some basic qns on VAC to VDC

Discussion in 'General Electronics Chat' started by zenite, Dec 21, 2009.

  1. zenite

    Thread Starter New Member

    Dec 21, 2009
    Hi, new here. I am doing a school project that requires some electronics knowledge, but problem is that I am in mechanical eng. So I need some help and advice on some very basic electronics.

    My project requires 3-5A, 12VDC depending on which component I use, and I have an AC adapter with output 5A, 12VDC.

    So if I use a peltier with rating of 1.5A, and connect the AC adapter output to the peltier, will it fry the peltier seeing that it only requires 1.5A and I am supplying it with 5A (I asked the store before buying the adapter, the guy says no problem...)

    For the peltier, if its rating is 5A but I supply only 1.5A, will it still work? I am assuming the lower current will give a lower heat transfer than at 5A, right?

    Another question, what circuit do I need to allow the AC from my home supply to charge my 12VDC batteries while allowing the batteries to power the device at the same time (the batteries are discharging and charging at the same time)?

    Also, I need a 12VDC battery that can last for 4 hours. Assuming my device uses 5A, the capacity needed is 20Ah right? What type of batteries can I look into? I tried asking around several electronics store, they do not sell such high capacity batteries. So I might have to make my own battery pack, though I am not sure if Li Ion, LiPo or NiMH is better.
    Last edited: Dec 21, 2009
  2. SgtWookie


    Jul 17, 2007
    What kind of "AC adapter" is this?
    If it is a "wall wart" (plug-in transformer DC supply), they are not usually regulated. Their output voltage will usually be within about 10% of specification when the rated current is drawn. However, under light or no-load conditions, the output voltage may be considerably higher. You might get 17VDC under no-load conditions.

    You should check your AC adapters' no-load voltage output before connecting the Peltier device. If it's 12v, no problem. If it's higher, you'll need a regulator of some sort.

    Yes. You will need to figure out how to efficiently regulate the current supply. A linear solution will not be efficient.

    It doesn't work like that.
    Batteries generally get charged at a higher voltage than their rated voltage.
    They are considered discharged when their output falls to some percentage of their rated voltage. Exactly where this is depends on the battery technology.

    That's a hefty load.

    Batteries are generally given an AH rating based on a 20 hour discharge. The higher the rate of discharge, the more power is dissipated within the battery itself due to the internal resistance. As a battery becomes discharged, it's internal resistance increases. Aging also affects internal resistance.

    Take a look at LiPo and sealed lead-acid (SLA) batteries.
    LiPo batteries seem to be very popular with R/C aircraft and boat modelers. They have their own unique charge requirements, as do all batteries.

    SLA batteries will last much longer if their discharge is limited to 30% of capacity. However, many people routinely discharge them to 50% capacity; this shortens their life considerably.
    Discharging a SLA battery past 50% will result in a very short life.
  3. zenite

    Thread Starter New Member

    Dec 21, 2009

    Sorry for the late reply, I was away abroad.

    I am thinking of how handphones work. When it is charging, the handphone can still be used. Thats how I want my product to be, functioning while charging. Is there anyway to do this? For the handphone, does the power source comes from the wall plug or the batteries while charging? I am guessing the battery charging rate is higher than the discharge, hence able to charge and power the handphone at the same time.
  4. maxpower097

    Well-Known Member

    Feb 20, 2009
    If you need a good battery contact these guys. They'll make you anything you want. I used to fly RC helis and used 22.2v 8000mah lipo's. Really only use lipo's for high high discharge needs, otherwise take a look at the A123 cells. They have almost the power of lipo's but can handle over 1000 charging cycles and tend not to explode when punctured or overcharged.

    As for charging and using battery's at the same time, sure you can. But you will need to first find out what kind of battery you will be using then the proper way to charge it. Many experts still disagree on the proper way to charge a lipo.
    Last edited: Dec 28, 2009
  5. thyristor

    Active Member

    Dec 27, 2009
    I have a boat and allow my house batteries (3 x 12v lead-acid; 135AH each) to discharge to 50% but no more. I typically get 6-7 years out of them charging them via a 3 stage charger with a bulk, absorption and float stage. The boat is used most weekends and for several continuous weeks in the warmer months.