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Propeller + ULN2803 question (schematic) — Parallax Forums

Propeller + ULN2803 question (schematic)

icecuberydericecuberyder Posts: 6
edited 2011-12-01 09:14 in Propeller 1
Hi All!

I'm going to need to drive up to 16 led strips from a single prop.
the previous design was based on transistor, but apparently 1 shorted on me and killed the prop, so I'm thinking about going to use ULN2803's instead as they seem to offer a bit more protection.

So, basically the question is, can the prop drive the 2803?
Input voltage is 12v and the average load is about 50mA per channel, with 5 being active at the same time at the most. (~250mA)
I decided on the 2803 so I can use 2 chips for the 16 outputs instead of 3 2003's

This is my initial board design.
please feel free to call out any mistakes if you see any.

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Comments

  • RaymanRayman Posts: 14,850
    edited 2011-11-29 13:12
    I think that should work fine. I did something similar for my PropRGB 8x8xRGB LED matrix board...
  • PaulPaul Posts: 263
    edited 2011-11-29 13:28
    Since all the LEDs should have a common anode to 12V you may want to add that as two more 8 pin sip connectors. (above where you have the LEDs.)

    Paul
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2011-11-29 15:09
    icecuberyder wrote: »
    Hi All!

    I'm going to need to drive up to 16 led strips from a single prop.
    the previous design was based on transistor, but apparently 1 shorted on me and killed the prop, so I'm thinking about going to use ULN2803's instead as they seem to offer a bit more protection.

    I'm rather curious as to how you had your transistors connected in that it ended up killing the Prop. If you had a suitable resistor from the Prop to the transistor base then this would have limited the current. The ULN2803 is a dumb transistor array and does not offer any "protection" that I know of. What does the ULN2803 seem to offer? I find that individual transistors are superior to transistor arrays and when implemented in SMD they take up less room and cost less too. Then ULN2803 does have a small advantage in that it's collector "diodes" allow it to drive small inductive loads (i.e. relays) without extra protection components.
  • RaymanRayman Posts: 14,850
    edited 2011-11-29 17:31
    ULN2803 does offer a lot of protection when driving inductive loads. May not be all that useful for LEDs though.
    But, it's much more convenient that 8 individual transistors and resistors...
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2011-11-29 20:45
    Resistors? No way......and what are 8 tiny little SOT-23 packs with built-in resistors at 6 cents each anyway, at least you can switch a total of 4A rather than the ULN2803 maximum of 800ma (or less). Anyway what prompted my reply was the premise that a ULN2803 could somehow offer more protection than individual transistors in the case of driving LEDS.
  • icecuberydericecuberyder Posts: 6
    edited 2011-11-30 06:59
    Thank you all.

    It appeared that the Prop was damaged because a wire got loose and hit the basepin of one of the transistor _before_ the protection resistor, thus putting the full 12v to the propeller and killing it.
    (and it was not the transistor that failed as I was initially led to believe)
    so when I mean 'protection', I basically mean that the prop and the ULN board would be enclosed so there would be "no user replaceable parts" to accidently break.

    As I am in Iceland and parts are (relatively) scarce and/or have limited selection, I would rather go with the ULN as a single chip costs about as much as 3 regular transistors (BC547 / 2N Series)
    (and SMD's IF available are much more expensive, if course I can order online, then I need to find a consistant seller that actually ships to Iceland, etc.)

    I also believe (for this particular project) that the ULN simplifies everything, including assembly.
  • RaymanRayman Posts: 14,850
    edited 2011-11-30 18:02
    Peter does raise a good point about why it is used for LEDs... That is listed in the datasheet as a target. But, I think there are really nice for inductive loads like relays, etc...
    So, why are they used for LEDs? They are usually not that inductive... My only guess is that the large size makes a good heatsink when PWM-ing LEDs.
  • Erik FriesenErik Friesen Posts: 1,071
    edited 2011-11-30 19:03
    The uln2803 includes built in resistors that work with 3.3v. Compared to transistors, its a good choice, its easy to wire up, handles 500ma, costs 3-10 cents per transistor, and has a nice footprint.
  • Jorge PJorge P Posts: 385
    edited 2011-12-01 08:46
    I have seen a stepper motor driver for the propeller using a ULN2803 someplace, its either buried in the forums or someplace on OBEX. You might be able to use that for your LED project.

    EDIT: Parallax sells these http://www.parallax.com/Store/Components/AllIntegratedCircuits/tabid/154/CategoryID/31/List/0/SortField/0/catpageindex/3/Level/a/ProductID/211/Default.aspx So I would assume you can drive them with the propeller.
  • NikosGNikosG Posts: 705
    edited 2011-12-01 09:14
    Look at the scematic in the following thread and replace the stepper motor phase pins with the leds positive pins. It will work!
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