computer simulation programs

[MrDEB]

how much should one depend on the results of these programs.
for instance I run TINA and simulate a transistor switch using a 2n222
I then run the same simulation using **broken link removed**
and the voltages are different.
I for one am trying to learn how people like Audioguru can caculate how a transistor will operate.
I have been trying to understand how my random cab light transistor switches work then the timer circuit I have been playing with lately.
Not debating if Audioguru is correct, just curious as to how??
Then while working with LEDs in series arrays how much of a load do they impose on the circuit.
really confussing. I believe the information that Audio presents is accurate (thanks) but why does a computer simulation come up with different numbers.
I have a hunch that the computer is only as good as the data input. Which in my case way off in left field sometimes.

 

[blueroomelectronics]

A basic understanding of Ohms law helps, plus Audioguru can read a datasheet. I don't use an analog simulator as they tend to be ideal (parts) or wrong (the model used).

Series LEDs are real simple. I'd start by wrapping my head around the very straight forward math.

 

[audioguru]

Simple arithmatic is easy.
Some people design circuits using "typical" spec's. Then some circuits work and others don't.
I design circuits using worst case guaranteed minimum and maximum spec's so that all the circuits I make work perfectly.

I don't buy cheap Chinese "floor sweepings" from RadioShack or E-Bay so the manufacturer's spec's are valid.

 

[MrDEB]

Simple arithmatic is easy kinda

I know ohms law but getting that decimal point in the right place.
so how does omne compute the resistive load of leds and a resistor?
supply voltage divided by the desired current?
In the simulation I get 2 different figures.
WHY?? if both use the same formulas??
I should learn more about reading a spec sheet.
take for instance how does one compute how much voltage and current to drive a transistor into saturation?
first you need the resistive load then base current or ???
if using a simulation program what would be the worst case so circuit at least has a good chance of working??

 

[MrDEB]

Tina vers transistor sim

here are two of the same circuits but slightly different results.
what are we supposed to look for for a good reliable circuit?
when using a spec sheet how does opne determine saturation, base current .
I know most transistors require .7 volts to the base but what resistance to use to figure out the base current? .7volts divided by ??=current at the base to achieve saturation.

 

[audioguru]

I know ohms law but getting that decimal point in the right place.

Common sense makes the decimal point in the correct place.

so how does omne compute the resistive load of leds and a resistor?

An LED has a range of forward voltage and you select a current that does not burn it out but makes it bright enough.

In the simulation I get 2 different figures.
WHY?? if both use the same formulas??

You are forcing a base voltage with unlimited current instead of applying a base current through a current-limiting resistor.

how does one compute how much voltage and current to drive a transistor into saturation?

Most datasheets show the max saturation voltage loss when the base current is 1/10th the collector current.

if using a simulation program what would be the worst case so circuit at least has a good chance of working??

A sim program works with "typical" specs, not worst case spec's.
Some parts (typical and better) will work but low spec parts probably will not work.