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Newbie

## Help with ODE's and Units

Hi, I am having trouble solving for this ODE or at least graphing a ODE. I am looking for voltage as a function of time v(t) or v(x) (i've tried both here) I keep getting unit errors, even when I solve this using unitless parameters. Also, is there any way to display the equation or does Odesolve only give graphical solutions. I uploaded in Mathcad 13 & 14.
5 REPLIES 5
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## Help with ODE's and Units

For whatever reason, MCad wants the prime notation used for the definition of y'(0). Go figure. I wasn't clear about your units and didn't do anything on the second half of the sheet. It seems to be the same problem.
As far as I know, ODESOLVE is not supposed to work with units. When the units you used are converted to SI, the equation goes to negative infinity in a hurry, tend has to be < 0.6. However, as shown in the attached sheet it is OK forever.

narlin
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## Help with ODE's and Units

Thank you for your help. I am still having problems however. The unit for C was in micro Farads, which means to be converted to base units, it should be 0.167*10^-6 (F) When i make that change into the vector you created however, it no longer works. Also, what is the "tend" for?
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## Help with ODE's and Units

Using C = .167 microF, which is what i was alluding to previously, causes the equation to be stiff. It is still numerically solvable, although I don't have much faith in these solutions. They may be OK.

The tend is just so I can remember what that number is in the ODEsolve. tend stands for time-end.

You have to right click on the word Odesolve (and not on the parenthesis following it) and then change the method to "stiff".

I have attached another that is stiffed.
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## Help with ODE's and Units

Show the DE in a *.gif,
some collab may have patience to construct, maybe interest in it too.

jmG
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## Help with ODE's and Units

Please do not post the same question in more than one place. It only serves to sow confusion as answers get split between the threads. I have answered your question in a response to your other post.
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� � � � Tom Gutman
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