
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
18th August 2010, 04:15 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, 04:30 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, 06:53 AM  #4  
diyAudio Member

Quote:
jd
__________________
An engineer designing a feedback amp is a mixed emotions creature  Hendrik Bode Check out Linear Audio! 

18th August 2010, 11:19 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, 12:09 PM  #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 
18th August 2010, 12:31 PM  #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.
__________________
Do wizards use spell checkers? 
18th August 2010, 12:48 PM  #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, 01:01 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.
__________________
Do wizards use spell checkers? 
18th August 2010, 01:13 PM  #10 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

yes, we agree.
__________________
regards Andrew T. Sent from my desktop computer using a keyboard 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Simple SE and Rectifier Manipulation  bigjppop  Tubelab  1  16th January 2010 12:11 PM 
simple se blowing rectifier and fuse  newtube  Tubes / Valves  15  16th May 2009 11:54 PM 
Tubelab Simple SE Rectifier Problem?  eddiemeddiem  Tubes / Valves  45  21st June 2008 04:27 AM 
simple transformer/rectifier problem...  maxw  Power Supplies  7  10th August 2005 11:55 PM 
Simple bridge rectifier question.  JoeBob  Everything Else  51  15th February 2002 08:46 PM 
New To Site?  Need Help? 