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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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27th Oct 2020, 3:37 pm | #1 |
Triode
Join Date: Oct 2020
Location: Tranent, East Lothian, Scotland, UK.
Posts: 17
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Two diode rectifier circuit in 1970s Bush radio
Hello
I'm new to electronics interested in audio analogue circuits. I'm slowly learning principles from a couple of books, alongside poring over a schematic for a Bush VHF102 that I have. I'm intrigued by the two diode circuit that I'd been understanding so far as functioning as a rectifier. From all the diode rectifier circuits I've seen in my books and online, the diodes are shown 'pointing' away from the transformer - i.e. they 'point' away from the secondary windings towards the load (excuse the non-technical language!) However, on the schematic, and on the circuit board, the BY126 diodes 'point' towards the transformer. I've attached a picture of the portion of the circuit on the schematic. So, this doesn't appear to be a rectifier circuit at all. Is the diode pair, with their parallel 1000p capacitors, simply managing the polarity of the AC output from the transformer? I also note that the arrangement is basically the same as a npn transistor. Many thanks for any explanations. Chris |
27th Oct 2020, 4:19 pm | #2 |
Octode
Join Date: Jan 2003
Location: Ware, Herts. UK.
Posts: 1,082
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Re: Two diode rectifier circuit in 1970s Bush radio
This a bi-phase rectifier as commonly seen in valve radios etc. What is different is that with the diodes connected cathode to transformer secondary as shown, the DC output is negative with respect to the transformer secondary centre tap 0V point.
The reason for this is that the circuit shown uses PNP germanium transistors and it is convenient to think of the power supply rail as negative for such circuits. The 1000pF capacitors are there to attenuate noise from the diodes. John |
27th Oct 2020, 4:40 pm | #3 |
Hexode
Join Date: Apr 2007
Location: W Yorks, UK.
Posts: 406
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Re: Two diode rectifier circuit in 1970s Bush radio
Notice that your rectifier produces a negative DC output voltage; transformer centre-tap is grounded as you normally see in textbooks. But you can get the same result whichever way the diodes 'point', depending on where you place the ground reference.
Last edited by daviddeakin; 27th Oct 2020 at 4:47 pm. |
27th Oct 2020, 4:46 pm | #4 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,783
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Re: Two diode rectifier circuit in 1970s Bush radio
The drawing is a bit confusing, but it's a conventional 3 terminal transformer rectifier circuit. They tended to be replaced with bridge rectifier designs, as an extra pair of diodes cost less than a centre tapped transformer winding.
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27th Oct 2020, 6:26 pm | #5 |
Heptode
Join Date: Jul 2009
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 722
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Re: Two diode rectifier circuit in 1970s Bush radio
Hi
What you will find is that circuit diagrams are often drawn by people who have no concept of flow or clarity, this will confuse you more than you should be confused. As long as the wires go to the right place then they don't care or possibly they don't know that they are being as plain as mud. Often re-drawing a circuit will reveal its secrets. Believe it or not corners on a circuit are possibly the main enemy of clarity - see how many there are on your circuit! Cheers James |
27th Oct 2020, 6:40 pm | #6 |
Triode
Join Date: Oct 2020
Location: Tranent, East Lothian, Scotland, UK.
Posts: 17
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Re: Two diode rectifier circuit in 1970s Bush radio
Many thanks all for the quick responses - they're helpful and appreciated! Well, at least I was right initially: it is a rectifier circuit. It does say in the schematic notes that all voltages shown are negative to the chassis...a clue that did have me wondering.
Cheers Chris |