I've been fooling around with LT spice .MEAS feature for a while now and I just can't seem to determine how to take the time measurements I need for an assignment.
I can look at the time values on the waveform viewer and determine an estimated time, however given that LT spice is equipped with a measurement feature I'd like to be precise.
I have a simple circuit of a voltage source in series with a resistor and capacitor and I need to find the time that the voltage at the capacitor reaches 63% of the Voltage input. I think this is called Rise Time?
the command I've been trying is:
.MEAS Tran res1 FIND time WHEN V(Vout)=0.63(Vin) cross=5 TD=1m
essentially I need to find the time that it takes Vout to reach 3.15v (63% 5V(Vin))
Thanks
Best Answer
You labeled the nodes
Vin
andVout
, and you are using the output node correctly (V(Vout)
), but not the input node ((Vin)
instead ofV(Vin)
).You also forgot the asterisk for multiplication:
0.63*V(Vin)
. If you need to be more precise, use(1-exp(-1))*V(Vin)
, impose a small timestep (1m of total time, or less), and, optional, add.opt plotwinsize=0
to disable waveform compression. It doesn't look like you need it, though.Finally, you use
cross=5
but you only have four rise/fall edges. Instead, userise=1
for the first rising edge,rise=2
for the 2nd, etc. You don't needtd
here.The new command should look like this:
As a minor addition, you could use a bit sharper rise/fall edges on your source, to avoid wrong readings.
1u
would do just fine, without sacrificing decimals.A little bit of attention can go a long way. This also implies no haste.