Re: [sdiy] Impedance / equivalent resistance calculation

[Roman]

I don't have the link to your schemo at hand, but trying to answer your questions from what I remember:
- you can safely treat the 10uF as short for AC signals above certain frequency, in this case about 40Hz
- the hipass is fromed from R9 AND R5 together with C9. For AC signal it doesn't matter if the resistor is connected to GND or power rail, so they are treated as parallel connected
- C1 forms lowpass filter which corner frequency is determined by impedance of the node with phototransistor and R1. That's assuming it has much lower capacitance than C9. Otherwise its use wouldn't make much sense. It would form a capacitive divider then with C9.

Roman

---- Wiadomość Oryginalna ----
Od: Bert Schiettecatte <bert at percussa.com>
Do: 'Roman' <modular at go2.pl>, synth-diy at dropmix.xs4all.nl
Data: Sat, 3 Sep 2005 02:19:51 +0200
Temat: RE: [sdiy] Impedance / equivalent resistance calculation

> Hi,
> 
> > your DC impedance at IRIN1 is parallel connection of IR5 and 
> > IR9, that's 750ohms. If you expect to detect the light at 
> > frequency highier than, say, 40Hz (twice corner frequency of 
> > filter made of IR5,9, IC9), cou could consider the cap as 
> > short. Your node with IR1 and IQ1 has equivalent resistance 
> > of 690 to 2.1k (again, parallel of IQ1 and IR1 in light and 
> > dark), so overal AC impedance is more or less (690-2100)||750 
> > = 360 - 550 ohms
> 
> That's what I thought too but I was not sure I could look at 
> The 10uF cap as a 'short'. 
> 
> Now, I do understand that IC9 and IR9 are forming a high-pass 
> Filter, and that IC1 and the equivalent resistance before it 
> Will form a low-pass filter. Are there any other filters I am 
> Missing here? I don't see how IR5 is involved in a filter.
> 
> Thanks,
> Bert. 
> 
>