PWM Power dissipation?

Discussion in 'General Electronics Chat' started by Bosparra, May 18, 2010.

  1. Bosparra

    Thread Starter Member

    Feb 17, 2010
    79
    3
    In another thread (http://forum.allaboutcircuits.com/showthread.php?t=38515) about PWM dimming of an LED, SgtWookie said in post #9:
    Could you please elaborate on this statement? I am in the process of designing some camping lights, using PWM to dimm them and as the lights will be battery powered, this statement caught my attention.

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    An ideal voltage source has zero impedance.

    Real-world voltage sources, like batteries, have an internal resistance/impedance.

    As the battery is discharged/consumed, its' internal resistance increases, up to the point where it can no longer do useful work.

    If, for instance, you tested a used 9v "transistor" battery with a DMM (digital multimeter) set to read on the 20v scale, you might read a fairly high voltage, but then be puzzled when it would not power a circuit. That is because the DMM has a very high impedance, perhaps 10 millon Ohms or more.

    However, if you tested the battery under load, you would find that the battery's voltage was far lower.

    Adding a capacitor across the battery terminals will help to extend the battery life when the load has high peak current, but a moderate average current. A PWM circuit is such a load.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    I've had circuits that absolutely require a large capacitor across the battery. If the load is intermittent, the capacitor can take up some of the slack. Doesn't work for continuous loads though.
     
  4. Bosparra

    Thread Starter Member

    Feb 17, 2010
    79
    3
    Thanks for the explanation. Just to be clear, are we talking here about actual damage to a battery or just a run down battery? Think 12V rechargeable lead acid (car) battery, powering a high current PWM controlled device. Are you saying that the high peak current and not the average current, will physically damage the battery or just run it down quicker.

    My understanding of the use PWM for speed control in motors or dimming control in lights, is the reduce the average power dissipation (heat) while still maintaining the current to actually drive the load?
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Both.

    Physical damage will occur to a battery that is drawn upon over manufactures recommendation for too long.

    SgtWookie was explaining about standard depletion of batteries. Re-charging reverses the effect to a point (You never get 100% back)

    Everytime you discharge a battery, the resistance goes up. If you discharge too fast, temperature will rise to much too fast and can sulphate the plates, evaporate water in the electrolyte, and can even physically damage the battery.

    But dealing with rising resistance with falling battery power is something to be designed into the circuit.

    You can incorporate a low battery cut-off if your circuit will be adversely affected by the higher resistances incurred.
     
  6. Bosparra

    Thread Starter Member

    Feb 17, 2010
    79
    3
    Sure, but this is to expected and is not specific to PWM right?. My question pertains to PWM specifically and any ill effects(damage) that might have on a battery, other than just running the battery down off course.

    To put it another way, I am trying to understand any adverse effects on a battery, when using PWM. I remember reading somewhere that some battery chemistries are not well suited to high current pulsed digital systems like cellphones and Bluetooth.

    Is this something to be concerned about when using lead acid batteries to drive LED's via PWM?
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Lower capacity batteries can take a little while to re-up a charge between pulses.

    Lead-acid batteries actually like it.

    You should also use a pulse-method to re-charge the battery.

    The pulsing gives a "shock" to the plates, and the very quick heating/cooling can help keep sulfation from taking hold to the plates.

    When this happens, resistance and available surface for acid/lead contact is reduced.

    Be sure to use batteries that are rated for twice what you calculate you will need for standard run time.

    If you calculate you need 10Ah, use a 20Ah battery.
     
    Bosparra likes this.
Loading...