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 Post subject: Modeling transformer windings in LTSpice
PostPosted: Jan Sat 08, 2022 2:18 am 
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I tried to model the circled power supply in LT Spice. Apparently there's no DC. Click on the LTSpice simulation and you'll see faintly a duplicate sine wave in blue across the bottom. These are voltage readings taken at the upper right and lower right rails with ground at the center tap as usual. I fed the diodes and caps a split AC power supply.

In a conventional two diode full wave power supply, the transformer center tap is at ground, as shown in the second example in the book. Yet the doubling circuit only works when I designate ground to be between the two capacitors and ignoring the center tap, effectively making the 12V CT transformer a single turn 24V transformer. What's going in here? I think I know but I'd like to hear what you think.


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 Post subject: Re: Modeling transformer windings in LTSpice
PostPosted: Jan Sat 08, 2022 2:42 am 
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Location: Detroit, MI USA
But the transformer center tap is not ground, it doesn't even need to be there since it connects to nothing.

What our college electronics professor explained was that the simplest way to look at that circuit is you have a positive supply, and a negative supply in series with each other so you effectively see twice the input voltage from the transformer. The reference point for that positive supply and negative supply is one end of the transformer winding which is the center point of the two supplies (capacitors).

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 Post subject: Re: Modeling transformer windings in LTSpice
PostPosted: Jan Sat 08, 2022 5:13 pm 
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That's what I have decided was going on. And it appears that the user has a choice of two ways of using this circuit.

Using the lower output as ground yields a DC output double that of the rated tansformer. Using the common capacitor connection as ground yields a positive and negative split supply at the rated transformer voltage.

Still it's counterintuitive to me that this provides full wave rectification.


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 Post subject: Re: Modeling transformer windings in LTSpice
PostPosted: Jan Sat 08, 2022 11:33 pm 
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Macrohenry wrote:

Still it's counterintuitive to me that this provides full wave rectification.



In one case it is a split supply with half wave rectification for each supply. In the other case it is a full wave doubler. The distinction is when the supply outputs you are using are "refreshed" on each half of the sine wave cycle at a 120Hz rate making it full wave, or only at 60Hz making it half wave.


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