PSpice question - Rectifing AC

[McGuinn]

I just download the eval of PSpice and started simulating AC full-wave rectification.
The problem is that the default (and only) AC supply seems to be half wave or something... I only get partial rectification through two of the diodes. Is there a way to change to a full wave setup... I tried editing the phase (to 360 degrees) but that gave the same results.

The AC and DC component of the AC supply is 6.4V, but when rectified is 5.59VDC. I expect to get 9.1VDC...

Any ideas?

 

[Styx]

Could you post yr cct so that we can see what is gonig on???

but anyway why do you have a DC offset???

 

[McGuinn]

That's a good question about the DC component. It didn't seem to work without it...!

If I set it to 0VDC, I get no currents, and no voltages...

 

[McGuinn]

I think i see part of the problem, but I am not too sure how to change it.
The AC supply is acting like a DC supply. There is the setup window for the supply... ACMAG is used during Sweep analysis, I'm not doing that...

What am I missing, before I head-but the monitor...??? :roll:

 

[Styx]

mmm that wont rectify, all you will get is the DC componet + some smooth AC

Remove the DC offset for starters. Also a rectifiying cct should look like this


This is how a single phase rectifier should look. Im talking abt the nodes for the diodes

 

[McGuinn]

I've tried that, and all I get is something close to rubbish.
How do I specify the 6.4VAC wave from peak to peak, with a frequency of 60Hz?
The other part of the problem is that I have to have a ground point... but my circuit is direct from a transformer... The ground point seems to be holding that side of the AC supply to 0VAC...
It's such a simple circuit, and it's just my lack of knowledge of PSpice at fault! I used it 8 years ago in EEng... but that was under Widows 3.11 and I forget.! :?

 

[McGuinn]

Gotta run... I'll get back to it in 3 hours... Thanks!

 

[Styx]

Ha you posted just before I updated my post.

Have a look at my last post.

As to the GND. Have teh GND on the AC side, but have a 10Meg resistor down to GND on teh DC side. Effectively this will be an open circuit, but it will help SPICE lock onto a solution

 

[Nigel Goodwin]

Well I've never used PSpice, nor ever seen any reason to? - but what you're trying to do is a very trivial circuit.

The circuit you posted originally was totally incorrect, it won't work in any case - the rectifiers were wired completely wrong. Styx posted the correct way to wire it, you need to try it that way.

Your problems with ground are because you are starting with the wrong premise, the transformer in a bridge rectifer circuit should NOT have any connection to ground. The ground is usually the negative output of the bridge rectifier, although it could be the positive if you wanted it that way.

The transformer would normally only connect to ground on a half wave recifier, or a full wave using two rectifiers and a centre tap - where the centre tap connects to ground.

 

[Styx]

Very true abt the grounding - transformers are used for the "transforming" of circuit parameters. Be it the voltage,current or impedance. But also for electrical isolation. The wrong plaing of GND in real life can cause no end of problems

However, from a Pspice point of view the internal algorithm's need a point of reference. Thus the placeing of GND nodes must be done.

Either referencing on the AC side and using a high impedance on the DC side so the SPICE algorithm can zero-in

 

[McGuinn]

OK, my bridge rectifier is totally wrong, that was a major mess-up. That's fixed now. Thanks Styx.
I still have a problem with setting up AC supplies though.
I've added a transformer, with the correct amount of turns on the L1 and L2, connected to an AC supply, but it's telling me that I have a voltage source loop, and only a series resistor will fix it. Once a series resistor has been added (of value 1 ohm), all is well, but no voltage originates from the supply...

Dunno, getting tired at this stage... I'll try a few online resources.

Nigel,
The reason I'm using PSpice is that I don't have a 6.4VAC transformer to hand, or a bridge rectifier for that matter. I want to learn how to use it again as it will save time in bodging circuits together...

 

[Styx]

ok here is another simulation tip

try putting a high Meg resistor (like a 10Meg) is parallel with winding 1 and also winding 2. Simulation tools have a problem with something called "Jacobian loops" where they keep trying to solve an ever decresing loop of parameters (nomally an inductive loop).

The resisitor in //el with the winding means that there is a tiny "resistive" loop which help the SPICE algorithm lock onto a solution

And Nigel teh reason I use simulation tools is because I work in the Aerospace sector and the AC supply that we have is 360Hz-800Hz, 96V->132V The only way to size DC inductors and DC caps w.r.t. THD is to run lots of analysis on the circuits to ensure that they operate within the specs for all load changes and supply changes lol

 

[McGuinn]

Styx,

Thanks for the tip, but this is what I get:
Where can I setup the frequency of the ac supply? Is this is what is missing?

Is there a good guide out there to assist with this?

 

[Styx]

you should have a VAC sourse block under teh SOURSE libary, that should solve your problem. If not post what you have and I'll fill in the blanks, simple cct so I dont mind

 

[McGuinn]

Styx,
I've used that too... this is what I get now...
Thanks for your time on this one!

 

[Nigel Goodwin]

Perhaps I'm missing the point here all together :?

But what are you actually trying to do?. I can see the reasoning behind trying to simulate a low pass filter or something like that. But for a simple bridge rectifier, what are you actually looking for?.

 

[McGuinn]

Simple, I want to use this to simulate an AC supply, and look at methods of rectifiing it to achieve good regulation and efficiency in a small space.

I first need to setup an AC supply with a simple circuit... I am so far unable to do that because I can't get PSpice configured correctly as regards VAC...

 

[Nigel Goodwin]

Simple, I want to use this to simulate an AC supply, and look at methods of rectifiing it to achieve good regulation and efficiency in a small space.

You've already got (in the circuit), a transformer, a bridge rectifer, and a reservoir capacitor - there's not a great deal you could do to improve it.

Regulation of a simple non-regulated supply like this is basically a function of the transformer (bigger the transformer, better the regulation), ripple is a function of the current drawn and the value of the capacitor. Neither of these really need much simulating - the value of the capacitor has been discussed a lot recently in other threads, and reduced to a very simple formula.

Obviously if you want decent regulation, use a regulated supply, a simple non-regulated one will always be comparatively poor.

 

[McGuinn]

Understood, but I also want to look at SMPS using a regulator you mentioned to me in a previous post... So the first thing I need to do is get AC working... no big mystery I would have thought...

 

[Exo]

Looking at how long this thread is running it seems to me it would be faster to just build the actual circuit for real on a test board and measure the information you want to know :lol: