How do I set an amperage value of 200mA in the design of a circuit

[don.farr]

Greetings,

I am a complete newbie to this software.  I am trying to demonstrate Kirchhoff''s Law, voltage drops around a simple series circuit.  The goal is to solve for the resistance value of R4, given the following:

Vs = 100 VDC - - Amperage is 200 mA  (negative to positive current flow), the Vdrops are V1- 2V,  V2 - 9.4V,  V3 - 20V, V4 - 68.6V.

What I cannot seem to do is to set the amperage to 200mA to demo the circuit.

Thanks,

Don

[pallen]

Hi Don,

It's been a really long time since I've used Multisim.  But I've been playing with it recently and that's how I spotted your post.

I worked up your circuit and I got pretty close to the 200mA we're looking for.  I'm sure the variance displayed is due to the simulation settings.

No offense meant if you already knew this, but you need to remember to put the meter in-line to measure current.  All of the current must pass through the meter in order for it to be measured accurately.   I did the same thing at first when I connected the meter to two points on the line without breaking the connection between them.  I measured 0V. 

It took a minute, but things came back to me and I realized the mistake in my connection. 

Hope this helps.

[_user32]

Hello,

Besides the ammeter, another method for measuring current is using the current probe (bottom right of the instrument, follows right hand rule) and attaching that to a scope.   The default setting is 1 V/ma (so 200V = 200mA on the scope) but I usually change scale to 1mV/1mA.

The current probe can be attached directly to the wire so no net breakage is needed (good if you are also intending to do PCB layout).

2 reasons for the accuracy difference... 

1. Simulation settings can impact accuracy.

2. A small sense resistor (~1mohm range) is used in the series circuit so this does change the results.

Regards,

Pat Noonan

National Instruments

[don.farr]

Pat,

Thanks for the scope suggestion, however, the class is not at a point where I can introduce a scope and scope procs.

I did place junction points and then break the connection so that I could get the ammeter probe installed.

Personally, I think it is me doing something dumb.  The overall purpose of the exercise is to show how everything is in proportion within the "Series Circuit" and how to manipulate the resistors you get different voltage drops.

I'm just hitting a wall.  My forte is repair.  I'm a wrench trying to be a pencil (design) and it is not working.  Personally, I think I am actually proving my point, just doing it backwards.

[Tom__Sedlack]

Change R4 to 343 ohms.

V4=68.6V

I=200mA

R=V4/I

68.6/0.200 = 343 ohms

Plug it into your circuit and the current will be 200 mA

[pallen]

Hmm, well if you're still not getting anything with the ammeter probe...and your resitor values are correct....and your voltage sorce value is correct....

Then, I'm not sure what it could be.  Are you clicking the "play" button to start the simulation?