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| LM334 constant current source "inverse" design? |
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| zdelarosa00:
Hello, I need some guidance for I am currently designing a constant current source for the following laser diode: OPV300 https://www.datasheetarchive.com/pdf/download.php?id=b08e595083910be444c389371b5b3171d8738d&type=M&term=opv300 And since it is going to be a fairly on/off/brightness control, I want to make a simple constant current source. I found some circuits online like this: http://www.bristolwatch.com/ccs/LM334.jpg And every other "lm334 constant current source circuit" has this configuration, since the LM334 pulls the current reference from V+ to V-, they all use PNP transistors (1st attachement), so, my question is, since every other example in the datasheet uses the same connection principle, can I "invert" the connection of this LM334 or would that mess stability/function? I came from the top of my head with the following examples... There is two points to take into account here, I want to know if I could use a NPN transistor first, and second, can I, similar to this circuit (2nd attachement) use at the end a MOSFET and if yes, is this a usable current-voltage translation (I know there are complete ICs and OpAmp circuits for current-voltage converters, but I know if this simple setup would work, why yes or why not). The thing here is just have a reliable current source, not an instrumentation grade military specification circit. Am I making sense here? Thanks in advance for your attention. :-DMM :horse: |
| Zero999:
The circuit you linked to is a bad idea. It depends on the Hfe of the transistor, which can very a lot, depending on the transistor and temperature. The same result could be achieve with an NPN transistor, by connecting the base to +V, via the LM334 current regulator and the laser diode between the collector and +V, with the emitter at 0V. The one with the MOSFET is equally bad. It'll just act as a source follower. Just use the standard LM338 regulator circuit, with no external transistors or an LM317L circuit which doesn't vary with temperature, like the LM338 does. |
| zdelarosa00:
But how are the options where a PNP transistor a good idea if they are also dependen on hFE?... And how the LM338 regulator with an NPN different than the LM334 current source and a PNP? That is why I certainly am not getting it. |
| Zero999:
--- Quote from: zdelarosa00 on March 25, 2019, 03:51:22 pm ---But how are the options where a PNP transistor a good idea if they are also dependen on hFE?... --- End quote --- As I said, the circuit using a PNP transistor is not a good idea, because it depends on the hFE. --- Quote ---And how the LM338 regulator with an NPN different than the LM334 current source and a PNP? That is why I certainly am not getting it. --- End quote --- Which circuit? All of the circuits posted in this thread are rubbish. You're better off using a current source made with the LM317L. Refer to figure 30, section 8.2.15 on page 18 of the datasheet. Change R1 to 180R for 7mA. The load is the diode laser. http://www.ti.com/lit/ds/symlink/lm317l-n.pdf EDIT: Schematic attached. R1 = 1.25/7mA = 1.25/0.007 = 180R. |
| zdelarosa00:
Ok. I would go to that LM317 circuit if needed. Thanks. But I have another doubt... What if I use an OpAmp to amplify the current source? Is that more stable? And on another idea: Is it a good idea to put a transistor as active load at RSET in the LM317 adj resistor and change current trought there? |
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