Led voltage indicator

Discussion in 'General Electronics Chat' started by Joeshmoe, Jun 4, 2007.

  1. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    I would like to make a led voltage indicator that works on a 400v capacitor. currently I have to hook a multimeter up to it and watch it charge. I have seen a circuit that uses an IC and one that just uses resistors and leds. And I am not sure where to start.
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
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    take your pick first, the ic way or the resistor way. then we can start from there. also pls provide where you saw the circuit.:)

    moz
     
  3. Joeshmoe

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    Jan 12, 2007
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  4. cumesoftware

    Senior Member

    Apr 27, 2007
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    You can base your circuit on a LM3914 chip. The LM3914 is a LED bargraph driver, so you can see the capacitor charging. They can be cascaded, so you can expand this meter to 20 or more LED's.

    See this: http://www.national.com/pf/LM/LM3914.html
     
  5. Joeshmoe

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    Jan 12, 2007
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    I did see that website but if I can I would like to use parts from radioshack and they dont have that chip
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
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    Why using parts from radioshack? I mean, radioshack is not universal. If they don't have, skip to others.
     
  7. mozikluv

    AAC Fanatic!

    Jan 22, 2004
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    what's the voltage your going to charge your cap?

    moz
     
  8. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    radioshack is the only store that I know of in my area and I prefer to buy things in a store rather than online.
    The highest the cap will be charged is 400v but I would like to see it charge up with multiple leds
     
  9. Ron H

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    Apr 14, 2005
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    Does your store have LM339's? You can drive 4 LEDs with one chip, 8 with 2 chips, etc. We can show you how to do this.
    What are you charging the cap with?
     
  10. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    ya they have the LM339's
    I am charging the cap from a transformer thats connected to the mains (in the US so 120v) I am using lightbulb as a current limiter and a full wave bridge rectifier to get DC
     
  11. Ron H

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    Apr 14, 2005
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    What is the wattage of your light bulb? How many LEDs do you want to use? You will need a DC power supply for the circuit. Do you have something in mind?
     
  12. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    The bulb is 3 way and 50-200-250 I switch it to 250 because it gives me the best charging time...As for leds well do the voltage increments have to be the same? If so maybe 12.. If it can be made so that as the cap gets closer to 400v the increments get smaller 8 should be enough. Can it run off batteries, like a 9 volt.
    Thank you for your help it is much appreciated
     
  13. Ron H

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    Apr 14, 2005
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    That must be one hog of a capacitor if the charge time is long enough to be seen on a multimeter. What is the capacitance? Is the peak voltage around 170 volts?
     
  14. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    the cap is 3900uf at 400v I have 4 of them and the current bank has 3 so 11700uf and these are my smaller caps I had to store the bigger ones (around 12in tall 3in wide)
     
  15. Ron H

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    Apr 14, 2005
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    I would expect it to be fully charged in 2 or 3 seconds. Does that sound about right?
    We could do a nonlinear scale, but I don't know what you are looking for. Seems like 12 LEDs in a linear scale would be easier, at least to start with.
     
  16. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    to charge 1 it takes a just a few seconds but when charging a bank of three it takes a little longer... I mainly want this so I dont have to have my multimeter hooked up to the caps every time I charge them... also I may eventually charge the caps from a battery based circuit so I assume that will take longer... the 12 LEDs in a linear scale sounds great.
    thanx again
     
  17. Ron H

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    Apr 14, 2005
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    I'm going to assume that the fully charged voltage is around 170 volts. If this is not correct, measure the voltage and post it here. I'll put in a pot for trimming full scale.
    A nine volt battery won't last long. I would use 8 NiMH cells, or 6 alkaline cells.
     
  18. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    I charge the cap up to 400V... and which is better 8NiMH or 6 alkaline
     
  19. Ron H

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    Apr 14, 2005
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    NiMH has a flatter discharge curve than alkaline, meaning the voltage will tend to be more nearly constant during usage. NiMH is rechargeable, meaning you will need a charger.
    Regarding the circuit, I have started drawing it up, but I think you should be aware that it will have a LOT more parts than if you used an LM3914. With the LM3914, it will require one IC, a pot, 10 leds (or one 10-LED light bar) and about 4 resistors. You can use it in either bar or dot mode.
    With the LM339s, it will take 3 ICs, the 10-LED light bar (or 12 discrete LEDs, if you want to use all 12 comparators), a pot, and more than 20 resistors. You will only have bar mode unless you add more ICs, which are probably not available at RS. Bar mode requires more current, because at full charge, all LEDs are on. This will reduce battery life, but this may not be an issue for you.

    I think I have seen LM3914's at RS stores.
     
  20. Joeshmoe

    Thread Starter Active Member

    Jan 12, 2007
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    geez I didnt realize what a difference not using a 3914 would be I thought IC were more interchangeable I feel bad now that I took your time for something that could have been done easier... I guess I will go with the 3914. http://www.national.com/pf/LM/LM3914.html has a schem that looks pretty simple my only question is how to use it with something thats 400v.
    thank you for all your help and patience's
     
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