Help On my mini dummy

[kirakun]

Hi guys, need pro help.. I doing a mini project for my job request.. becos everytime we need to install this convertor into a machine, that machine will show the amount of quality inside the converter. The measuring reading is take from the converter, where there r 2 connectors. The converter inside have 2 capacitance probes that measure the quality inside the converter. As i come out this mini project, that will act as a dummy converter. As u know, the reading is take from e converter capacitance probes. So i buy non polar ceramic caps for my project and coaxial cable for the input and output.

The problem is after i finished and tested. The result not very accurate.. Y??? I buy the correct reading caps. How can it give me unacceptable reading. Ex, 63PF from my mini pro, i install it to the machine, it's give a reading of 70PF.. Wow.. That out of range.. How come?? I need ur HELP guys.. Really thankfully if you guys help me solved the problem..

Here is sche. diag i draw..

[audioguru]

63pF is a very small capacitance. The wiring to it and terminals for it are a few pF and the shielded cable also adds capacitance. The switch has a few pf between its terminals.
All the extra capacitance is called stray capacitance and must be compensated for in calculations for making an RF circuit.

[kirakun]

63pF is a very small capacitance. The wiring to it and terminals for it are a few pF and the shielded cable also adds capacitance. The switch has a few pf between its terminals.
All the extra capacitance is called stray capacitance and must be compensated for in calculations for making an RF circuit.

oic.. How 2 reduce spray capacitance?? Stray capacitance is it a virtual cap??

[audioguru]

Stray capacitance can be reduced by making wiring as short as possible.

[kirakun]

Stray capacitance can be reduced by making wiring as short as possible.

wah audioguru, u r good man..  Thx..  ;D Reducing wire.. Hmm.. i tried b4.. i juz take the caps directly connected to the machine, it give correct value. Got more info on spray capacitance? The books or websites where i can take a reference..

[audioguru]

Two wires beside each other have capacitance between them.
A shielded cable has a fairly high capacitance because the shield surrounds the center wire.

[kirakun]

Is there a need to ground the shield? Btw, i calculate e coaxial capacitance value. dont know correct or not.. U see, i take the reading minus the caps value that i put. The  result will be the coaxial value.. Den i take the actual reading that i want minus the coaxial value, the result will be my actual cap value.. Am i right?

Ex.. Actual Reading :63PF (I want the reading on that machine)
      Reading taken from the machine meter is 70PF (the reading i dont want)
      Caps that i solder is 63PF.

So to calculate the coaxial cap= 70-63 =7PF
To get the actual cap value = 63-7=56PF

The correct caps value i should buy is 56PF, Right?

[audioguru]

The shield of a shielded cable should be connected to the circuit's ground to work properly. Some circuits connect the shield to the output of a buffer amplifier that is carrying the signal so that the capacitance of the cable is cancelled.

The stray capacitance changes when you change the capacitor so you cannot calculate a value that will be accurate. Use a trimmer capacitor in parallel with a smaller capacitor and adjust it to the exact value that is needed.

[kirakun]

The shield of a shielded cable should be connected to the circuit's ground to work properly. Some circuits connect the shield to the output of a buffer amplifier that is carrying the signal so that the capacitance of the cable is cancelled.

The stray capacitance changes when you change the capacitor so you cannot calculate a value that will be accurate. Use a trimmer capacitor in parallel with a smaller capacitor and adjust it to the exact value that is needed.

So i cannot juz calculate the value. If i change there will still have that unactual reading. Ur suggestion is get a trimmer cap. By parallel with those caps (show below), does it need to putting to all caps? I do a search on website, there r many types of trimmer cap value.. Which value should i get??  :-\



The shielding part can i do that?

[kirakun]

guru... help... tis sat i need get those components again :P

[audioguru]

Why are you switching capacitors with such small values?
Why is the exact value of each capacitor important?

What is the input signal?
What is the output signal?

[MP]

The problem is after i finished and tested. The result not very accurate.. Y??? I buy the correct reading caps. How can it give me unacceptable reading. Ex, 63PF from my mini pro, i install it to the machine, it's give a reading of 70PF.. Wow.. That out of range.. How come?? I need ur HELP guys.. Really thankfully if you guys help me solved the problem..

What is the tolerance spec on your capacitors? If you use a cap with 10% tolerance, it would be normal to see 70 pf from a 63 pf cap. If you use a 20% tolerance cap you could see worse.
You need to find 1% tolerance capacitors to make this work properly. Then follow good grounding guidelines. Depending upon the specifications of the instrument you are connecting to the input, you could still need a buffer or a drive circuit to help with any signal loss. What tolerance does the input instrument allow for ? In other words, how much error can you allow? There will always be some error. You have to allow for this margin. You also have to use a metal enclosure which has the same grounding potential as the shield on your cables. Even with this, since you are working with capacitance, you can see differences with the box open as opposed to what you see when the box is closed. Depending upon the test equipment used, you could even see different capacitance when the test probe is connected and when it is not.

MP

[kirakun]

Why are you switching capacitors with such small values?
Why is the exact value of each capacitor important?

What is the input signal?
What is the output signal?

The machine is used to measure the quality inside the converter. When we do troubleshoot on that machine. we will take those different quality converter to do our test.. Like my diagram got a small value capacitance 63PF, e machine will indicate the converter is 0 litre. Others capacitance values like 69PF will show 1 litre.. This is very important when doin troubleshoot on the machine. Nevermind, i had bought trimmer caps the already, caps value is from 20-120PF..

[kirakun]

What is the tolerance spec on your capacitors? If you use a cap with 10% tolerance, it would be normal to see 70 pf from a 63 pf cap. If you use a 20% tolerance cap you could see worse.
You need to find 1% tolerance capacitors to make this work properly. Then follow good grounding guidelines. Depending upon the specifications of the instrument you are connecting to the input, you could still need a buffer or a drive circuit to help with any signal loss. What tolerance does the input instrument allow for ? In other words, how much error can you allow? There have to allow fwill always be some error. You or this margin. You also have to use a metal enclosure which has the same grounding potential as the shield on your cables. Even with this, since you are working with capacitance, you can see differences with the box open as opposed to what you see when the box is closed. Depending upon the test equipment used, you could even see different capacitance when the test probe is connected and when it is not.

MP

The caps tolerance, the cap dint stated anything.. The color is brown..  i think is 20% as search on internet.  Even with this, since you are working with capacitance, you can see differences with the box open as opposed to what you see when the box is closed. i agree wif that..