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Need Help with a Multisim circuit

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lycosa

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Hello everyone,

I'm new here and I have a question hopefully someone can help me with.

After a long break from the hobby, I'm getting back into things and I'm attempting to teach my son at the same time... hard to do when I keep forgetting so much.

Anyhow, I decided to use Multisim for a simulator.

The circuit I've built is just a simple oscillator using a 4001 NOR gate. It seems that Multisim isn't calculating the RC time constant correctly. The frequency Multisim is showing on the Oscilloscope is way faster than it should be when I built it on the breadboard with my son. I've been pulling my hair out all day trying to figure out what's going on.

Here's a screenshot of the Oscilloscope and the circuit:

https://i613.photobucket.com/albums/tt214/stevenbeckman/NANDOscillator.png

If someone could throw me some advice, I'd really appreciate it. If I do the math for C1 to charge through R3 I get 220 milliseconds which is right for what I'm seeing on the breadboard.. the scope in Multisim is showing a tim/div of 200nS which is way faster than it should be.

Thanks in advance for any help that can be offered.
 
Multisim is old and doesn't know if it is coming or going. It frequently makes errors.

Where did you find a non-polarized capacitor for the circuit?
Here are the calculations for frequency:
 

Attachments

  • Cmos oscillator.PNG
    Cmos oscillator.PNG
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Thanks for the help. That attachment really helped clarify a question we had.. mainly how to figure out the frequency from the RC time constant in that circuit.

So.. if you have another moment, could you verify the math in the circuit?

If C1 = 10uF = 1.0^10-6 = 0.000001 = 10 * 0.000001 = 0.00001 F
R3 = 22000 ohms

The RC time constant = 0.00001 * 22000 = 0.22 seconds or 220 milliseconds

Since R2 < R1 the frequency for the circuit is 0.455 / 0.22 = 2.07 Hertz

2.07 Hertz is very close to the flash rate of the LED we observed when we built the circuit on the breadboard.

Multisim is showing a period of about 120nS which is of course way way faster than what I observed when my son and I put the circuit together on the breadboard.

I just wish there was a way to fix Multisim so the program output was correct. Very frustrating. I'm guessing there is a way to run the simulation correctly, but I certainly can't figure out how to do it so far.
 
Are you sure the capacitors and resistors are the correct values in multisim? What are their indicated values on the schematic?
 
Are you sure the capacitors and resistors are the correct values in multisim? What are their indicated values on the schematic?

The values are 10mF for the Cap and 22k for the resistor. I attached a link to a screenshot of the schematic so you can see the values and the reading on the vitual oscilloscope on the OP.
 
mF is millifarad not microfarad (µF).

Edit: That link to the screenshot is blocked by my firewall. If you attach it directly to this thread, I can see it.
 
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I'll see if I can attach the schematic to this post.

**broken link removed**

Hope this helps.

Still not sure what went wrong..

Since it's hard to see, the scope tim/div is at 200nS not milliseconds.
 
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You are still linking to a PhotoBucket website. Instead you should upload the schematic to here from your hard drive.
 
You are still linking to a PhotoBucket website. Instead you should upload the schematic to here from your hard drive.

Sorry about that.. it shows full size on my post preview so I thought it would look the same for you.

So here's the pic uploaded to this site.

**broken link removed**
 
I finally was able to view your circuit and simulate it with my old copy of Electronic Workbench (now part of Multisim) and found the apparent problem. You need to add a small capacitance from the input of the first inverter to ground to simulate the normal input capacitance (10pF will work, see below). Somehow without that to suppress the high frequencies, the CMOS model goes into some strange high frequency mode where the input and output of the NAND gate are in-phase rather than inverted as they should be. This causes the high frequency oscillations.
Nand Osc.jpg

P.S. Audioguru, I've used Electronic Workbench for many years and only infrequently has it not simulated a circuit properly.
 
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