voltage output (output 0 - 0.4 V)

I have designed with LM317 and LM337, but I want to try with LM7805 and 7905. Simply I will have 5V output for 7805 and -5V output for 7905. However the output current is low. Therefore I should somehow increase the output currents. I've seen somewhere that it can be done with external transistor but I did not understand exactly how. Could you please give me any ideas?

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I don't get why you would want a 7805/7905 pair when you need 0..0.4V output? You state that the output current is too low. Which output current do you need? The 7805 current booster circuit can be found in the typical 7805 datasheet, for instance here on page 21, bottom left. Note that in its simple form this circuit has no current limit! – Wouter van Ooijen Jul 4 '12 at 10:01
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I'm under the impression that you are mixing up things: "Inreasing the output current", "single low voltage output (-0.5 to 0.5 V )", "small voltage output (output 0-0.4 V)", "have 5V output for 7805 and -5V output for 7905", "output current is low". How much do you want to increase current, what is your requirement? – jippie Jul 4 '12 at 11:24
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What current do you need? Why do you want to try another way (Was there some problem with the 317/337 way, or maybe just for fun?). Do you want to use the +-15V as input, or teh +-24V? – Wouter van Ooijen Jul 4 '12 at 12:46
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Certainly not without you telling me how much current you will need! – Wouter van Ooijen Jul 4 '12 at 13:12
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Sorry, that does not compute. 2.5V over 10 kOhm results in 0.25 milli ampere, which asks for a very different approach (a simple ompamp would do) than 2.5 ampere. But you wording and context gives me the idea that we don't understand each other. You want a +/- 0.5V range, yet you talk a bout a 2.5V drop? – Wouter van Ooijen Jul 4 '12 at 13:40
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closed as not a real question by W5VO♦ Nov 20 '12 at 4:47

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, see the FAQ.

1 Answer

up vote 4 down vote

Increasing the output current of a voltage regulator can be done with an outboard transistor.

     +--------     ------------+
     |        \>__/            |
     |          |              |
IN --+---/\/\/--+----[ REG ]---+-- OUT
          Rsense        |
                        |
                       ---

When current flowing across the small-valued current-sensing resistor develops a voltage greater than 0.7V, the outboard PNP transistor turns on and begins to shunt extra current around the regulator.

The resistor is chosen such that the transistor starts to turn on before the regulator's current limit is reached. For instance, for 1A, it would be around 0.7 ohms.

There is no upper limit on this current. Whereas the regulator protects itself, the outboard transistor has no current limiting. That can be added with more components. Another transistor circuit with an even smaller current sensing resistor can kick in at some upper current value, and redirect current back into the regulator, thereby triggering its shutdown mechanism.

enter image description here

For instance if Q1 cannot handle much over 10A (with the heatsink we put on it and all) Rtiny can be chosen with a 0.07 ohm value. When 10A passes through it, Q2 is turned on, and starts to pass collector current into the regulator which takes care of the rest.

For the negative voltage regulator, the circuit is similar, but with NPN transistors rather than PNP.

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are you still on a z-80 or a commadore 64 text only display? somebody donate this poor guy a computer with GUI system. – sandun dhammika Nov 18 '12 at 17:31
@sandun: Yeah, the ASCII art was reasonbly well done, but it's still ugly ASCII art nonetheless. However, +1 anyway for a good explanation. – Olin Lathrop Nov 18 '12 at 18:22
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I'm a software developer, so everything I do is ASCII (or, increasingly, UTF-8 nowadays). This was not worth whipping out an EDA and go through all the hassles of exporting an image, cropping, uploading ... Anyway, someone with a Commodore 64 is technologically way ahead of someone cobbing together a small voltage supply from a larger voltage regulator with outboard transistors. :) – Kaz Nov 18 '12 at 18:59
I made graphical version of the second schematic. Phew, a lot of trouble. TinyCad's export makes all vector graphics one pixel wide, regardless of scale. I exported at 400%, and then in GIMP, I turned the image from indexed to RGB, added an alpha channel, turning the white color transparent, added a white background layer, and then duplicated the image over itself at various offsets to crudely thicken up all the one-pixel-wide curves and lines. That was followed by shrinking to 25%, resulting in a small, smooth image. All in all, more time consuming than the ASCII diagram. – Kaz Nov 20 '12 at 5:33

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