Rectifier for AC to DC circuit

Discussion in 'General Electronics Chat' started by Uman, Jun 14, 2007.

  1. Uman

    Thread Starter Member

    May 2, 2004
    11
    0
    Hello,
    I am designing a transformerless linear AC 120v to DC 5v power supply with an output of about 500ma for a PIC microcontroller and LCD.
    I am considering a Fairchild bridge rectifier. The Fairchild data sheet lists 7 different models (DF005 - DF10M) with the same specs but with different max reverse voltages (50 -1000v).
    Should I just use the 1000v max reverse voltage rectifier or does a lesser one have an advantage?

    Jeff in Florida
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Without a transformer to isolate you from the line voltage, the project is extremely dangerous. Is there some overriding reason not to use an inexpensive wall transformer and regulator?
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,175
    1,798
    For what it's worth I think this is a stupendously ill advised idea. It's bad enough to make one for your personal use, but it would be a mistake to productize this thing and subject people to a true safety hazard.
     
  4. Uman

    Thread Starter Member

    May 2, 2004
    11
    0
    I have struggled with the notion of using a transformer and hoped to eliminate it from the circuit. I have researched the circuit and have seen views to use a transformer and not to use one. The power supply will use a regulator and will be fused. I was planning on using a zener diode with resitstor to drop the voltage. Why is it dangerous without a transformer?

    Jeff in Florida
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,175
    1,798
    One of the dangers is plugging into a wall socket that has been wired backwards. Lights and curling irons don't care. You will care when you touch a piece of exposed metal and find that it is AC Hot. Don't they teach anything in school anymore?
     
  6. Uman

    Thread Starter Member

    May 2, 2004
    11
    0
    The AC input connects directly to a bridge rectifier. I believe the rectifier does not care which input terminal is hot or neutral; the output will be rippled DC. How does a plug reversal cause concern here? The circuit is fused and case grounded.

    Cheers,
    Jeff in Florida
     
  7. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    Why bother doing this when a decent switch mode 5VDC 1A wall wart costs like $15?

    What is it about the transformer that is troublesome? The size?
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I'm hoping you know that you might die if you touch the hot side of the AC line and earth ground at the same time. Look at the schematics below. Do you see the problem?
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The resistor would have to dissapate more than 80 Watts. Your regulator could do double-duty as a space-heater.

    I very strongly suggest using a nice simple transformer.
     
  10. Uman

    Thread Starter Member

    May 2, 2004
    11
    0
    I am convinced a transformer is essential. But what about the original question about the bridge rectifier reverse voltage? Any thoughts here?

    And yes, I have considered the alternate molded case power supply.
    My design was to consolidate all components on one PCB, and the transformer will be a challenge to locate, due to its size and weight. I like the idea of an modular power supply, but I have not found one with terminal inputs and terminal outputs in the low cost range.

    Jeff in Florida
     
  11. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    The bridge rectifier will go on the secondary side of your transformer which will
    have a voltage that is well under the 50V reverse voltage of the rectifier.

    (* jcl *)
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    It depends on the peak voltage of the transformer, but in your case, you are fine using a DF005. Also, there is no vantage on using a bridge with more reverse voltage rating, since the voltage drop and voltage drop behavior
    with different currents is exacly the same.
     
  13. Uman

    Thread Starter Member

    May 2, 2004
    11
    0
    Since they cost about the same, is there any disadvantages with the 1000v reverse voltage? Why not use the 1000v in all cases?

    Jeff in Florida
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    There is no need for that. Also, if you are doing a project for others to see, asking for a DF005 will leave people with other options. Asking for a DF10 won't. Especially with newbies that can't quite understand your project.

    But I think this issue is more a question of taste.
     
  15. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Because with the higher reverse voltage usually comes a higher forward voltage
    and lower forward current. The higher forward voltage increases the power
    dissipation in the bridge.

    (* jcl *)
     
  16. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Where did you saw that? In all datasheets I see, the I/Vf curves are the same for diodes of the same type with diferent reverse voltage ratings.
     
  17. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Something like this
    http://www.pulseeng.com/products/datasheets/LT2006_06.pdf
    You should pick the bridge based on max current and max voltage and overrate it at least 25 percent. 1000v reverse is ok. The reason to derate that is cost.

    Another space saving gadget is a three terminal switching regulator. The pinout fits in a standard three terminal linear pinout.

    Of course TI are blithering, ****** ***** morons for discontinuing this but someone will gut them unmercifully and offer an alternative.

    http://focus.ti.com/docs/prod/folders/print/pt5041.html

    All you can get is their proprietary puke.
    http://focus.ti.com/docs/prod/folders/print/ptqa420050.html
    It is totally isolated and it's ok, I just like their three terminal ones better.
     
  18. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    The last diodes I needed to specify where Schottky diodes for a DC-DC converter.
    If you take a look at the On-Semi MBR0530T1 and MBR0540T1. The
    530 has a Vrrm=30V, If=0.5A and Vf=0.43. The 540 has a Vrrm=40V, If=0.5A
    and Vf=0.51V. I believe they may have a 20V device as well with an even lower
    Vf. Sizing the output rectifier for the proper breakdown voltage makes a big
    difference in efficiency.

    The Fairchild DFxxx specification does have the same Vf and If specifications for all
    of the parts. Perhaps it is a wide specification or maybe the parts are screened for high voltage operation. I am not sure why.

    (* jcl *)
     
  19. niftydog

    Active Member

    Jun 13, 2007
    95
    0
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