Bipolar Power Rectifier

Discussion in 'General Electronics Chat' started by CurlsOnKeys, Jan 3, 2015.

  1. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
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    2
    Hi all,

    I've just started an electronic course in evening school and I'm studying a schematic of an old Moog synthesizer. I've got a question about the power rectification as attached in the image. As you can see, we go from 24VAC to +12V and -12V DC (necessary since the synth uses a lot of opamps). As I understand it, this is called "bipolar rectification", since you "rectify" AC voltage not to a single postive DC voltage, but to a positive and negative (hence "bipolar") voltage, correct?

    Now, when I google "bipolar rectifier", most example circuits I find consist of a diode bridge (like for example the circuit in figure 8 (bottom) of this page: http://www.bristolwatch.com/ele/power_supplies.htm).

    So the question is: are there any "advantages" or reasons why Robert Moog chose only two diodes, while "classic" bipolar rectifiers use a diode bridge? Cost effective? As I understand it (but correct me if I'm wrong), your capacitors have to be larger when you use only two diodes and the cost of two larger elco's is higher than using two extra diodes, no?

    Any insights would be greatly appreciated!

    Thanks,
    Senne
    Screen Shot 2015-01-03 at 11.50.21.png
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Your figure produces half-wave rectification at C37 and C38, meaning they have to be twice the capacitance (physically bigger, more expensive) than would be required with full-wave rectification to produce enough filtering. The ripple is at the line frequency 50/60Hz. Only two wires need run from the transformer to the box.

    Fig 7/8 at Bristolwatch produces full-wave rectification across their respective filter capacitors, meaning the ripple is at 2Xline frequency, so 100/120 Hz. This is easier to filter, meaning that for the same load current, the filter cap can be half the size as above. It takes a center-tapped transformer, so there must be three wires from transformer to the box. Wall-wart plug-in transformers with CT brought out are uncommon.
     
  3. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
    47
    2
    Mike,

    thanks for your clarification! I understand the difference between the two schematics a bit better now (overlooked the center-tap in figure 8). But I still don't really get why that choice is made. If I understand you correctly, you are basically saying the only reason they use half-wave rectification is because of the wall-wart-transformers that don't come center-tapped (only 2 wires), right?
    Which leads me to the conclusion that wall-wart-transformers are cheaper than ehm... "other" transformers (built in the box I suppose than?)

    Thanks!
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    They sure are. I can go to my local thrift store (Habitat for Humanity), and they have boxes and boxes of them. These come from retired electronics that people donate. I can buy one for less than a third of the price of a cup of coffee at Starbucks.

    Be aware that any wall-wart built in the last ten years is likely to be a switcher (SMPS), while the older ones use real iron and copper. Switchers are to be avoided around low-level audio and radios. You can tell the iron ones from the switchers by weight.
     
  5. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
    47
    2
    Cool! I knew it was just basic economics :) Thanks for the insights!
     
  6. crutschow

    Expert

    Mar 14, 2008
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    For small currents a half-wave rectifier and a larger capacitor is probably cheaper than a full-wave bridge, center-tapped transformer and a smaller capacitor. But for larger currents a full-wave rectifier and smaller capacitor is generally preferred, especially if the plus and minus currents are significantly different.
     
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