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Which transistor to choose for my regulators !!!

Discussion in 'General Electronics Discussion' started by Farukh Khan, Jul 20, 2015.

  1. Farukh Khan

    Farukh Khan

    154
    2
    Jun 12, 2015
    Hello Guys,

    Once again am here with my beginner questions :p

    I was trying to make some voltage regulator circuits for my setup. I want to make a 9V regulator for arduino and a 5V regulator for Raspberry Pi and some 12V regulator for running my relays. I have got a 24V 20A power supply using which I will power up all my equipment. So guys please suggest me a very good quality accurate voltage regulator transistors which I can use to convert 24V to 9V for my arduino and 24V to 5V for my raspberry pi and also 24V to 12V for the relays. I used some LM regulators without checking their datasheet and burned them down. I burned a 5V and 9V one. When I tried to use the 12V one it worked but gets really hot. Therefore please suggest me some good quality regulator transistors which will not get hot enough to get a heatsink on my these voltage conversions. And also using which I will get very accurate voltages on the output.


    Thanks in advance.....
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    you don't want a transistor, you want a regulator.

    a 7805 is a 5v regulator, a 7809 a 9v regulator, and a 7812 is a 12 volt regulator. The voltage is the output voltage. Most regulators of this general type can withstand an input voltage over 30 volts, but are usually operated with a smaller margin between the input and output voltage.
     
  3. duke37

    duke37

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    Jan 9, 2011
    The temperature of the series transistors or regulators will depend on the power to be dissipated and not on the particular device. If they get too hot, you will need a heat sink.
    A switch mode buck convertor can reduce voltage with little power loss.

    Linear regulators can probably be found to do what you want or can be boosted with a series power transistor.

    A resistor in series with the relay will allow a 12V relay to be used on 24V. Relays do not need an accurate supply voltage.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    These regulators week not burn up unless you wire them up incorrectly. Site is how you wierd them up.

    depending on your power source and load you may need filter capacitors and a heat sink.
     
  5. BobK

    BobK

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    Jan 5, 2010
    Converting down from 24V to 5V with any but very small currents is a bad idea. Get a 5V USB charger to power your 5V electronics.

    Bob
     
  6. AnalogKid

    AnalogKid

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    Jun 10, 2015
    P = E x I Power equals voltage times current. Example:

    If your 5V circuits draw 1 amp total, and this is coming from a 7805 and a 24 V source, then the power in the regulator device is (24-5)V x 1 A = 19 watts. Without a substantial heatsink, the 7805 will overheat in a few seconds. So for each of your outputs you need to figure out how much current will be needed, then calculate the power in the linear regulators, then get some heatsinks and insulation mounting kits. Without heatsinks, it won't matter what you use as the linear regulator device - transistors, adjustable, fixed, whatever, no heatsink equals quick death.

    An alternative is switching regulators. These use a more complex circuit to reduce the heat dissipated in the regulator device.

    ak
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The 78xx regulators will shut down rather than suffer a quick death.
     
  8. AnalogKid

    AnalogKid

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    Jun 10, 2015
    My experience indicates that that is more of a concept than a rule.

    ak
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sure, but unless you do something really silly, you're far more likely to see a shutdown than smoke. However, a 24 volt input for a 5v reg along with an excessive load may verge on "really silly"
     
  10. Farukh Khan

    Farukh Khan

    154
    2
    Jun 12, 2015
    So what If I use a LM2678T-ADJ regulator circuit for getting a constant voltage without any drop at the output? Is this regulator is a good one for regulating different voltages? And what amount of heat this regulator might generate? And If I use quite a bit of large heatsink along with cooling fan with my regulators like 7812 or 7805 or LM2678T-ADJ to keep it totally cool then will it help me to reach out accurate voltage and also keep my regulators safe? Does over cooling will harm anything?

    Guys please help me out with my questions and confusions.

    Thanks in advance.....
     
  11. duke37

    duke37

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    Jan 9, 2011
    The switch mode regulator will drop the voltage with little power loss, you can calculate what this is by finding the details of components. Power loss will be mainly in the inductor and catch diode.


    If you use the LM2678 to provide 12V, then you could use a 7809 and 7805 to give 9V and 5V.
    The power loss will be the voltage drop multiplied by the current. This will be much less than getting the input from 24V.

    You cannot keep the parts totally cool, power loss will generate heat.
    You cannot overcool in Bangladesh unless you live on liquid oxygen !
     
  12. Farukh Khan

    Farukh Khan

    154
    2
    Jun 12, 2015
    Got it but a little bit confused. Are you trying to say that I will use the LM2678 to regulate 12V and then use 7809 or 7805 in series to produce the regulated 9V or 5V without any loss?
     
  13. duke37

    duke37

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    Jan 9, 2011
    If you have current, then you will have loss. Resistors and semiconductors will have significant loss and there will be some loss in capacitors also. No circuit will operate without any loss.

    The LM2678 will drop 12V (24V to 12V) with little loss, the input current will be less than the output current, most of these things will have an efficiency of 80% or more.

    If you feed the 12V into a 7809, the power loss will be the output current times three so at 1A maximum, the chip will have to dissipate 3W.

    You could feed the 7805 from 12V or 9V. It depends on the currents involved as to which is best and where you wish to supply cooling.
     
  14. BobK

    BobK

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    Jan 5, 2010
    I don't understand why you remain fixated on using this inappropriate 24V supply, when you can get inexpensive more efficient regulators for the voltages you need.

    Bob
     
  15. Farukh Khan

    Farukh Khan

    154
    2
    Jun 12, 2015
    I remain fixated because I already bought this 24V 20A power supply and it is quite expensive for me. But what regulators are you suggesting which are more efficient regulators and also inexpensive for the voltages conversion I need? Can you specify the models or datasheet links?
     
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