Rectification on Multisim

Thread Starter

seanbein

Joined Mar 16, 2010
21
my input ranges from 160vrms~270vrms

Hi all, based on the picture posted, I have 2 questions to ask:
1.Where should the ground be?in pic 1 or pic 2?
2.This is intended for a smps with flyback converter. The output of this rectification will be fed to the primary of flyback transformer.On my transformer design, which voltage should I look to?the Vp-p (eg:160*root two)
or the vdc which is the output of this rectification?this is confusing because the vdc is about 100v on my channel B oscilloscope regardless of input 160~270 and the measurement probe indicates 49.5 at the second picture where the ground is changed.
 

wayneh

Joined Sep 9, 2010
16,384
Be careful with "ground". Usually, the ground in pic1 becomes ground (the lowest voltage pole) of the DC side. This isolates the DC side from the AC side. But problems arise when connecting "grounds" from different devices that may not share true earth ground. Oscilloscope ground is different than device ground, and current can flow. Bad.

Something's wrong if your DC voltage is not changing with the AC voltage, unless maybe the sim knows something about your diodes. Maybe they can't handle that voltage?
 

crutschow

Joined Mar 14, 2008
24,730
Without a mains transformer the actual earth ground will be as shown in pic2, but the DC reference ground (common) is as shown in pic1.

The measurement probe voltage will change with a change in ground connection since it is referenced to that ground point. The oscilloscope reading should not change (assuming it has an isolated differential input capability). But the output voltage should change with a change in the input voltage as wayneh noted.
 

Adjuster

Joined Dec 26, 2010
2,148
3N249 is made in several versions, rated from 50V VRRM upwards. http://www.datasheetcatalog.org/datasheet/vishay/3n252.pdf You need to have a VRRM (PIV) of at least twice the peak value, plus some margin.

It is suspicious that your output DC voltage is so low, but even more suspicious that the AC input current is displayed as 14.9kAp-p (!) Something is obviously wrong.

Note that your "AC" supply also seems to have a DC value, which seems very odd - is this deliberate, and if so, why?
 

Adjuster

Joined Dec 26, 2010
2,148
Oops, missed that. No isolation without a transformer.
Yes: Pretend "oscilloscopes in simulation programs often have inputs that behave as if they were fully isolated from each other - differential inputs, effectively.

Real oscilloscope inputs are normally single - ended, and share a common reference, firmly connected to mains ground.
The sort of measuring set up shown in the diagram in practice would result in a short-circuit.
 

Thread Starter

seanbein

Joined Mar 16, 2010
21
hi,i have replaced the rectification diodes with 1n4004 in practical and there is no problem with the amplitude changing with varying ac input.however, my output is only a half bridge rectified waveform.any suggestion on what to look at?
 
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