Bridge Rectifier Help

Discussion in 'The Projects Forum' started by guitarguy12387, Jul 1, 2008.

  1. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Hey i'm kinda a noob. I am looking at building something like this:

    http://www.aaroncake.net/Circuits/supply.asp

    But in my search for an appropriate bridge rectifier, i am somewhat dumbfounded. I have been looking on Newark's website and there's a ton of different parameters when selecting bridge rectifiers and i don't know what i would need for this particular circuit. If anyone could help me select an appropriate one, it would be much appreciated.

    Thanks in advance.
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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  3. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Wow thanks for the great reply. Very helpful. And i'm glad those are cheap! hah. If you don't mind, could you give a bit of an overview as to how you came to narrow my choices down to those? Thanks again!
     
  4. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Also, I am in a similar situation for the transformer. Too many options, and i have extremely limited experience with transformers. Any help would be great. Thanks again!
     
  5. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I just filtered on rectifiers with a sufficient current and voltage rating. The transformer is specified at 24V 5A, so a 50V 8A regulator will be fine. The specified transformer is a 120VA unit (IV). Choosing transformers and rectifiers is not an exact science since they only make them in certain sizes and ratings which means a bit of rounding-up is usually necessary.
     
  6. guitarguy12387

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    Apr 10, 2008
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  7. Wendy

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    Mar 24, 2008
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    Something to think about, a computer power supply is relatively cheap, has oodles of current, and can be filtered for smoother output (the output ripples a lot as is). I'm also fond of wall warts, as they get the hottest part of any circuit away from the rest of the electronics.

    Are you building this to learn how they work, or for practical use?
     
  8. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Thanks, that's true. I think i'm doing it a little for both. I would like to learn a bit more about it, but i'd also like to be able to use it to mess with some other elementary circuits. I figured that since it's variable voltage and relatively cheap, it would be alright. I do wish it was variable current as well, but ya gotta start somewhere i guess haha.
     
  9. Tahmid

    Active Member

    Jul 2, 2008
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    Hi,
    In my opinion, it is best to start with 0.5A or 1A power supply.
    It will work for basic electronic and digital circuits.
    If bridge rectifiers are unabailable for your 5A power supply, you could make one with 4 6A10 diodes.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Go to National Semiconductor's site and download the datasheet for the LM117/LM317, or click here:
    http://cache.national.com/ds/LM/LM117.pdf

    There are lots of application schematics in the last half of the datasheet, including a constant current, constant voltage regulator. There is also Application Note 181, available here:
    http://www.national.com/an/AN/AN-181.pdf

    There is also their datasheet for the LM338, accessible from this page:
    http://www.national.com/mpf/LM/LM338.html
    or directly:
    http://www.national.com/ds.cgi/LM/LM138.pdf
    Many more applications in that document.

    Now if you'd like to build a simple switching step-down voltage regulator that won't require a huge heat sink like the others will, look at the LM2679:
    http://www.national.com/pf/LM/LM2679.html
    With the LM2679, you can build a much more efficient supply than you could with the LM317 or LM338.

    The LM317 and LM338 are venerable linear regulators; they've been around a long time (since the early 1970's!) and are relatively simple to use. The trouble with linear regulators is that they waste a great deal of power as heat. The greater the differential between the input and the output, the more power they have to dissipate. The worst-case scenario comes when you have a 37v input and you're supplying a high-current load with the lowest voltage output setting from the regulator. You will need a very large heat sink with a fan to get rid of the heat, or the regulator will go into thermal shutdown, which is very stressful for the regulator.

    Switching regulators are far more efficient because they're either on or off. The output will have some ripple, which can be reduced considerably by filtering. The construction of a switcher is somewhat more complex, but the efficiency is greatly improved over a linear version.

    I suggest that you build both types. The linear version will be handy when you are experimenting with linear circuits that need very low supply noise. The variable voltage switcher will be handy for many less-sensitive circuits.

    But one item that you'll probably use the most is a converted ATX-form factor supply from a junked computer. This Google search will give you 50k+ hits:
    http://www.google.com/search?hl=en&q=convert+atx+bench+supply
    The biggest limitation of the converted ATX supplies (aside from being fixed voltages) is that the negative voltages (-5,-12) generally have low current output compared to the positive supplies - that's because those are only used for RS232 com ports. However, they can still be very useful for testing most of your small projects.
     
  11. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Thanks. Ya i figured the amperage was a little higher than what i'll need. Maybe i'll keep looking for better schematics. Is it terribly difficult to make a power supply that is both variable voltage and variable current output?

    I dunno if its a weird coincidence or something... but i just tried to order some of those rectifiers from Newark and they are out of stock hah.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    It's been awhile since I ordered from Newark, but don't they have a minimum $ amount order requirement? Mouser does not, neither does Jameco. You might find some deals at Electronic Goldmine. MPJA.com has lots of power supplies and transformers for decent prices. They also have occasional sales; I bought double-sided PCB's on sale there for less than I found anywhere else.

    For your electrolytic caps, I suggest that you buy them from a large supplier like Newark, Mouser, etc - their stock will be fresh. You don't want to put "new old stock" electrolytic capacitors in a new project, because they are very likely to blow up in your face! :eek:

    There is a way to "restore" older aluminum and tantalum electrolytic capacitors called "reforming"; you supply them with their rated voltage at very low current through a resistor over a period of time until either their leakage current is within tolerance, or they are discarded due to their leakage current being too high. However, since you don't have a supply yet, you can't do that!
     
  13. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Nope. I just placed an order yesterday actually and no minimum order. But thanks alot for the advice. I will check out those suppliers when placing orders. I actually just ordered a few of the LM317s. They are for another project though, but i think i ordered a few extra that i'll play with.

    Thanks again
     
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