Darlington Pair in TO92

Discussion in 'General Electronics Chat' started by tracecom, Feb 17, 2011.

  1. tracecom

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    I want to drive a unipolar stepper motor from a PICAXE. I have the circuit working via a ULN2803A, but the space I have available won't readily accommodate an 18-pin dip. In addition, I only need four darlington pairs, so I am thinking about using four darlington pair IC's in TO92 packages.

    I have attached a schematic of one pair from a ULN2803A. Does anyone know of a TO92 package that is equivalent to one of the pairs in a ULN2803A?

    Thanks.
     
  2. Bernard

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    Gheck specs on MPSA13 or 14.
     
  3. Wendy

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  4. SgtWookie

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    Keep in mind that TO92 packages have a maximum power dissipation rating of 625mW, and Darlington transitors have a high Vce compared to typical bjt and MOSFETs; Vce starts at around 0.7v.

    You really should tell us what voltage and current your stepper requires.

    Note that there are no built-in protection diodes in the MPSA13/MPSA14; you will have to provide external fast recovery diodes or the transistors will get zapped.
     
  5. russ_hensel

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    There is a 16 pin version if 2 pins help. Has 7 darlingtons.
     
  6. tracecom

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    I am using one of these (AXE021 project board.)

    [​IMG]

    I can fit the ULN2803A on the PCB, but I will have to modify the tracks on the bottom. Still, that's probably easier than trying to source TO92 replacements. Thanks to all for the input.
     
  7. SgtWookie

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    Rather than chopping up the board, you can bend up or clip off the pins for the channels you won't be using; for that matter you could even flatten out pins 5 thru 9 so they stick straight out from the side; then you could insert the input side of the driver vertically.

    Make sure that you keep pins 9 and 10, as 9 is the ground (common) and pin 10 needs to connect to your stepper +V supply.
     
  8. tracecom

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    I will look at the possibilities you suggest about leaving some of the pins unconnected since I actually don't need pins 5, 6, 7, 8, 11, 12, 13, or 14. It might free up some pads and board space.

    The major mod that I am going to have to make is because there are only 9 rows of pads on the board, so I will have to use all 9 rows to accommodate the 18 pin IC. The row at the top (in the pic) is connected to +5 and the row on the bottom is connected to ground. That means I have to isolate pins 1, 9, 10, and 18 from the busses. In addition, I need to keep the input side (pins 1-9) of the ULN2803A facing the PICAXE. Furthermore, I need to locate space on the board for a LM7805 regulator and the associated caps. I may have to piggyback the regulator circuit on a separate piece of perfboard.

    As I think about it, it might be easier just to put the whole thing on perfboard. Or solder to the ULN pins dead bug style. I am going to take a good look at it tomorrow and figure the best way to do it.

    Or maybe somebody wants to make a PCB for me? Just kidding. :)

    Thanks.
     
  9. SgtWookie

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    Ahh, wait a minute.

    Instead, don't use pins 1 thru 4 or 15 thru 18; just:
    5 through 8 for the inputs
    9 to ground
    10 to stepper +V
    11 through 14 for the outputs (stepper current sink).
     
  10. tracecom

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    I had actually decided after logging off last night to build on your earlier suggestion and do exactly that. I am going to mount a 14 pin socket on the board, turn up (or cut off) pins 1, 2, 17, and 18, and plug the remaining pins in the socket. Therefore, there will be no modifications required to the PCB. The ULN2803A will hang over the end of the socket of course, but I think it won't extend past the perimeter of the PCB.

    I am going to place the ULN2803A socket at the end of the board and I think I can then get the regulator circuit on the center section between the PICAXE and the Darlington array. Instead of an LM7805 (which I only have in T0220), I have an LM317 in the TO92 package and I will see if that's a better fit.

    Thanks for the help. I'll post pics if it works out.
     
  11. tracecom

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    Does anyone see any problems with the attached schematic?

    P.S. I am posting it on the PICAXE forum as well.

    Thanks.
     
  12. SgtWookie

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    You'll only get about 4.7v out of the LM317L.

    Change R3 to 360 Ohms; that should give you 5v.

    The LM317L only requires 5mA current from it's output in order to regulate properly, so you can use 220 or 240 Ohms for R3, then re-calculate R4. For example, if R3=220 Ohms, then R4=660 Ohms; 680 Ohms is the closest standard value.

    Reduce C4 to 10uF.
    Add a 100uF cap across pins 9 and 10 of the ULN2803.
     
  13. tracecom

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    Thanks for the help. I made the changes you suggested above. The voltage turned out a little high (5.17v) but it's below the PICAXE max of 5.5V.

    Attached is a photo and final schematic. (I actually forgot D1, so I deleted it from the schematic.)
     
    Last edited: Feb 23, 2011
  14. SgtWookie

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    You can reduce the voltage closer to 5v by using a ~15k fixed resistor in parallel with the 680 Ohm resistor if you'd like.

    Unfortunately, that forum requires people to register and log in to view attachments. Please attach the images to your post instead.
     
  15. tracecom

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    Images added to post 13.
     
  16. SgtWookie

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    Looks good, guy :)
    The socket for the ULN was a good idea. Glad you got it working.
     
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