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creating a dummy circuit for impedance analysis?

 
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PostPosted: Tue Nov 06, 2007 12:41 pm    Post subject: creating a dummy circuit for impedance analysis? Reply with quote

I need to make a simple RC circuit made up of 3 resistors and 2 capacitors in order to validate data being given by a Solartron 1260 Frequency Response Analyzer (FRA). It is constructed with 1 resistor in series with 2 series RC circuits. The values of resistance and capacitance I need are:
R1 - 2.5 ohms
R2 ~ 0.07 ohms
C1 ~ 0.02 F
R3 ~ 0.06 ohms
C2 ~ 1 F

My questions are simple:
1) Is it possible to find resistors this small?
2) 1 F capacitors seem fairly common...how can I get a .02 F capacitor (or equivalent capacitance using the least expensive route)?
3) The capacitances seem quite high...will this be a problem for testing the response? Do I need to be concerned about voltage ratings, etc (I am in the US and the FRA is being plugged into a standard wall outlet)? Or should it be straight forward?
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Tue Nov 06, 2007 6:04 pm    Post subject: Reply with quote

It is possible to get low value resistor & higher value caps.("super caps" are within that range") As far as resistance, you are dealing in super low value territory which is practical with SMD stripes--- mainly used in high precision military/space application.

Vishay manufactures such product; http://www.vishay.com/resistors-discrete/list/product-63089/

Not sure of the practicality of your R/C ntwrk at such low value resistances?

Unless you are litterally dealing with gold contacts wiring/etc, the inherit resistance from ANY connection to&fro, will easily override the resistor values needed ---making your "overall" R/C network completely different.
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Nov 12, 2007 12:55 pm    Post subject: Reply with quote

Such small resistances is often best made of a suitable length of copper cable. Although copper has a high TC, it is no problem when the hook-up is only to be used at room temperature and if the power losses are low enough to not cause any notable self heating. You can calculate the wire lengths roughly by knowing that the resistivity of copper is 0,0175 ohms x mm²/m (i.e. a copper wire of 1 m length and 1 mm² cross section has a resistance of 0,0175 ohms at room temperature), and then measuring with a low-ohm LCR meter and adjusting as necessary.

But wouldn't be possible to re-scale the network using larger resistances and smaller capacitances?
If you increase the frequency by 100 times (i.e. 6 kHz instead of the 60 Hz from the electrical mains), then you could use 100 times smaller capacitors.
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