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little problem with a simple schematic

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I was trying the new Multisim 11 and so I decided to reproduce the schematic below, which I took from a book. It should be led flashing at intervals, but I can't get it to work. Can someone kindly tell me why it isn't working(flashing)?

Image1.jpg

Message Edited by poldoj on 01-14-2010 06:59 PM
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Hi Poldoj,

 

Thank you for contacting National Instruments. I have been looking at this circuit you supplied. I think the issue is probably due to a number of factors. 

 

When I built the circuit, I actually used slightly different values for the circuit. I used a capacitor of 0.01uF in series with a resistor of 100k. This seemed to make the LED flash quite rapidly. When I replaced the values with the ones that you have used I found that the RC value seems to make the flashing incredibly slowly. 

 

The other factor to consider is that this circuit works by very slowly charging the capacitor. Once the capacitor has charged, it will switch the 2N6027 which will rapidly discharge the capacitor through the LED. This means that the LED on time is very, very short. This is particularly evident if you don't have any series resistor to try and slow the rate of discharge.

 

I've attached my circuit to this post so that you can have a look at my results.

 

Please let me know how you get on,

 

Many thanks,

Andrew McLennan
Applications Engineer
National Instruments
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Accepted by topic author poldoj

Hello Poldoj and Andrew. I agree with Andrew. It didn't work with 470 kOhm (the time is long, but nothing at all happens - even if you wait a long time). It seems that the simulation never reaches the flash-point. I lowered 470 kOhm to 220 kOhm and then it works. In the simulation, the LED will flash (briefly), but it will not discharge the 2.2 uF entirely. I added a 100 Ohm resistor in across the LED to discharge the capacitor more. It is easier to see what happens in the graph then, but is probably not needed in the real circuit. See the two graphs with and without the 100 Ohm.

 

 

W.kr.

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Thank you so much, it helped me a lot :smileyvery-happy:
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Hey!

 

I have same problem.

 

So why is multisim acting so badly here?

I am slowly losing my faith in multisim. Why do i have to modify circuits(that work real life) alot to work in multisim? Bigger problem is that when ever i design anything in multisim i have no idea how it works in real life.

 

I really cant understand this. Multisim looks like very good program but now i really dont know what to think about this.

 

Is there a  way to make multisim simulate this?

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