why voltage regulator overheat?

Discussion in 'The Projects Forum' started by mermaid09, Feb 20, 2010.

  1. mermaid09

    Thread Starter New Member

    Nov 21, 2009
    9
    0
    hai everyone..

    i need help about voltage regulator,
    i have done built my circuit using PIC18F252 that have 4 switchs,1 potential meter and 1 LCD grafiq(i use 4 bit to transmit data), 1dc motor(12v supply)

    my problem here is my voltage regulator become heat when power supply on,but all my input and output of my PIC working normally,i did not expect short circuit because this circuit working as i desired.

    But before this,i only put switchs and dc motor,the voltage regulator not heat but after i add 1 potential meter and lcd,it become heat..i have check my connection,no short circuit.
    i use 78L05 voltage regulator, and use 12Volt at input voltage regulator,and i have measure the output of voltage regulator =5V(even voltage regulator over heat).

    i do not know why it happen and how to avoid the voltage regulator not over heating..pls somebody help me
     
  2. russpatterson

    Member

    Feb 1, 2010
    351
    16
    Put your meter in circuit and see what current is passing from your 12V supply through your regulator. Could be your sinking some current somewhere in your circuit and the extra amps are causing the over heating.

    The 12V to 5V gets reduced by turning the extra into heat for a linear regulator like that. How hot is it getting? Burn your finger/melt your bread board hot or just warm to touch?
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,651
    632
    And while at it, check the temperature of other components in the circuit to see if you can figure out which ones are drawing the most power -that can give you a clue as to what is wrong.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    This is a typical problem when using a three-terminal linear regulator.

    I reviewed the datasheet for the LM78L05 and it looks like the maximum power without a heatsink for the TO-92 package is capped at 0.8W. That means that the maximum safe current available to the load with an input voltage of 12V is around 114 milliamps.

    I_{max}\ =\ \frac{0.8W}{(12V-5V)}\ =\ 114\ milliamps

    Even with load currents slightly less that this figure there is bound to be a noticeable amount of heating of the TO-92 package.

    You have a couple of options that can relieve the heat. The most obvious is to reduce the voltage applied to the input of the regulator. Drop the voltage to the minimum of 7V for this 5V regulator and you will notice a dramatic reduction in the heat for the same load current. If this is not practical then you will need to make use of a heatsink.

    You can replace your 78L05 with a 7805 in a TO-220 package which allows for more output current to the load. It too has an eventually top end current at which it will begin to get hot.

    If you look at this linear regulator technique on the whole, you will see that it provides convenience at a substantial cost to power supply efficiency. There is more power being consumed in the form of wasted heat than is being delivered to the load. That translates to an efficiency of less than 50%.

    hgmjr
     
  5. wingerr

    New Member

    Oct 24, 2005
    3
    0
    A switching power supply is better when you have a large difference between the input voltage and the output voltage; I came across a nifty little prebuilt adjustable regulator on ebay for $7.99 that works well and delivers an amp. Unless you want to DIY for the fun of it, just buying one might be the best way to go.
    Search for
    AX3022 Based Switching Regulator and see if you might want to go with that.
    It won't get hot, much more efficient, and easily tweaked for the voltage you want.


     
  6. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Good advice.

    Also note ( and people seem to forget this ) If the input voltage has reasonable regulation you can put a dropping resistor in series with the input of the regulator. Size it for a voltage drop that still leaves headroom for the regulator at maximum current. Use this current to calculated the power rating of the resistor. This way the heat load ( some of it ) is moved from the regulator to the resistor.

    Yes switching regs are more efficient, but linear regs are so cheap and easy.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The regulator might be oscillating if its input and output capacitors are too far away from it. It gets hot when it is oscillating.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Good point Audioguru. Three terminal regulators are prone to oscillation if filter caps are not kept close to the regulator's input and output terminals.

    hgmjr
     
Loading...