Selecting the Proper Transformer

Discussion in 'The Projects Forum' started by hondabones, Mar 26, 2010.

  1. hondabones

    Thread Starter Active Member

    Sep 29, 2009
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    I learned in school about the number of turns, step-up, step-down, and all that jazz. I am at a loss when it comes to finding the correct transformer. Does anyone know or can explain an easier way to go about doing this? Once I learn this we'll talk about the application I will use it for.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    You generally select a transformer that can supply the necessary power to your load. Generally, the load wishes some specific voltage, so the current capability drives the transformer selection.

    After the rectifier (assuming DC usage), the nominal AC secondary voltage will increase by roughly 1.4 on the filter capacitors.
     
  3. hondabones

    Thread Starter Active Member

    Sep 29, 2009
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    I want 120 V step-down to 5 V. I will use the 7805 to regulate it. As far as the rest. I don't know that. I need to learn about it. i.e. The filter caps you are referring to. Yes it will convert to DC
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    The conventional solution is to either get a wall wart and regulate it, or get a 12 volt center tapped transformer and full wave rectify it.

    If you are running old TTL, the rule of thumb is 80 ma/IC for figuring current.
     
  5. hondabones

    Thread Starter Active Member

    Sep 29, 2009
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    Something like this?

    [​IMG]
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    You need a center tapped transformer for a bridge rectifier to work. 1N4148's are moderately fast signal diodes, good for something like 30 volts and less than 50 mills. For power rectification, use larger diodes, like 1N400x series, or an encapsulated bridge.

    BTW, the transistor U1 is an attempt to generate a 60 Hz clock. The BE junction might withstand the reverse voltage, or not. It's much better to use an additional diode to block the negative going excursion.

    For a single secondary, you may as well take one lead to ground and simply half wave the rectification. Take a look at how the secondary current flows to see what I mean.
     
  7. hondabones

    Thread Starter Active Member

    Sep 29, 2009
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    A funny thing, I just learned this stuff in class today. lol.

    Your right, I need something like the 1N4001 diode. When I came up with this circuit I wasn't aware that the type really made a difference, in this case.

    I want to generate a 60 Hz clock. You don't think this is the best way to do it? I'm open to suggestions if you could draw up a schematic that would be great.

    This will be my first power supply that I have built. I want it perfect before I throw the switch and start dealing with 120 V.

    I think I understand the current flow, so I'm not sure why you say I should half-wave rectify it. Granted I just learned it today, but I think a half-wave won't give me what I want.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    It may not, but that transformer will not let you full wave rectify the output. You need one with a center tap on the secondary or else two secondaries that can have leads tied together to make a center tap.

    Add a 4148 diode to the signal path to the base of your transistor to keep the signal level from going negative. 680K is way too high. Figure the peak voltage and adjust the resistor value to pass 3 - 4 mills. That will be on the order of 3 - 4000 ohms.

    I have used a 2N7000 FET with a 1K resistor (plus diode) to generate a 1 Hz clock. Same general arrangement.
     
  9. hondabones

    Thread Starter Active Member

    Sep 29, 2009
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    Could you explain this? According to my professors and the textbook, it will.

    The 4148 diode is a good idea.

    I haven't learned transistors yet, but the professors say that the voltage across the emitter and base is 0.7 V. β = 170, using these numbers and a 9 V peak sine wave at the base we came up with those sizes of resistors.
     
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