Bridge Rectifier Problem when connected to Wall Power Outlet

Discussion in 'General Electronics Chat' started by richeek.arya, Nov 5, 2009.

  1. richeek.arya

    Thread Starter New Member

    Oct 21, 2009
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    Hi all
    I know this is a long post but I tried to put everthing...

    I am making a bridge rectifier to give DC voltage to another circuit. At the output of the rectifier I am connecting 0.47uF-250V, mylar capacitor and across the capacitor I am connecting resistive load.
    The bridge should rectify 120 VAC from a WALL OUTLET to obtain a DC voltage. I am trying to acheive as close as possible to 170 VDC.
    Now the problem is that I want to get this voltage with respect to circuit ground. What happens is if I try to connect other end of the capacitor to ground some diodes blow off. Even if I try to connect oscilloscope between two terminal of capacitor some diode blows off.
    I searched it over the net and found out similar post: http://www.electro-tech-online.com/.../5568-full-wave-bridge-rectifier-problem.html
    I think I am also facing the similar problem. Could anyone please tell me why this is happening? Why the other end of the capacitor is not always at potential 0V? What should I do now to get a 170V DC power supply?
    I mesure the voltages at both ends of the capcaitor by just connecting positive terminal of a 10X probe to the circuit and floating the probe's ground. And I saw that capactor terminal that I think should be at 0V is actually oscillating between 0V and -170V.
    At the same time other end of the capacitor is oscillating between 170V and 0V. Hence together they will give me 170VDC. But how to connect them to another circuit?
    Kindly provide some inputs I am so confused.
    Thank you.
    Sincerely
    Richeek Arya
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You are experimenting with mains power, and it will soon kill you if you do not use at least an isolation transformer.

    You can buy 1:1 isolation transformers rated for 120v quite inexpensively.

    Please do not continue to try using mains power directly, as it is very dangerous. Your rectifier bridge does NOT provide isolation from the mains power.

    We are very concerned about safety here at AAC.
     
  3. richeek.arya

    Thread Starter New Member

    Oct 21, 2009
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    Thanks SgtWookie for showing me the absoulte danger. I will buy an isolation transformer. But could you tell me why the other terminal of the cap is not at 0V permanently?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I don't know where you are. Mains power is different worldwide.

    In the USA, typical household current is supplied at 120v. Coming in to the breaker panel, there are two "hot" lines called L1 and L2, a "Neutral" line, and an earth ground line that is attached via a large conductor to a copper-clad stake (or two or three) driven deep into the ground.

    The "Neutral" line (which is actually the center tap on a step-down transformer) is connected to earth ground at the panel for safety. If there is a fault in the transformer, the fuse should blow instead of electrocuting people in the dwelling.

    Power is fed via circuit breakers to various outlets around the home. If there is heavy loading on one side of the circuit (either L1 or L2), the "neutral" line may stray several volts away from ground.

    I do not know why you were seeing such a large imbalance. I suggest that you have an electrician look at your wiring to fix the problem.
     
  5. richeek.arya

    Thread Starter New Member

    Oct 21, 2009
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    Thank SgtWookie. I am also is USA. Here in my design I am using black wire and white wire of the power outlet. So ideally speaking, if I connect oscilloscope across capacitor will it short the circuit?
     
  6. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Yes I have always been told to use an isolation transformer for safety too. I just always accepted the fact and never really considered how an isolation transformer performs this important function.

    How does this happen?

    Trying to reach back trough all of those cobwebs in my memory of electronics theory, I would guess it has something to do with interrupting the magnetic field in the secondary of the transformer thereby limiting the current. am I close?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    A transformer removes any direct connection to the mains power. Power is inductively coupled instead. The primary winding must be fused. The secondary winding is referenced to ground. If a short develops between the primary and secondary, the fuse will blow.

    The amount of power coupled from the primary to the secondary in a 50Hz/60Hz transformer is directly related to the physical size of the transformer.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    if you draw your circuit out, you will see that if you reference either DC terminal to ground, that you are effectivley shorting out your diodes. The oscilliscope does the same thing because your reference lead is grounded.

    The NEC is clear on this by stating that your circuit (consumer) cannot be grounded anywhere other than at the service entrance.

    As Sgt.Wookie indicated, you must isolate your circuit for safety sake. You can isolate your scope for measurement sake, but my suggestion would be to immediately cease any work on live mains.

    Another consideration is that you want 170 volts. On a 120 volt main, you will only get this without load. You may want a different ratio isolation transformer if your drawing any load and want to maintain the 170 volts.
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
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    Besides solving the potential shock hazard issue, the isolation transformer before your bridge will solve your problem as well. Notice that, without an isolation transformer, both bridge outputs would virtually be shorted to live half the time, in a 60Hz cycle. That is why your diodes are being blown when you try to ground either output.
     
  10. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Thanks yes I know it is inductively coupled. But what is it about the induction that results in minimal risk? Or is it just simply as you stated that the amount of power is relative to the size of the transformer? Limit it's size and limit it's hazard? If you have a huge 1:1 transformer are you then back in the same danger zone as you would be if you were connected direct to the AC?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    If you had a very large 1:1 transformer, you could get quite a bit of current out of it. However, you can ground either end or the center tap of the secondary, or connect it to a bridge rectifier. You ground either the - or + output of the bridge, depending on whether you need a positive or negative voltage supply.

    Then you fuse the primary "hot"/"live" lead. If an overload (excessive current draw) occurs, whether it's just too low of a resistance on the output, or a short from the primary to the secondary winding, the fuse blows and prevents damage to the equipment.

    Keeping the physical size of the transformer small will limit the amount of power that can be inductively coupled to the output.
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    When you learn about voltage sources there is the concept of source resistance. The power company has specifically designed the power distribution system to have an incredibly low source resistance. They can and will supply virtually unlimited amounts of power to the service entrance by virtue of their extremely low source resistance. What this means in safety terms is that very high currents will be supplied with little or no drop in voltage.
     
  13. richeek.arya

    Thread Starter New Member

    Oct 21, 2009
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    I have understood all the risk associated but one rather silly doubt that I have is neutral is connected to ground in the domestic power supply. And in the rectifier my one input is the hot wire and the other one is neutral. I checked that by measuring each input waveform by oscilloscope (not connecting the ground pin of it). I got sine wave on one input and 0v on other. Now if neutral is grounded how does connecting one end of the diode to ground shorting the neutral? Now if I go with my theory that one end of the ac power supply is grounded I can not explain the the working of rectifier. How to explain the working of bridge rectifier when one of the input of it is at constant ground potential. Because in this case one diode will be permanently reverse biased. I think I need a break!!!
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    You can't use a FW bridge rectifier when one of the AC inputs is connected to ground. On one-half cycle, the bridge will work as expected; however on the reverse half-cycle, one of the diodes in the bridge will cause a short circuit to ground.

    If you use an isolation transformer, you won't have that problem.
     
  15. richeek.arya

    Thread Starter New Member

    Oct 21, 2009
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    Is it true that neutral is always grounded? Means its potential is always at 0V. If this is true how can we explain the working of Full Wave Rectifer. I am attaching a fig.
    [​IMG]
    In this fig. during negative clock cycle top right diode will become Reverse biased if we consider neutral to be at 0V as the capactior was charged to high voltage(appx 170VDC) during the positive cycle. So how does FW rectifer works in this case?
     
  16. beenthere

    Retired Moderator

    Apr 20, 2004
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    With AC neutral tied to ground - as it is if the wiring is to code - the bottom diodes short the negative cycle to ground - although only the left diode will ever conduct. Point B, ground, and the AC neutral line are all at the same potential.
     
  17. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    if your nuetral is grounded (which it is), and your point B is grounded, as you've indicated, that would be the same as moving your nuetral to point B. That then should show you clearly the shorting potential.

    you will explain it by not referencing your DC common and AC ground as the same point.

    if your AC coming in is isolated, then the 'ground' becomes a reference and not an absolute. A rectified DC supply is typically not connected to ground, but left floating. The 'ground' is a bit of misnomer, and the term 'common' is more appropriate.
     
    Last edited: Nov 7, 2009
  18. willy1usa

    New Member

    Feb 9, 2007
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    Ditto to the other posts - please get isolation transformer.

    Make sure you've got the rectifier hooked up correctly. DC voltage from rectifier will be 1.41
    X mains AC voltage. 110vac = 155vdc. My mains outlets in US usually have 120v. 120vac X
    1.41 = 169 vdc.

    PLEASE !! Be careful. I forgot the exact figure, but a fraction of an amp across your heart
    can STOP IT !!
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    At 120VAC, somewhere between 20mA and 50mA current through the heart can be lethal; even lower current under certain circumstances. I'm not a medical doctor; I just know enough to not allow that current through my chest!
     
  20. richeek.arya

    Thread Starter New Member

    Oct 21, 2009
    12
    0
    Thanks guys for all the help. Problem is resolved now. Until I get a transformer I am using a half wave rectifer with all the necessary precautions.
     
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