# LT Spice - Bridge Rectifier

1. Dec 10, 2008

### lukesowersby

1. The problem statement, all variables and given/known data
I am to build a linear power supply in four stages. However, at stage two, i have to add a bridge rectifier to the ouput of my transformer. My lecturer at university says i should get a graph that is a sine wave, with the negative values reflected into the positive x axis (Volatage). I was wondering were am i going wrong?

2. The attempt at a solution
http://img399.imageshack.us/img399/1367/ltspice1gm7.jpg [Broken]

Any help would be greatly appreciated.

Thanks.

Last edited by a moderator: May 3, 2017
2. Dec 10, 2008

### The Electrician

You need to plot the difference between rectifidp and rectifidn. Or, alternatively, plot the voltage across your load resistor, R5.

3. Dec 10, 2008

### lukesowersby

Yeah, had a suspicion it might have been that, but it took me a while to work out how to enter an expression. Only started using the program a few days ago, so i'm still getting used to it.

http://img218.imageshack.us/img218/138/ltspice2rc0.jpg [Broken]

Many thanks for your help.

Last edited by a moderator: May 3, 2017
4. Dec 10, 2008

### lukesowersby

I've got another problem. For a full wave bridge, with a capacitive filter, we are told the following relationship between Vdc [V(Regout)-V(Rectifidn)] and Vac [V(Secondary)]:

Vdc = 1.41 x Vac

However, i'm obtaining the following results:

http://img386.imageshack.us/img386/5310/lt4vr1.jpg [Broken]

I'm just wondering if i have the wrong formula, or taking incorrect readings?

Thanks.

Last edited by a moderator: May 3, 2017
5. Dec 10, 2008

### Redbelly98

Staff Emeritus
A couple of things to keep in mind here:

That formula uses the rms voltage, not the amplitude, for Vac.

Also, it does not account for the 2 diode drops in the rectifier voltage.

6. Dec 10, 2008

### lukesowersby

Ah, forgot about rms. Thanks.

That equation was given a bridge rectifier (4 diodes) with a capacitor and load resistor, but exluding the regulator. So i assume to calculate Vac, if i want Vdc of 5, i simply calculate 5+the voltage acrross the regultor, all divided by 1.41.

7. Dec 11, 2008

### Redbelly98

Staff Emeritus
You can check that on your Spice circuit, "transformer1 stage 5".

If you plot

V(rectifidp) - V(rectifidn) ​
and
V(secondary) ​

you can see if the regulator input voltage equals the amplitude of the transformer secondary.

8. Dec 11, 2008

### lukesowersby

Yeah, thanks for your help.