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18th August 2010, 03:15 AM  #1 
diyAudio Member
Join Date: Sep 2004
Location: Sweden

LTspice  problem simulating simple rectifier
This is a simple question, but at this hour I can't figure out what I'm doing wrong.
I'm using LTspice to simulate a simple rectifier circuit using a sine wave generator (24 V, 50 Hz), a full wave diode bridge, a reservoir capacitor (2200 µF) and a load resistor (1k). I was expecting to see approximately 24 * sqrt(2)  2 * 0.65 (diode drop) = 32.5 V, across the load resistor (plus some ripple). Instead, I'm seeing 23.3 V. What am I doing/thinking wrong? Screenshot attached. Regards, Alexander 
18th August 2010, 03:30 AM  #2 
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Join Date: Feb 2003
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24 V is the amplitude of the sine source  ie the peak values are +/24 V
you have to multiply your argument in the sine source parameter list by sqrt(2) if you want "rms" 
18th August 2010, 05:53 AM  #4  
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Quote:
jd
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18th August 2010, 10:19 AM  #5 
diyAudio Member
Join Date: Sep 2004
Location: Sweden

Well, of course. Thank you!
Ever since I started playing/working with electronics (which isn't many years, but still), I've had the intuitive view of the RMS voltage of an AC signal, as a rectangle which occupies the same area under the graph as a sine wave does over the same range. Mathematically speaking; integrate abs(sin(x)) (a rectified sine wave) over the range of 0 to a (a = 2*pi*k), and divide the sum by a to get the height/amplitude of the rectangle that you've created. Example: Integrate abs(sin(x)) over 0 to 2*pi = 4. Divide 4 by 2*pi = 2/pi ≈ 0.637. 0.637 V would then be the RMS voltage of an AC signal with a peak voltage of 1 V. I did some calculations and found this terribly wrong. The real RMS voltage of an AC signal with a Vpk of 1 V is 1/sqrt(2) ≈ 0.707 V. What am I missing? 
18th August 2010, 11:09 AM  #6 
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Join Date: Jul 2004
Location: Scottish Borders

Why is LTspice reporting one diode Vdrop?
24Vpk and 23.3Vdc equates to a 700mV Vdrop?
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18th August 2010, 11:31 AM  #7 
diyAudio Member

Probably because those are Schottky diodes, and because the current is quite low. Forward voltage drop is a function of the forward current.
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18th August 2010, 11:48 AM  #8 
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Join Date: Jul 2004
Location: Scottish Borders

so that means the simulator is reporting 350mVdrop for each diode passing 23.3mA.
Correct?
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18th August 2010, 12:01 PM  #9 
diyAudio Member

I'm sure Spice aficionados can chime in here, but I'd prefer to say that "the model used for the diodes result in a 350mV drop at 23.3mA". LTSpice isn't really reporting the voltage drop across the diodes. It uses whatever the model tells it to.
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18th August 2010, 12:13 PM  #10 
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Join Date: Jul 2004
Location: Scottish Borders

yes, we agree.
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