power supply schematic

Discussion in 'General Electronics Chat' started by Kelp, Jan 17, 2015.

  1. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    Hi All, I've been messing around in the KiCad schematic editor. I've come up with this schematic for a power supply.

    I've got a 40VA transformer going to a bridge, the output of which is about 37V. Then I've got a capacitor, a voltage divider (to get the voltage into the range of the 78XX which is 35V max), then capacitors on the outputs.

    I've breadboarded this and it works, but the resistors I used are the wrong rating, it works but they get hot. Is this all I need for a simple regulated supply?
    Anyone have a recommendation on capacitor sizes?

    I attached the schematic as a pdf.
    thanks
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You would be better off scrapping the voltage divider and going with LM317's.
    Max.
     
  3. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    Is that just because the 317 can handle the voltage?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    40v I/O difference.
    Max.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
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    LM2576
     
  6. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Besides the voltage divider is not a voltage divider. Wired wrong.

    I agree - scrap it!
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Resistors in series with the input of a 3 terminal regulator could even defeat the whole object of the exercise as the volt drop on them varies with the current load.

    If you have a large input/output differential on the regulator; it will also get hot for any appreciable load current.

    Possibly the best solution is a pre-regulator - a big fat TO3 (or TO3P) transistor to cover the expected dissipation - just an emitter follower with a base resistor and a zener to GND. It doesn't need much accuracy - the 3-terminal regulator does that, all it does is drop the voltage down to what the chip can handle.

    With the small amount of excessive voltage mentioned - a really simplistic way of dropping some of it is to use a series chain of silicon power diodes - but this approach is not without a few issues!
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Using a voltage divider isn't the way to go for several reasons. When you say that the output of the transformer is 37V, I assume that that is RMS. That means that the peak voltage (which is what is going to destroy your regulators) is more like 52V. Second, your two resistors aren't even configured at a voltage divider.

    Let's assume that your input filter capacitor is large enough to keep the average voltage around 50V. That means that your 2.2kΩ resistor is going to be dissipating a bit over 1W. While it is rated at 1W, that is a lot of heat and it IS going to get hot. The other resistor is going to be dissipating power only to the degree that the two regulators are providing current to a load. Ignoring the small quiescent current, the power in that resistor is going to be the resistance times the square of the combined output currents. In order to get the voltage down to the 35V rating for the regulators, you need to drop 15V across that 330Ω resistor, so the combined current draw needs to be at least 45mA. But also note that you need a couple volts of overhead so you don't want it to drop below about 14V meaning that your maximum combined current draw can't exceed about 110 mA otherwise the 7812 will drop out of regulation.

    The best solution is going to be to get a transformer that is better matched to your needs. If you get one with a 12V RMS output that should cover your overhead since it will peak at about 16V. Or get a 16V or 18V output transformer. With the transformer you have, you are going to be generating about 40mW of heat for every milliamp of output current you deliver.

    If you have to use that transformer and therefore need to drop some voltage but don't want to dump a lot of excess power doing it, then use diodes in series between your bridge output and your input filter. If you are using a TO-220 case without a heatsink then you are looking at about 20°C/W and so you can only let them dissipate about 5W of power (which is really pushing it). Since you need to keep at least 14V for the 7812, you will have 9V across the 7805 meaning that it can only be asked to output about 500mA. The 7812 can provide the full 1.5A that it is rated for. So that's as much as 2A -- which will mean that you are drawing about 100W from the transformer but only delivering about 20W to the loads. To drop from 50V down to 14V would mean dropping 36V which would require about 50 diodes, each dissipating as much as 1.4W. But that's if you need that kind of current output. If you only want, say, 200 mA of current from each supply then you can run a lot higher overhead on the regulators. So if you get the input voltage down to 25V that would require about 30 to 35 diodes and each would max out at about 1/4 W. You'll still be wasting a lot of power because it is a linear supply.

    Bottom line -- get an appropriately sized transformer.

    For capacitor sizing, look at most any 78xx data sheet. The TI data sheet recommends 0.33uF on the input and 0.1uF on the output. Since you have two regulators, you might put 0.68uF or 1uF on the input. But that's just for the regulator IC and it assumes a reasonably descent DC input voltage. You also need to consider your ripple voltage and that will depend on the maximum current draw you want to support. So, what is the maximum current you want to be able to deliver from each of your regulated outputs?
     
  9. WBahn

    Moderator

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    Wow! When I started my post I there weren't any responses. The OP is getting great customer service on this one!
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    30 Minutes! You must be a slow typer?:p
    Max.
     
  11. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    WBahn,
    I am measuring 36VDC at the between the voltage side of the 330 ohm resistor and ground.
     
  12. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    What I am looking to do is be able to drive an Arduino at 12V which I am guessing could need 700mA, as well as 5 SPST relays (directly off of the the 36-37V DC supply), and 5V for a particular sensor that has a heating element.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    25 minutes or so. I'm definitely not the world's fastest typist, but I went out and downloaded the data sheet, looked for the several key specs, ran a bunch of numbers, and considered a few options. Plus went down and started a fire in the stove because the temp in the house is only 53°F so if I didn't get it going soon the furnace would come on and we don't have the propane to let it run unless it has to.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    So I figure you have around a 24vac secondary, for the relays you could simply use a separate bridge for them with no capacitor, then you could spec in 24vdc coils.
    Max.
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    That's the voltage as measured by a DC voltmeter? That's only the average, not the peak. It's hard to say what the peak is. If you have no current being drawn from the regulators, then if you tell us the value you art using for C1 we can take a good guess based on the current in the 2.2kΩ resistor. A better way would be to remove everything but C1 and then measure the voltage across C1, which should then stay very close to the peak voltage.

    Also, you indicated that both resistors were getting hot. But the 330Ω resistor, as you have it in the schematic, should only get hot if you are actually drawing current from the regulators.
     
  16. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    Yes thats right on the 24VAC secondary.
    It sounds like the resistors, though they give me the right voltage limit my current.
    I'm leaning toward the LM317 option.
     
  17. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    When all 5 of the relays are on the voltage between the input to the 330ohm resistor and GND drops from 36VDC to about 33VDC.
     
  18. WBahn

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    Are you talking about powering the coils from the 36V DC or using the Arduino to control the coils and having the relays switch the 36V DC to power something else?
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    But it might be best to do as I suggest and run the relays off of a straight bridge for 24vdc what voltage relays do you have for 36vdc?
    They will not load up any regulator then also.
    Max.
     
  20. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    Here is the relay schematic
     
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