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modifying a Velleman kit

Discussion in 'General Electronics Discussion' started by DennisJ, Mar 23, 2011.

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  1. DennisJ

    DennisJ

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    Mar 23, 2011
    I bought some K8063 kits from Velleman and I like the way I can program the 7-seg LEDs from my PC. Now I have acquired some larger digits, and I'd like to use the same PCBs to drive them, so that all displays can work using the same protocol. The new digits are 5'' tall and I think that they draw about 200 mA. (The old ones drew 120.) I'm wondering what mod I should do to the PCB to make this work. I don't have a lot of electronics knowledge and would appreciate any help. I do know that I would use a different power supply for every pair of digits, to keep current levels down in the PCBs. I tried to put the circuit dgm here, but I couldn't get it to print. It's at www.velleman-kit.com as part of the K8063 assembly manual. I'm really stuck here and would value suggestions. The kit runs at 12v, which is what my new digits require.
     
  2. shrtrnd

    shrtrnd

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    Jan 15, 2010
    I'd google the manufactuerer of the larger 7-Seg LED's. When you've got data on the LED's themselves, you can google for cirucits using them, then you'll find the most commonly used display drivers for them.
    With the data sheets on the displays, and data sheets on the display driver, you should be able to work it out.
     
    Last edited: Mar 24, 2011
  3. Resqueline

    Resqueline

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    Jul 31, 2009
    Ok, so the Velleman display kit draws max 120mA per digit, using 9V, 100 ohms, a PIC16C630, & a common anode display.
    That divides down to 16mA per segment plus 8mA for the decimal point. What I'm unsure about is the 100 ohm resistors which might set a 60mA segment current.
    The current depends on if there's more than one LED (in series) per segment.
    I'm unable to find datasheets on the '630. I wonder how much current it can sink. Can you check to see if the segments are pulsed, and maybe the voltage drops?
    What's the exact (comprehensive) data on the new display?
     
  4. DennisJ

    DennisJ

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    Mar 23, 2011
    Here is what I've been able to find out: The Velleman kit digits are said to draw 20mA per segment, and the power supply is supposed to be 9-12 Volts. I run mine with an input of 12 volts, but the Velleman digits are rated with a max forward voltage of 9.6, and I guess that's why the circuit board has a 9V voltage regulator. I think that the Velleman digits have 4 LEDs per segment. The larger digits are rated at 15-20mA per segment, and have 5 LEDs per segment, so I'm guessing it would need more like 20 than 15. The forward voltage is 13 V. They are not the same brand as the Velleman-supplied digits. I'm not sure how to check on whether the Velleman segments are pulsed; they're not multiplexed because there's a driver for each digit. All digits are common anode. So is the problem that the 9 V out of the Velleman boards won't drive the larger digits? The interest of both of you, and any others, is much appreciated.
     
  5. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Ok, it adds up then; 1.6V drop in the resistors and 1.8V for each LED chip, leaving 0.2V for the PIC output. No need for pulsing then.
    You can check for pulsed LED's by shaking them fast. Pulsing will appear as dotted lines of light.
    So, for the new digits; 5 * 1.8V = 9V. Running them from 12V - 9V - 0.2V leaves 2.8V for the resistors = 28mA. But it all depends very much on the forward voltage per LED.
    I suggest you can do a direct replacement, except running them off 12V instead of 9V, then check the voltage drop across the resistors to get to know the exact current.
    Adjust resistance or drive voltage values accordingly (if neccessary) to get the current within the rated range. The DP zener will need to be a couple volts (or more) higher.
    Yes, 5 chips in series requires 9-10V (+ the resistor stabilizing headroom) for full brightness, while 4 chips only requires 7-8V (+ the resistor stabilizing headroom).
     
  6. DennisJ

    DennisJ

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    Mar 23, 2011
    I just need to check that I understand your suggestion correctly. I take it to mean that I should take out the UA7809 voltage regulator, and replace it with a 7812, and then see if the board will run the new digit. Then I should check the voltage drop across one of the 100 ohm resistors? (Or did you mean to check the voltage drop across one of the segments?)(I presume that if it's too low I would have to replace the resitor(s) with one(s) of lower value.)

    By the way, to check the pin arrangement of the new digit(s), I did run one of them straight from the 12V supply, through a 560 ohm reisistor, and the various segments do light up, so they are functional. I will not be using the dp in my application. Thanks for your continued interest; I'm at the outer limits of my knowledge here!
     
  7. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    If you have a 15V PSU you must replace the 7809 with a 7812, but if you start out with a 12V regulated PSU then you just replace the 7809 with a piece of wire.
    The voltage drop across the 100 ohm resistor tells you the current (2.8V=28mA, 1.6V=16mA) and that is the most important thing.
    Even if the current turns out to be 28mA the display will be completely safe for at least 10 seconds (long enough to take a reading) and it won't burn out for a loong time.
    If the current is too high you increase the resistance value (f.ex. 100*28/16=175 ohms, use 180 ohms). Replace the numbers in the formula according to your finds/needs.
     
  8. DennisJ

    DennisJ

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    Mar 23, 2011
    Ok, here's a little more info. I checked out the voltage drop across the 100ohm resistor for segment "a" (top of a "7") for the original digit, and I get a drop of about 1.4V.( It's hard to be really accurate because my multitester is analog.) But when I test current across the same resistor I get about 30 mA. Doesn't this seem twice as high as would be expected?
     
  9. Resqueline

    Resqueline

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    Jul 31, 2009
    No, you're geting about 14mA. I'll bet if you had noticed it, the segment brightness increased (with 30mA) when you shorted out the resistor with the meter's mA range..
    You don't measure currents across current limiting resistors (or anything), you measure it in series with.. ;)
     
  10. DennisJ

    DennisJ

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    Mar 23, 2011
    Thanks for setting me straight on that...I'll try the suggested remedya soon as I can, but it may be a couple of days before I get the chance. Thanks so much for lending your expertise on this!
     
  11. DennisJ

    DennisJ

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    Mar 23, 2011
    Didn't think I'd have time today, but I did..it worked! Thank you so much! I did the voltage drop across the 100 ohm resistor, and it was about 1.2 V. That's spot on isn't it? I just wired through the VR terminals, but I think that I'll get some 12 volt ones in case I ever change the power supply. It's just great to see it work!
     
  12. DennisJ

    DennisJ

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    Mar 23, 2011
    I have a new problem with my Velleman display board. When I use a transformer putting out 12 V 1000 mA DC my system runs fine, with all digits bright. (The smaller ones are brighter, but they are better quality than the larger ones.) I need to use this where this is no plug-in for the transformer, so I bought a 12 V 7.0 A/h "gel cell", but the result is poor, as the larger dfigits are way too dim. I went back to the store and tried a 12 V 10.0 A/h battery, with the same result. Why is 12 V dc from a transformer OK but [email protected] v from a battery no good? the battery is fully charged... and I re-checked the plarity on both systems. Help would be really appreciated!
     
  13. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Did you ever measure the off-load (or even with the kit load?) voltage? I'm sure it's in the 14-16V range.
    An ordinary unregulated power supply delivers the stated voltage (+/- 10%) at the stated current. With no (or less) load it's substantially higher.
    A 12V SLA battery may deliver 14V off-load immediately after charging, but this soon drops to ~12.5V under load. The average discharge voltage is 12.0V.
    Did you end up running without a regulator on that kit?
     
  14. DennisJ

    DennisJ

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    Mar 23, 2011
    Sorry for the delay, I waited to get a digital meter. OK, you were right on the money.I tested in 3 states; no load, load comprised of the PCB circuits and the 7-seg LEDs, load of PCB circuits only. My transformer supply readings were 19V, 14.5V, 16.75V repectively. (I was amazed!) The gel cell ones were 12.87V, 12.6,V 12.7V . So it seems that I need more voltage than the gel cell provides to run this project . The PCBs for my small digits have 9V regulators on them; there are no regulators now on the PCBs for my large digits. What sort of battery should I head to, and should I be installing any sort of regulator on the large digits? Thanks for your continued interest!
     
  15. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    There are often more ways than one to "skin a cat".
    You could put three 6V SLA's in series (= 18V) and use a 15V regulator.
    You could use a four-cell Li-ion battery (= 14.4V), but these are more picky on the recharge procedure.
    You could perhaps still use the 12V SLA but instead reduce the segment resistor values to get the needed current.
     
  16. DennisJ

    DennisJ

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    Mar 23, 2011
    OK; thanks for the ideas. I don't really want to take it apart and replace 28 resistors, so I think for now I'll get a different battery. It will be a couple of weeks before I post again, as I'm going to be away all next week. If I replicate this scoreboard, I would consider the different resistor option. I'm thinking 75 or 80 ohms instead of the 100. Do you think that would be reasonable? Those 18V batteries that power drills and saws might work, and they come with a re-charger. Thanks for the help!
     
  17. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Last year it seems you measured 1.2V across the resistors, using a ~14.5V supply, so using a ~12.5V supply you might have to go way down low (22Ω?).
    You'll have to measure a set of voltages to get a basis for estimating what needs to be changed. It would also be nice to know if the segments are pulsed or not.
    Measure across resistor, LEDs, driver, and supply, - for both transformer supplied and battery supplied conditions (8 measurements in all).
    Yes, 18V drill batteries and a 15V regulator sounds ideal for the job.
     
  18. DennisJ

    DennisJ

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    0
    Mar 23, 2011
    I decided the cheapest way to get battery operation was to buy another 6V gel cell and put it in series with my 12 V...a little bulkier than I would have liked, but it works great. I put in a 15 V VR as you suggested. The battery shows about 18.7 unloaded, and gives a good display when run throught the VR. I tried it very briefly without the VR - the display was great, but when I turned it off, the digits continued to fluoresce, so I decided that was not good. Here's the funny thing: my transformer shows 19.1 V unloaded,yet it does not work well throught the 15V VR! The digits aren't bright enough. How can this be? Hence I put in a second plug for the transformer; that circuit bypasses the VR. Haven't done that 12 V testing yet...will get back to you then. Thanks for all the help!
     
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