# PSPICE help

Discussion in 'Electronics Resources' started by damdam, May 10, 2010.

1. ### damdam Thread Starter New Member

May 10, 2010
2
0
Hey

I just got pspice and I'm just trying to figure out how to use it. I want to start out simple and make a circuit where I know the currents flowing through resistors, and I know some voltages, and I want the program to tell me what the required resistor values are.

I can't seem to get the program to let me make a netlist without specifying resistor values, and I don't know how to specify currents flowing through various resistors. Can anyone help me?

Thanks

2. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
Dont we all brother.

Resistor use is dependent on supply voltage and the rating and desired effect of each particular component.

For instance, you add resistors to a 555 IC to tell the 555 how long you want to run the timer for.

And for OPamps, the resistors in the feedback loop 'tell' the opamp the gain you want it to produce.

LEDs use resistors so too much current doesn't flow through them in order to change brightness and extend life.

The datasheets of each component will tell you how resistance, capacitance, etc.. will affect them.

HOWEVER, Ohms Law will TELL you what resistance you need to meet a certain current.

For instance, If you have a 5v battery and want to limit a LED with a 2.2Vf to a particular current, you would use Ohms law to figure out which resistor values would work.
a 330 ohm resistor.

Here is a post I did earlier showing Ohms law as a diagram. You should save this, print it out, book mark it, and memorize it. You WILL use this law every time you design or adapt or try to understand a circuit.

3. ### damdam Thread Starter New Member

May 10, 2010
2
0
Suppose all I have is a bunch of resistors in parallel. If I know the voltages on either side, and I know the currents flowing through each resistor, can I get pspice to tell me the resistor values? I can do it by hand, I just want to see if the program can do it too, because eventually I want to do this for complicated circuits with series and parallel combinations of resistors.

Thanks

4. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
You can measure the amperage and the voltage. Then using ohms law get the resistance

If the amperage is 22mA and the voltage is 14.8v

Using E / I = R where 'E' is Volts, 'I' is Amps, and 'R' is resistance in Ohms.

So 14.8v / .022A = 672.72 Ohms

You should have a list of standard resistors, if not, here is one:
http://www.logwell.com/tech/components/resistor_values.html
E12 and E24 are the standard used resitors used these days. More accurate the component, the more expensive.

Last edited: May 10, 2010
5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
If you use a passive component (R, L, C) it must be assigned a value that is 0 < <value> < ∞

It's more restrictive than that - it must be within the limits of the simulation settings. If you exceed the limits, you may receive helpful messages like "Timestep too small"