problems with my new dso 5 in 1

Discussion in 'General Electronics Chat' started by ninjaman, May 3, 2016.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello,

    i bought one of these
    http://www.ebay.co.uk/itm/USB-2CH-2...919203?hash=item3a8c98ad23:g:pTUAAMXQlgtSysjQ

    and i plugged it in to a half wave rectifier circuit that i am learning about and got 4 volt peak to peak. the sine wave didnt look too great either. the frequency was correct at 50Hz. i had the probe set to 1x. i tested the dso using the 1kHz test signal and that seemed ok. the circuit i built is a 240v / 12v transformer 6va. i measured 16 volts without a load. i tried to measure this using the oscilloscope and got the 4 volts mentioned above. am i doing something wrong (which is my guess) or is the dso cheap rubbish (another guess).

    any help would be great, hope your all doing well by the way!!!

    thanks

    simon
     
  2. SLK001

    Well-Known Member

    Nov 29, 2011
    811
    224
    Where is your schematic? And a picture of your circuit? Without these, all we can do is guess, too.
     
  3. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    here is the circuit the diode is a 1n4004, 1k ohm resistor 1/2 W.
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
    1,981
    388
    The cap is the load?

    What are you trying to see?
     
  5. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello
    i have to measure the supply voltage and the load voltage. so far i have just the diode and resistor connected, i want to get the measurements right first before moving onto the other stuff. i measured the output voltage of the transformer with a multimeter and got approx 16.4 volts. i take this as rms and so multiply by 1.414 i would get the peak value of 23.189. when i connect the oscilloscope, i set the probe to 10x and to measure AC. i get 22.9 volts and sometimes 23.1volts, it flickers between the two. this is my max voltage, the peak to peak value is 46 volts. then i connect the probe to the resistor, otherside of the diode and set the osciolloscope to DC. i get 22 volts max and 22.9 volts peak to peak. the sine wave is rectified so i am only getting the half wave this time. when i measure with a multimeter on 20v DC range i get 7.2 volts approx..,.....so if i have a peak to peak value of 22 volts, i would halve it to get the peak value of 11 volts and then to find the rms value i would multiply it by 0.707 to get 7.77 volts.
    although i dont think this is correct. the diode is cutting the waveform in half so i am only getting the one half which is one half the pkpk. but the value of DC voltage across the resistor is weird. i dont understand if this is right?

    after i put the capacitor in, i measured the voltage across the capacitor using the 200vDC range on the multimeter. it measured just over 20 volts. then i measured the capacitor using the oscilloscope and got 3.8 volts peak to peak. but i imagine this is the bottom to top of the ripple. is this the correct result. the waveform looks right. however the oscilloscope seems a little jumpy with its readings. leaving me to think that i am not going to get a good accurate result. there has been some error with calculations and the oscilloscope. not to mention my lack of knowledge experience with this equipment and process. i will put some error down to cheap equipment and components that are not 100% accurate. but how far from accurate is all this?

    any help would be great!!!
     
    Last edited: May 3, 2016
  6. BR-549

    Well-Known Member

    Sep 22, 2013
    1,981
    388
    I am no design expert, but for what you have connected, looks good to me.

    This should change with filtering and load.

    A new scope is like a new mule. You have to get use to one another.
     
  7. SLK001

    Well-Known Member

    Nov 29, 2011
    811
    224
    A meter measuring RMS is only correct when measuring a sine wave. Also, the formulas you are using are also sine wave equations. Once you rectify your waveform, the RMS value isn't correct (though in this case, it is close to the measured values).

    The waveform may look better at closer scales, say one or two cycles. There is no need for as many as you have shown. Also, if your are comparing the two, both channels should have the same scale.

    I'm not sure what your goals are here. Perhaps you should elaborate.
     
  8. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello,

    thanks for the replies!
    i am trying to learn about electronics. i have a book called "discovering electronics" which has a lot of practical and theory. however the theory doesnt cover the problem that i am having here. i am on the first project in the book which is building and testing a power supply. there are a number of tests, each expands on the last and your required to write down your findings. the book is aimed at students taking a course and is meant as a supplement to the course. no course in particular, just a book written with the intention of providing an already written course of electronics. i have both of the new art of electronics books and wanted to use the student manual to learn from. though i ran into some problems and found it a little confusing.
    thanks for your help.
    i would like to understand why i am getting half the voltage of the rectified wave across the resistor. i measured across the diode and i think that the diode and resistor act as a voltage divider. is this correct?
     
  9. BR-549

    Well-Known Member

    Sep 22, 2013
    1,981
    388
    Yes it can be confusing. Don't get flustered, just take your time. I would like to make a suggestion.

    Don't try to build a power supply yet. A diode and resistor is not a voltage divider. There is a lot to learn first. You need to learn what a diode is and how it works, before attempting to use one. But while you are studying, you can practice using your scope.

    Find an AM radio or a audio player that meets these conditions.

    Working unit.

    Battery power.

    A schematic for the unit.

    Make sure you can easily get to the circuit board.

    Now you have something that works, and you can learn all your scope functions safely, and follow the different signals thru the different circuits.

    It will take some time to learn the scope. Study the manual. Look up un-familiar terms.

    Learn to use it on a working battery powered circuit with good signals first.

    Looks like a nice scope. Enjoy.
     
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