# AC resistance & Dc resistance

Discussion in 'General Electronics Chat' started by aamirali, Mar 2, 2013.

1. ### aamirali Thread Starter Member

Feb 2, 2012
415
2
What is diff b/w two.

1. I have a graph showing i-v curve & have to calculate ac resistance at I = 2mA.
& I noted that at I = 2ma, V = 2V.

What should be ac resistance here.

2. ### #12 Expert

Nov 30, 2010
18,076
9,678
You're having trouble with word definitions. Capacitors and inductors have reactance, resistors have resistance.

If 2 volts causes 2 milliamps, the reactance must be 1000 ohms. I think the math guys call it j1000 or -j1000. I never learned that part because I get along pretty well without it.

Anyway, the voltage applied to the capacitor is a sine wave, and 2V is the RMS value of it. The frequency of the sine wave must be right for the capacitor size to have 1000 ohms of reactance, and the capacitor will only have that amount of reactance at one frequency.

3. ### DC_Kid AAC Fanatic!

Feb 25, 2008
792
17
caps have capacitance
coils have inductance

AC "resistance" is a function of frequency and is expressed as "impedance" Z (vector sum of pure resistance and reactance in the circuit, where reactance has component values 180 degrees apart, one for capacitance and one for inductance).

keeping in mind, a wire wound resister is pure resistance + inductance in an AC circuit due to the windings, etc.

Last edited: Mar 7, 2013
4. ### MrChips Moderator

Oct 2, 2009
18,202
5,718
Capacitors have capacitance.
Inductors have inductance.

Both capacitors and inductors have reactance which are a function of frequency.

5. ### Papabravo Expert

Feb 24, 2006
11,993
2,574
Don't listen to those other guys; they've gone off into the weeds. The DC-resistance is related to the DC-load line used to bias an amplifier stage. The AC resistance is related to the AC load line. The DC load and the AC load are different so the load lines are different and so on.

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
The AC resistance would depend on the slope of the VI characteristic at the nominal DC operating condition.

7. ### MrChips Moderator

Oct 2, 2009
18,202
5,718
You are correct. I did not read the op carefully.

8. ### #12 Expert

Nov 30, 2010
18,076
9,678
I thought it was about "a graph showing i-v curve".
At least I know more about AC resistance now.

(I learn something almost every day on this site.)

Last edited: Mar 3, 2013
9. ### Papabravo Expert

Feb 24, 2006
11,993
2,574
In the days before computers the I-V characteristics of a vacuum tube or a transistor were an important design tool. In a common emitter stage it is the bypass capacitor around the emitter resistor that accounts for part of the difference in the AC and DC load lines. It is important to remember that they intersect at the Q-point.