My Bench top power supply project : Reference voltage from 78xx ?

Discussion in 'The Projects Forum' started by prashanthb, May 17, 2015.

  1. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    Hello everybody :)
    I am new on this forum, from Bangalore, India.I am actually formally educated in Mechanical field, but was always interested in electronics. What ever I know is from the internet and my crazy/simple experiments. I lack the basics and hence I require help from the seniors here. Thanks in advance :)

    I am building a benchtop power supply (30V/3A x 3 outputs). Just a simple circuit, not transistorized !
    I am using cheap multimeters as panel volt and ammeter. I want to calibrate them first. Can I use the 78xx (5V to 24V) and their schematic from their datasheet, as reference voltages ? I will be powering them up with batteries and not on rectified AC.

    1) Upto 2 decimal place accuracy is enough for my project(s).
    2)Will 7812 output exactly 12.00V? or 12.01V or 11.98V or .........
    3)If they can be used as (rudimentary) reference voltage sources, how much excess voltage should I feed them ?

    Thanks in advance :)
    regards
    prashanth
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Download and read the data sheet for the 78xx regulators to determine how precisely they conform to their nominal output voltage, out of the box, and over temperature.

    Since you are using a digital panel meter to display the voltage, the precision of the regulator is not that important. The accuracy of setting voltage will depend on the voltage reference used inside the dpm.

    The data sheet for the regulator in question will tell you the "dropout voltage", which is the minimum headroom required to keep the regulator "regulating". This becomes important when selecting a transformer, and sizing the filter capacitor...

    You will need huge heatsinks to mount the regulators on...
     
    prashanthb likes this.
  3. crutschow

    Expert

    Mar 14, 2008
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    What is this un-transistorized power supply?
     
  4. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    How dumb must I be. I didnt read the proper datasheet.Electrical Characteristics table has the answers. It does specify the input voltage range, current and Voltage vs temp relationship clearly.
    Heat sink : I plan to dip the LM317 regulators and MUR860 diodes in liquid for cooling :D I bought the required high temp silicone paste yesterday for electrically insulating it. The MUR860s became too hot at 12V/1.9A during testing. I still have to experiment. If it fails I have to settle for a fan.
    Thank you Mike.
    @crutschow :
    Tx > B.Rectifier > 2mH inductor > LM317 regulator > Large inductor > DPM > 10,000uF capacitor reservoir >Audio/LED light projects
     
  5. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    What do you mean here sir ? Do you say that I use a better quality DPM ?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Silicone paste is for providing a low thermal resistance between the device and the heat sink only, it is not used to provide electrical insulation. That requires separate insulating washers and hardware.
     
  7. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    Sorry I should have mentioned this : I will include mica sheets insides before sealing it.

    EDIT : Tx > B.Rectifier > 2mH inductor > LM317 regulator > Large inductor > 10,000uF capacitor reservoir > DPM >Audio/LED light projects
     
  8. crutschow

    Expert

    Mar 14, 2008
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    It's likely that the DPM has sufficient accuracy for you requirements without calibration.
     
  9. ebeowulf17

    Active Member

    Aug 12, 2014
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    In the first post it says "cheap multimeter." Will the meters be actual panel meters or just multimeters hacked in?
     
  10. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    DT830D multimeters available cheap on ebay. I want to modify these. They cost INR130. Decent DPM with good accuracy will cost INR621. But I will not be needing that kind of accuracy. My present DT830D shows +0.05 V higher reading than my other better multimeter. I will save some money if I can trim this excess reading.
     
  11. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    (continued....)
    I just have to change/replace one fixed resistor with a pot inside and then I can adjust. But first I need reference voltage & that is what this thread is about.

    @MikeML : I was just watching EEVblog's lab power supply video on YT, I suppose I sort of know what reference voltage is now.

    I want to ask one new question related to my project. Kindly answer.
    As opposed to a "current limiting" powersupply, will a "constant current" powersupply give out constant current while voltage is being varied ? Eg 1.5V/3A to 20V/3A.
    I thought if I control voltage the current also gets dragged up/down(viceversa).
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,416
    3,354
    A number of people don't understand how to use a constant voltage/constant current power supply.
    Threat both as limits.

    "current limiting" and "constant current" are the same thing.
    "voltage limiting" and "constant voltage" are the same thing.

    Apply Ohm's Law: I = V/R

    For a fixed load resistance R, I and V must follow.

    Yes, the voltage must drag down the current and vice-versa. You cannot have constant voltage and constant current both at the same time.

    The supply will hit constant voltage or constant current depending which limit is reached first.

    When in constant current mode, you can set the voltage limit to be higher than the V = IR value. You make this adjustment when there is no load or the load current is less than the current setting. When the proper load R is connect, the voltage output will follow the V = IR value.

    To set the current limit setting, short the output of the supply and adjust the current setting.
     
    Last edited: May 18, 2015
  13. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    Thank you MrChips.

    As a demo of the method you just explained :

    @12 min
     
  14. crutschow

    Expert

    Mar 14, 2008
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    For what purpose do you need such high accuracy on you power supply voltage?
     
  15. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    Here is a 0.1% accuracy dual voltage reference:

    http://www.adafruit.com/products/2200

    Presumably you could get the relevant parts yourself cheaper if you're comfortable working with surface mount components.
     
  16. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    I guess working with small leds will require small current measurements. And audio circuits will need to be accurate as well. I am a bit of audio fanatic :)
    I will try to build it as accurate as possible but wont fuss over it too much if it isnt. Only now I have some free time, later I cant spend time on it.
    Yes but first I will try with 78xx series. That accuracy is enough for me.

    Thank you
     
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