[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Current Limiting and Impedence

Original poster: "Gerald  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Mark,

This can be a confusing area.

Series resistances add:
Rtotal = R1 + R2

For parallel resistances, the conductances (G) add:
Gtotal = G1 + G2

or expressed as resistance:
1/Rtotal = 1/R1 + 1/R2

Now the tough part:

Impedance, in the general case, has resistive (R) and reactive (X) components (sometimes refered to as the real and imaginary parts). For series impedances, the resistive components add up and the reactive components add up (keeping in mind that capacitive reactance is negative and inductive reactance is positive) so you get the following:

Ztotal =  Rtotal + jXtotal  = Rtotal + j(XLtotal -XCtotal)

You can't linearly add the resistive and reactive components together. The impedance is a complex number denoted by the j prescript on the reactive part. What you can do instead is determine the magnitude of the impedance using:

Z = sqrt(R^2 + X^2)

You can think of the R and X terms being two sides of a right angle triangle and the Z being the hypotenus (sp?). Series impedances add similar to resistances in that:

Ztotal = Z1 + Z2
but one needs to keep to the rules of complex math.

Parallel impedances also behave similar to parallel resistances in that:

1/Ztotal = 1/Z1 + 1/Z2
and again one needs to keep to the rules of complex math.

Hope this helps more than being confusing.

Gerry R

Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>

I still don't quite get it.

1.  Not that it matters for this question, but I thought I could sum
series impedence.  Could you re-confirm? Are you sure you haven't
confused with  Z^2 = R^2 + X^2?  I don't mean to question and I am not
an expert so I am just making sure.

If it is Z^2 then my formula for parallel Z must be wrong.  I'm using
1/Z = 1/Z1 + 1/Z2 + ... .  Just like resistance.