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New To Electronics - Problems with 12V Regulator

Former Member

Hello,

I am new to the forums and new to electronics. I am an apprentice Electrical engineer in my 2nd year and I quite fancy getting into electronics and start working more with my raspberry pi but firstly the I want to get the basic stuff down. I already have a few bits of kit and I recently bought some more stuff to build a little power supply for 12V and I am following this drawing....

 

image

I Got all of these exact components but the issue that I am having is that the output is 17V DC but the output from the transformer is 14V AC does anyone have idea what I may be doing wrong or how I could fix this issue?

 

Thanks

  • 0

    That is the correct voltage to expect when using a full wave bridge rectifier.

     

    I would have thought by year 2 that might have been covered.

    Google will help you and this link will explain how it does it.

    Full Wave Rectifier and Bridge Rectifier Theory

     

    there is also information here

    Rectifier - Wikipedia, the free encyclopedia

     

    You need to add a capacitor on the output, and a good rule of thumb is 1000uF per Amp.

    This is the absolute minimum so I suggest 4700 (along with the 0.1uF). based on your 250mA transformer.

     

    However things tend to get replaced (ie the transformer), and to be safe a 10,000uF cap might be a better option.

     

     

    You are fortunate that the rectified voltage is 17, since the LM7812 (and most 3 terminal regulators) have a 2.5v internal drop (it varies between manufacturers), meaning that the input voltage needs to be 2.5v higher than the output.

    Therefore in your case the minimum is 12 + 2.5 = 14.5v DC.

    I would add a heatsink to be safe as you are dropping 17-12 = 5 @ 250mA = 1.25w at full load, and who knows what the future holds.

     

     

    Mark

  • 0 in reply to mcb1

    I know how rectification works ect just not sure of voltages. Thanks I will get some more capacitors and a heatsink I was thinking i would need one. But what I don't understand is that I have 14V going into the regulator then the 2V drop would be 12 volts so where does the 17 come from? And it's actually a dual output 500mA transformer so that's a total of 1A (12VA)?

  • 0 in reply to Former Member

    Alistair Fyfe wrote:

     

    I know how rectification works ect just not sure of voltages. Thanks I will get some more capacitors and a heatsink I was thinking i would need one. But what I don't understand is that I have 14V going into the regulator then the 2V drop would be 12 volts so where does the 17 come from? And it's actually a dual output 500mA transformer so that's a total of 1A (12VA)?

    Where are you measuring 17V?  If it's at the input of the regulator (IC2 pin 1), it's probably right.  From the Wikipedia link, 12V RMS is 17V peak, so if the LED is the only load you'll see 17V at IC2 pin 1.  However, as you add load at Vout, the ripple at IC2 pin 1 will increase and its average will start to go down.  At some point the low end of the ripple will go below 14.5V and you'll start to see ripple at Vout.  This is hard to observe with a multimeter -- it's much clearer what's going on with an oscilloscope.

     

    If you're seeing 17V at Vout (IC2 pin 3), something is wrong.  Either you have a bad regulator, something is wired wrong, or the small value of C6 is screwing up regulator function.

  • 0 in reply to johnbeetem

    I See 18V out of the rectifier and 17V at where it says 12Vout

    i added a small 555 Circuit to the Vout and the voltages don't really change. Also the regulator gets really hot. I will maybe try changing C6.

  • 0 in reply to Former Member

    Alistair Fyfe wrote:

     

    I See 18V out of the rectifier and 17V at where it says 12Vout

    i added a small 555 Circuit to the Vout and the voltages don't really change. Also the regulator gets really hot. I will maybe try changing C6.

    The regulator should not be getting hot with just the LED as its load.  That's only 3 mA through through IC2, or 3 mW with a 1V drop.  So something nasty is going on.  I'd try (1) disconnect everything from IC2 pin 3 and see what you get, (2) see what you get with just C6, (3) see what you get with C6 plus whatever 25V cap you have handy that's at least 1 uF.

     

    Oh, do make sure all your caps are rated for the voltages.

  • 0 in reply to Former Member

    I know how rectification works ect just not sure of voltages.

    With all due respect I don't think you do know how rectification works given the rest of the statement.

     

    Did you read the links, it explains in pretty simple language why you will be seeing a voltage higher than the nominal AC voltage when you have a bridge rectifier and capacitor.

    image

     

    Your DC meter (and I'm assumming you have something decent since you are intending to be an Electrical Engineer) will be measuring the black line, which is 1.4 times the rms voltage.

    Cheap meters could be measuring the ripple component or any combination of the two.

     

    Page 11 of this link should help you for any future designs.

    http://www.hammondmfg.com/pdf/5C08.pdf

     

     

    Mark

  • 0 in reply to johnbeetem

    Thanks for your help everyone but i figured it out! I check the pins for the IC and i had it the wrong way round! Thats what happens when you build these things when tired. and a bit of a noob thing really. but thanks for your help.

  • 0 in reply to Former Member

    That will do it. ....

     

    Can you please mark the answers helpful, and tehn selct answered for the post.

    This then helps others find it.

     

    Cheers

    mark

  • 0 in reply to mcb1

    Why should he mark the answers as helpful - he has thanked people nicely for their help with a message rather than just a box tick, but he may not wish to play the points game. I don't buy into the "answered" bit either - several of the replies in this thread are interesting but not directly helpful or answers.

     

    As DAB would say, just my 2C worth.....

     

     

    MK