LTspice - problem simulating simple rectifier - diyAudio
 LTspice - problem simulating simple rectifier
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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
Attached Images
 rectifier.png (63.8 KB, 403 views)

 18th August 2010, 03:30 AM #2 diyAudio Member   Join Date: Feb 2003 Location: .. 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, 03:32 AM #3 diyAudio Member   Join Date: Nov 2006 Location: Indiana Blog Entries: 1 Yoiu should enter the PEAK value for the sine's AMPLITUDE field, in the AC voltage source, not the RMS value.
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Quote:
 Originally Posted by gootee Yoiu should enter the PEAK value for the sine's AMPLITUDE field, in the AC voltage source, not the RMS value.
Yes. The entered value shows 24V which is the peak value. For 24VRMS he should enter about 34V peak (or 24V RMS; not sure you can spec RMS in LTspice).

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 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Why is LTspice reporting one diode Vdrop? 24Vpk and 23.3Vdc equates to a 700mV Vdrop? __________________ regards Andrew T. Sent from my desktop computer using a keyboard
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Join Date: Jul 2003
Location: Huddinge, Sweden
Quote:
 Originally Posted by AndrewT Why is LTspice reporting one diode Vdrop? 24Vpk and 23.3Vdc equates to a 700mV Vdrop?
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 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders so that means the simulator is reporting 350mVdrop for each diode passing 23.3mA. Correct? __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 18th August 2010, 12:01 PM #9 diyAudio Member     Join Date: Jul 2003 Location: Huddinge, Sweden 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. __________________ Do wizards use spell checkers?
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Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by runebivrin 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.
yes, we agree.
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