[sdiy] LTspice - exponential voltage curve

[Tom Wiltshire]

He did say "Taking my first few steps with LTspice". That includes doing maths with it. Mostly I don't understand what the hell even the interface is doing well enough. Getting the actual models to do what I want is a nice idea, but probably not happening without some pointing in the right the right direction. I think that's what he was after. It's not the world's most helpful software. On the plus side it's free, which is probably why we use it, like Eagle (similarly counter-intuitive and unhelpful).

T.


On 29 Oct 2013, at 16:31, Neil Johnson <neil.johnson71 at gmail.com> wrote:

> Justin,
> 
> In future it would REALLY help if you ask what you actually want.  In
> this case it's an easy bit of mathematical manipulation to come up
> with the right exponent formula to plot what you want:
> 
> V=5*exp(3.466*(time-1))
> 
> Neil
> 
> On 29 October 2013 16:15, Justin Owen <juzowen at gmail.com> wrote:
>> Still not having any luck with this.
>> 
>> I've tried scaling the arb voltage based on a 5V ramp using V=exp(V(Ramp)/2.8)-1 where (Ramp) is a 5V Pulse that ramps up from 0V to 5V across 1 second.
>> 
>> I've tried scaling exp() against time V=exp(1.7917*time)-1.0 and a variation on Neil's suggestion of V=exp(time/0.56)-1.
>> 
>> These all give pretty much the same results - but none of them have the exact curve I'm looking for which is:
>> 
>> 0s = 0V (OK 0V = 0.15625)
>> 0.2s = 0.3125V
>> 0.4s = 0.625V
>> 0.6s = 1.25V
>> 0.8s = 2.5V
>> 1s = 5V
>> 
>> Any other suggestions? Thanks.
>>