recreating dc with ac component

Discussion in 'General Electronics Chat' started by jimmiegin, Apr 5, 2014.

  1. jimmiegin

    Thread Starter Member

    Apr 4, 2014
    49
    0
    Hi guys, I am currently learning about regulator design and transistors and other stuff in general. What i want to do is experiment with things like values of filter caps and line regulation and bypass caps and so on. I had an idea that i could supply my circuits with regulated dc and use my little miniosc sig gen in addition to put some ripple in there so i could easily control frequency and amplitude. But the output voltage only goes up to about 2v sine. So i think what i am looking for is a function generator? Can someone tell me if this is what i need and also if there is such a thing as a good, but cheap one. I found that when i was recently in the market for a good but cheap o'scope....well, you cant have your cake and eat it! Thank you guys. Jimmie
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Be very very careful of connecting the outputs of different devices together, unless you like the blue smoke.

    If you really must then make sure the generator feeds into the power supply through a large series capacitor.

    Far better to learn about rectifiers, filters and loads by making one and playing with it.

    The books by Michael Tooley are really excellent in this respect, combining practice and theory.

    http://books.google.co.uk/books/about/Electronic_Circuits.html?id=8fuppV9O7xwC&redir_esc=y

    As a matter of interest the ripply from a power supply or regulator is not a sine wave but a sawtooth wave at twice the mains frequency.
     
    Last edited: Apr 5, 2014
  3. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,061
    To add to this caution, be aware that you can get the smoke by merely connecting the "grounds" of two power supplies, with no other apparent connection. Don't ever connect any part of two power supplies without first checking for a voltage between them, and then use progressively smaller resistors to verify current is not flowing from one ground to the other.
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    It is great that you want to experiment, but first, understand where power supply ripple comes from. Look at the ouput of an un-filtered 1/2 wave rectifier circuit. It us just a series of humps with dead time in between. Very high ripple. Now, look at the output of a full wave rectifier circuit. Again, just a series of humps with the diffeence being that the gap that existed in the 1/2 wave circuit is now filled in with another hump. Now for the ripple. Adding a filter capacitor to either will improve the output. If you assume zero load on your power supply, the capacitor will charge to the peak value of the hump and hold that value. When a load is applied, the filter cap will still charge to the value of the hump, BUT, during the time the hump voltage is lower than the capacitor voltage, the capacitor becomes the source of power for the load. Since it only stores power, the amount will reduce until it gets another boost from the hump out of the rectifier which means that its voltage drops below the voltage coming from the diode(s).

    Now, after that ramble, to your original question.

    In order to impart a signal on top of an existing DC level that can be filtered out, you would need some method of isolating that signal. In a power supply, the rectifier would isolate the output from the power supplying the diodes.

    Your best bet would be to get a small transformer, some diodes, capacitors and resistors to experiment with. Much easier to deal with and, hopefully you have a scope so you can see the results of your experiments.
     
  5. jimmiegin

    Thread Starter Member

    Apr 4, 2014
    49
    0
    Lol your ramble is appreciated billb thank you. I was aware of the frequency after rectification and have a little knowledge of formulae for calculating capactors but had not thought (dumb) about the waveform. And to studiot and wayneh, would there only be a problem with elevated ground if caused by loops creating current flow and voltage drop across wire resistance? Or may there be another reason for elevated ground?. also I have another matter I would like to ask about regarding this power supply

    http://diyfan.blogspot.dk/2013/03/adjustable-lab-power-supply-take-two.html

    I think that 30v 3A supply like this should have maybe rather more than one 2n3055? What would be your thoughts please guys? Thank you for your replies :)
     
  6. jimmiegin

    Thread Starter Member

    Apr 4, 2014
    49
    0
    ps. I am currently building up my little workbench so if anyone can recommend a function generator that will not cost a whole wage packet (I have rather modest wage packets) it would be very much appreciated. Has anyone got or tried any of these small kit jobbies or are they a waist of time?
     
  7. tindel

    Active Member

    Sep 16, 2012
    568
    193
    I bought an early 90s Tektronix function generator off of Ebay for $100USD I thought that was pretty reasonable. With what you are trying to do make sure the shield of the generator is floating... How much current is you're load?
     
  8. jimmiegin

    Thread Starter Member

    Apr 4, 2014
    49
    0
    Yes Tindel I would say at 100usd you got a sweet deal. Although I am rather far from an expert! Unfortunately there seems to be an abundance of function gen's at good prices USD, and very few in GB£'s, And the Issue being postage. So I guess the god of all things great and electrical shall decide my fate on that one.

    May I ask to what part of my last few posts you are enquiring "What is the load" please? Thanks for the reply
     
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