Multisim rect' DC with 0V ripple! How to inject/simulate AC ripple please?

Thread Starter

G.Beard

Joined Jun 22, 2015
18
Hello!
Firstly thanks for looking.

I have 240 RMS > Fully wave rectifier > 170uF cap.
200mA ILoad
5% Vripple = 17Vpp

The above is what I believe should be happening more or less. Im my sim I get 0Vpp.
How do I introduce this ripple? I thought maybe I could slap another AC source 17Vpp into the mix but it seems a bit crude.
 

DickCappels

Joined Aug 21, 2008
10,124
Per MikeML you should see ripple if the simulation is representative of the real-life circuit.

Please post circuit as shown in your simulator and also the simulation file. We might be able to spot a way to help you.

Note: This thread moved to General Electronics Chat for better response.
 

Thread Starter

G.Beard

Joined Jun 22, 2015
18
here is a screen shot, I am running my simulator in VMware fusion Windows 7 guest and my Mac will not let me upload the .MS14Screen Shot 2015-06-26 at 14.13.37.png file.
 

MikeML

Joined Oct 2, 2009
5,444
There is no ripple because there is no load current. The capacitor charges to the full line voltage on the first half-cycle, and there is nothing to discharge the capacitor. Try a resistor load, or a constant current source wired to discharge the capacitor.
 

Thread Starter

G.Beard

Joined Jun 22, 2015
18
Hi, I put a pot across the cap until I got roughly 0.2A
Please have a look at my file.
Thanks.

EDIT. That was the wrong one!
Here is the one with the load attached.
 

Attachments

Thread Starter

G.Beard

Joined Jun 22, 2015
18
This virtual scope is not like my little Velleman pocket scope and I think maybe I will stop tis simulation stuff and just breadboard my experiments and stand back :)
 

DickCappels

Joined Aug 21, 2008
10,124
You are looking at microamps and millivolts. You need to increase the current (decrease the load resistor) to get 200 milliamps, then you should see "better looking" ripple.
 

crutschow

Joined Mar 14, 2008
34,063
This virtual scope is not like my little Velleman pocket scope and I think maybe I will stop tis simulation stuff and just breadboard my experiments and stand back :)
Yes, the next room would be good.
Never experiment with line powered electronics. It's way too dangerous. :eek:
Always use an isolation transformer for such testing.
 

Thread Starter

G.Beard

Joined Jun 22, 2015
18
I have an isolation traf with a variac in front of it.

The pot I used in the sim had the wiper tied to another leg and was NOT set to the full 2M value. I just trimmed it wound. I would not attempted to recreate this circuit in the same way in real life... I like my life!
 

Thread Starter

G.Beard

Joined Jun 22, 2015
18
Screen Shot 2015-06-27 at 09.05.47.png Hi, I have another question. The first attachment, how do we calculate the average current draw over time?
Here is the exact same circuit in LTspice, except for the 2M pot that I dialled to 1k695 in multisim.... I used a fixed resistor in LTspice. When I used LTspice I got the exact results I was expecting. But Screen Shot 2015-06-27 at 08.44.17.pngScreen Shot 2015-06-27 at 08.44.29.pngScreen Shot 2015-06-27 at 08.46.09.png
 

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Last edited:

DickCappels

Joined Aug 21, 2008
10,124
There are large differences between the ideal voltage source in LTSpice and the output of a real-world transformer. The transformer effectively has resistance and inductance in series with its secondary, which means it takes longer to restore the charge in the capacitor.
 

Thread Starter

G.Beard

Joined Jun 22, 2015
18
Ok, I understand that this is a far more complicated matter in real world due to series resistance, inductance. I would like to get a very rough and ready idea of what size transformer I can safely use. Is there a basic rule of thumb?

Thank you very much for your time, it is greatly appreciated.
 
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