Help on LM3914

Discussion in 'The Projects Forum' started by soda, Mar 3, 2010.

  1. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Hi,
    I setup this diagram to turn a battery charger off when the battery is fully charged and then again to turn it on when the voltage dropped the a lower voltage. Please check to see if i made any mistakes on this diagram.

    Thanks for all the help
     
  2. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Hi,
    Because i don't get any reply's on this thread i when back to the drawing board and then see what i did wrong. My relay was on the wrong side of the pnp transistor.

    Can somebody please tell me if i need a darlington or a normal bipolar pnp transistor to drive the relays.

    I attached my new setup

    Please help me on this question
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Hi Soda,
    That's an interesting idea to use an LM3914 as a charge controller.

    I think one of the reasons why you didn't get a response is that not many people really understand the LM3914 to begin with.

    In your 1st schematic, you used the PNP transistors as voltage followers rather than saturated switches. That would not have worked well. If you put the transistors on the 12v side of the relays' coils, it would work much better.

    The LM3914 can control the amount of current sunk from the LED's outputs. If you don't need much current (using MOSFETs or Darlingtons), the IC can handle regulating the base current or sinking the gate current, and the base resistors are therefore not needed.

    You are drawing fixed resistors as inductors. This makes the schematic confusing.

    In all reality, using the LM3914 as a battery charge controller is not really a wonderful idea. You would be better off simply using a couple of voltage comparators, like LM2903's, to decide high/low voltage limits. Better yet, just use a dedicated charge controller IC.

    Lead-acid batteries start sulphating at around 12.5v. It's best to maintain batteries using a "float" controller, keeping them in a range of 13.3 to 13.8v, depending on chemistry, temperature, and manufacturer's recommendations. An occasional "equalization charge" (as often as daily) will help to extend their life span.
     
  4. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Sgtwookie,

    Thanks a lot for helping me.The reason for usesing the Lm3914 is that I've already use a similar project but there i use an led and a photo resistor to switch a transistor.the thing is,here in SA one's not able to get all those fancy ic's you talking about.
    I did as you suggested and remove the resistors and it looks like it's working now.
    You say the battery want's to be charged between 13.3 and 13.8. Is this for sealed lead acid too because what i try to do is to keep the S.L.A battery fully charged. I use it with my home alarm project.
    Thanks again for your input.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Is the LM3914 circuit powered by the battery? If so, it will be a constant drain on the battery, which is not so good. I do not know how reliable the electric power is in S. Africa. If the circuit tries to turn on the charger and mains power is not available, the current through the relay coil will drain the battery. Once the battery is below the "begin to charge" range, you will have to manually charge the battery, or replace it.

    You really should look at the manufacturer's datasheet for the battery you have. There are many different chemistry batteries on the market today, and the charging requirements are different between the chemistries. If you don't follow the manufacturer's recommendations, the battery will have a short life.
     
  6. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Sgtwookie,

    Let me explain exactly what i wanted to do. I design this home alarm in shush a way that when the power fall, this diagram will cut out by means of a relay that disengage.when this all happen, the battery has to kick in and supply the power to the alarm system.The battery only has to supply the amplifier, the control board(a555timer) and the speaker and after a 10min delay, it will turn itself off waiting for the next burglar.All that this diagram is suppose to do is to keep the battery full of charge when the 240v ac is on.The SLA battery is a 7a/h. So, this diagram will be part of the SLA charger pc board after i figured out all the problems.

    You know Sgt. before i find this forum, i had no one to ask for advise, because every one is busy playing either games or upgrading their computers. The youngsters is just not interesting in electronics anymore. I'm only an electronic hobbyist and have to do the best with what i have.This is why my ideas is a bit weird, but at the end of the day, with lots of sweat, i usually get it running.

    Thanks again for your interest in my project.
     
    Last edited: Mar 5, 2010
  7. SgtWookie

    Expert

    Jul 17, 2007
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    That sounds like a reasonable plan. Since the overall plan was not included in the original schematic, I had to play "devils' advocate" and see what might cause your circuit to break, or otherwise malfunction, as it was originally posted.

    I am still concerned that if the battery becomes discharged below the thresholds that you have pre-set, the charger will never turn on, and you will be left with a dead alarm system.

    The input voltage to the 3914 should be via a resistive divider. I don't know offhand how much current the voltage level sense input requires, but the less drain on the battery the better. Hopefully, a decent compromise can be reached where the LM3914's drain on your battery can be well under 1mA.

    I still suggest that a low-power dual or quad comparator would be a better choice than the LM3914 for this particular task. There are many available. The LM339 quad comparator has been manufactured for years, and is very cheap. The LM393/LM2903 dual comparators have also been around for many years, and are also very cheap.

    You could also use a microcontroller that has an on-board comparator to turn things on and off. This would eliminate a lot of parts, and they can be very "stingy" on power. You can program them to "sleep" for a period of time, then wake up, take a voltage reading, and an appropriate action. If you decide to take up microcontrollers, you will find many exciting possibilities opening up for you.

    The learning curve for the programming end can be a bit steep, but once you get over that learning curve, you will wonder why everyone else isn't doing the same thing.

    It's rather sad, isn't it?

    In the 60's and 70's, it seemed like there were well-stocked electronics stores every few miles in a medium-sized city. I could walk into one of these stores, and spend hours (or a whole day) looking over everything they had.

    Nowadays, there are very few such stores. I was in one of my favorite electronics surplus stores this afternoon. I only spent three hours there today, because I had other things to do.

    But in my journey through the store, I helped several people who knew what they wanted to do, but not how to achieve their goal.

    It's fun, and nice to help people.

    See if you can find some low-power dual or quad comparators like those I mentioned.

    Try to find the manufacturer's datasheet for the battery you have. Post it.

    Then we can see how close we can get to the manufacturer's recommendations for battery maintenance, and your battery will live longer - saving you money.
     
  8. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Sgtwookie,
    I looked in my linear data handbook for the LM393 and was staring at it knowing i don't have a clue how to use this IC. On the other hand i was thinking of what you said and that you want to help me.

    Now with all my respect for you and knowing that you try to help me i just have to turn my back on you for this time. The main reason for doing so is only because i have already bought all the components for this project and can't see a way to dump it and start all over .That will then cost me a lot of money.

    PLEASE Sgtwookie, don't be mad at me this time for doing this to you. I promise next time if i have a similar project, I'll first come to you for advise before i start anything.

    The other thing is ,that our power supply here in South Africa is 95% reliable, so the battery is only there for the 5%. The risk is just not so big and if you now thinking that i'm trying to get out of this situation, then you quiet right, i will admit it.

    I can upload a block diagram of my alarm system if you insist of seeing it, but the thing is ,it's all written in Afrikaans languages and to do it over will be a use job

    Have a nice day
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I'm not mad at you, Soda.

    You have a long-time first-hand experience of what life is like in S. Africa. I have only a vague idea that parts may be very expensive and hard to find there, and really have no idea what components, might be available to you.

    It sounds like you have put a lot of thought and time into using what parts that you do have. You are a clever and resourceful person.
     
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