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Regulated 12v supply

Discussion in 'Power Electronics' started by colum, Mar 23, 2021.

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  1. colum

    colum

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    4
    Jul 25, 2013
    I was building a circuit with 4000 ser and timer ics but I was getting curious hi frequency pulses even when the timer and capacitors was not connected..I believe it was the power supply which did die soon
    Could someone suggest a good capacitor combination for a 12+ ic well regulated power supply
    Some Cts show polarized some not polarized caps some show large value first cap and small output caps so I would apprecioate help...Thanks..colum
     
  2. dave9

    dave9

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    Mar 5, 2017
    More info needed. All info needed. Schematic wouldn't hurt either.
     
  3. colum

    colum

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    Jul 25, 2013
    I abandoned the 4000 ser circuit the 12v reg power supply is for the future so I just need to understand the capacitors in a regulated circuit. I'm sure they can be oversized or undersized Thanks..colum
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,153
    2,547
    Nov 17, 2011
    There is no universal formula for choosing capacitors in a power supply. It all depends on the topology of the power supply circuit, output current, components used for building the regulator etc.
    A regulated power supply can be built in so many different ways, there is no good chance you'll get useful information without first supplying more information on the power supply.
    If you use an off the shelf regulator chip (which is what I suggest short of buying a complete power supply module) the datasheet of the chip should provide all the information required to select the proper components.
     
  5. colum

    colum

    155
    4
    Jul 25, 2013
    Hi folks This power supply is meant to be a bench item to test ICs and at most recharge a small battery
    The capacitor requirement puzzles me The intake side is 25v so I imagine the cap needs to be 2X and the output at 12v therefore a 24v cap but the Amp rating of the caps for a general light duty supply is puzzling
    The full rectifier is heavy duty the Transformer is 1.2 amps the the 7812 is off the shelf. I need to know the best combination of caps or 2 or 3 to give some leeway Thanks..colum
     
  6. colum

    colum

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    Jul 25, 2013
    402720210323_21010888.jpg 402720210323_21010888.jpg
     
  7. dave9

    dave9

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    Mar 5, 2017
    There are some real engineers here that can calculate amount of capacitance needed for amount of ripple at current, but you didn't mention the current your circuit needs to be capable of. However with 7812, some would use a generic rule like 1000uF between transformer and that regulator for each amp current.

    The cap after it, could be anything low ESR like a 10uF tantalum, but if there is a long run to the powered device, it should have capacitor(s) local to it too.

    You're starting with a quite high 25V input. Put that through a bridge rectifier and you end up with around 1.4X that minus two diode forward drop losses, so around 34V, regulated down to 12V, is a SUBSTANTIAL loss. For this reason, I suggest that your PSU circuit proposed is not very good for more than a few tens of mA current and really, long term it's still better to find a lower voltage transformer.

    Ideally you only want your transformer rated for a couple volts (technically more like 2.5V IIRC) over your output regulated voltage at max forward drop for the regulator or instead you can calculate based on max actual load it will see (see its datasheet, should have a graph about Vf vs current). This will reduce power loss, meaning less wasted electricity on your power bill, and less heat to deal with which linearly reduces the amount of heatsinking needed. Voltage drop * current = watts heat dissipation. So, with a 1.2A load, at a 22V drop, that's 26W lost as heat, way too much to be reasonable.

    Must you use this center tap 25V output? If so, then I would insert a buck switching regulator between the bridge rectifier and the linear regulator to more efficiently lose most of that excess voltage, if you need cleaner power than a SMPS output produces (so can't just get rid of the linear 7812 regulator and use straight from the buck SMPS output) and can't calculate a sufficient filter to quiet down the ripple.
     
    Last edited: Mar 24, 2021
  8. colum

    colum

    155
    4
    Jul 25, 2013
    Sorry not sure how that got duplicated...colum
    Many thanks for that information and it does surprise me the technical engineering with lo voltage DC
    I am more familiar with 3 and 4 phase AC Throw the switch and stand back and don't be grounded
    and hope the installers did a good job...colum
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    11,153
    2,547
    Nov 17, 2011
    The transformer should be labeled VT (voltage transformer). A voltage transformer in this application steps down a high input voltage to a low output voltage. That is what you want here.
    A CT (current transformer) is used to step down (or step up) an input current to an output current. Usually used to measure large AC currents. Not what you want here.

    Right so.
    The 25 V are usually RMS. That's approx. 35 V peak. The capacitor a the output of the rectifier will be charged to that peak voltage. To regulate to 12 V from this 35 V, the regulator as to drop 23 V between input and output. Even a meager 100 mA will lead to 2.3 W power dissipation in the 7812. This is not an issue with the 7812 but with any linear regulator. A switch mode regulator is a much more efficient solution.

    As for the capacitors:
    The one between rectifier and regulator has to supply current while the input voltage is lower than the output voltage (in the valleys of the sine wave). A rule of thumb is 1000 µF / 1 A. There are online calculators that allow you to fine-tune this value according to your needs (current, max. allowable ripple voltage). Note that usually electrolytic capacitors are used in this position. These have a wide tolerance. When in doubt use the next higher standard value. You will also need a low value ceramic or foil capacitor (100 nF or so) directly next to the input and output pin of the regulator (from pin to GND). These are for suppressing high frequency oscillations. Look in the datasheet of the regulator you use, the value of these capacitors may vary for different types of regulators (some, but not all, are even built to work without).
     
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