voltage regulators heating

Discussion in 'General Electronics Chat' started by yassser, Dec 19, 2012.

  1. yassser

    Thread Starter Member

    Jul 25, 2011
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    0
    hello,

    I have a circuit that needs several voltage regulators with several voltage values . 5v , 12v , 15v , 18v , when I used a big 24V transformer to supply the circuit , the 5v regulators heated up severely , when I used a smaller 20v transformer , they didn't , but I have to use the big transformer for other circuits in the project , so I don't want to use 2 transformers .

    is there an explanation for that ? and is there something I can do about it?
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Maximum input voltage is 35Volts Dc for a 7805, so any more and it will get hot.

    use the big transformer for the supplies but take the 5V feed from the 15V supply
     
  4. yassser

    Thread Starter Member

    Jul 25, 2011
    91
    0
    @Dodgydave , yes I'm aware its input is 35v maximum , I only put 24V

    and what does the size of the transformer have to do with the regulator heating , I can't see why would it heat with a big transformer and not heat with a smaller one?
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    When you put 24V AC in it will get upto 1.5 times bigger when its across a smoothing capacitor, so thats 36V which is on the limit of the input. Better take the 5v feed from the 15V or 20V regulators.
     
  6. yassser

    Thread Starter Member

    Jul 25, 2011
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    @scottwang , wouldn't that make each regulator load all the following ones , isn't there another way?
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    If your highest voltage is 18Vdc, then you can choose 18Vac transformer.
    I'm not sure how big the current you want, if you use LM317, then you can setting several sections for the output voltage as you like.

    You can setting different R2 shown as below to change the output voltage, R2 can be from 0~5K.

    LM317 datasheet, FIGURE 3. Regulator with Protection Diodes, page 6.
    http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS009063.PDF

    LM317 - 1.5A.
    LM1085 -- 3A
    LM1084 -- 5A
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The heat is caused by power dissipation. The higher the Vin value is, the higher the power dissipation in the regulator.
     
  9. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    For the lower voltage regulators you can add a power resistor in series with the regulator's input. That will transfer some of the power dissipation from the regulators to the resistor. The value of the resistor and its power rating depends upon the amount of voltage you want to drop and the maximum output load current you need from the regulator. You probably don't want the resistor drop to be more than about 80% of the difference between the raw DC voltage from the transformer filter and the regulator output voltage at the desired maximum regulator output current.
     
  10. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Subtract the output voltage from the input voltage, and multiply by the output current -- this is the power dissipated by an analog regulator.

    For example:
    24Vin - 5Vout = 19V
    19V * A_maxload = Wdiss (Pd) in regulator
     
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