### Author Topic: parasitc capacitance.  (Read 1158 times)

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#### rainman1

• Beginner
• Posts: 1
##### parasitc capacitance.
« on: March 06, 2009, 05:34:53 PM »
Hello, i'm new to this forum and hope to fit in will .

I would like to ask 2 questions regarding parasitic capacitance:
1. Is it true that wires or PCB traces contain inside capacitance of pico Farads?
If its true, than how come that they dont create a cut-off in DC circuits?

2. In my PCB there are traces that carry 220Vac and 16Aac, 50HZ and 60Hz frequency.
Is it considered a high frequency, such that parasitic capacitance between close traces can significantly influence the circuit?

Thanks.

#### paulstreats

• Supreme Robot
• Posts: 1,381
##### Re: parasitc capacitance.
« Reply #1 on: March 06, 2009, 09:03:33 PM »
1) the capacitance is usually created in a parallel style, it doesnt exhibit series capacitance. (joining capacitors in series blocks dc but in paralell it doesnt, they become quite useful.)
parasitic capacitance affects components and connections too. It can simply be 2 wires running next to each other with opposite signals would use the air and other insulators as a capacative medium - not a very good medium but it still holds capacitive charge. Some high freq/low voltage circuits are extremely sensitive because of the job they do and the components they use , so in a transistor if you have capacitance between the base and the collector it would turn on a small bit. in a music amplifier setup this would produce a hum out of the speakers.

2)Its not really considered high frequency at 50 - 60hz. with it being ac parasitic capacitance would have an effect of turning the square wave into a semi triangle/square wave for an extremely small amount of time. the incoming voltage and current would soon break the fields down

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: parasitc capacitance.
« Reply #2 on: March 06, 2009, 10:00:13 PM »
Hi,

1. Is it true that wires or PCB traces contain inside capacitance of pico Farads?
If its true, than how come that they dont create a cut-off in DC circuits?
Two conductors running in parallel IS a capacitor, but a very small one.
They DO create a cutoff for DC, which is quite fortunate, as otherwise you could just as well solder all your components to a single plate of metal.

In high frequency it becomes a real problem, but as the saying goes: If someone gives you a lemon, squeeze it and make lemonade.
So, in high frequency (UHF and beyond) circuits, that capacitance (and the likewise inescapable inductance) of PCB tracks are turned into components - Striplines is used quite a bit in UHF and in micro wave circuits you need specialized programs to design your PCB, where parameters for the PCB carrier material . like thickness and dielectric constant, has to be entered to calculate the components. A microwave filter can be made with just a PCB and no further components besides what you create by the parasitic capacity and inductance - perhaps the word "parasitic" shouldn't be used in those applications though.

2. In my PCB there are traces that carry 220Vac and 16Aac, 50HZ and 60Hz frequency.
Is it considered a high frequency, such that parasitic capacitance between close traces can significantly influence the circuit?
No, it's considered pretty low in the audio frequency spectrum, so you won't have a problem with parasitic components in close traces... However, anyone using close traces in a mains voltage PCB should be shot by sight (or at least electrocuted  ), since they're endangering other peoples life (by electric shock or by fire etc.).

There should be an absolute minimum distance of 10 mm between mains voltage and low voltage circuits and at least 3 mm between any two points of mains voltage tracks.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: parasitc capacitance.
« Reply #3 on: March 06, 2009, 10:17:26 PM »
Hi,

1) the capacitance is usually created in a parallel style, it doesnt exhibit series capacitance. (joining capacitors in series blocks dc but in paralell it doesnt, they become quite useful.)
An interesting point of view. I assume it's a parameter of the flux capacitor used in the deLorean in "Back To The Future"?
In the real world however...

capacative medium
Dielectricum!

[...] so in a transistor if you have capacitance between the base and the collector it would turn on a small bit. in a music amplifier setup this would produce a hum out of the speakers.
You allways have capacitance between base and collector (as well as any other two "pins" of a transistor, whether it's got pins or not).
Why do you think that capacitance would turn on a transistor or create hum?

with it being ac parasitic capacitance would have an effect of turning the square wave into a semi triangle/square wave for an extremely small amount of time. the incoming voltage and current would soon break the fields down
Had a "wet" evening or are you just really really tired?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### paulstreats

• Supreme Robot
• Posts: 1,381
##### Re: parasitc capacitance.
« Reply #4 on: March 07, 2009, 01:07:12 PM »
Quote
a mixture of that and being exasperated by our new baby constantly crying, i think my explanations are worded a bit confusing... (spaghetti brains at the minute )

#### want2learn

• Robot Overlord
• Posts: 189
##### Re: parasitc capacitance.
« Reply #5 on: March 08, 2009, 03:45:19 AM »
Off topic but:

Quite a lot of my internet time is when I'm feeding my kid (our youngest) and it sometimes takes me like 10 minutes to post.
Even after double checking my posts I still make (sometimes really basic) mistakes, just check out the posts on torque as an example.

Man, kids can really fry your brain
The question that drives me hazy:

Am I, or the others crazy?

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: parasitc capacitance.
« Reply #6 on: March 08, 2009, 11:56:34 AM »
Hi,

Wouldn't know what you're talking about... Just got me a grandson on the 25. of february and well... I just give him back to his parents when he cries - which he only does when he's hungry - which he is when he's not sleeping

They may be a bit tiresome at times, but they are the best thing in the world!
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

• Supreme Robot
• Posts: 11,585
##### Re: parasitc capacitance.
« Reply #7 on: March 25, 2009, 12:31:13 AM »
Just an on topic interesting note . . .

When you add a crystal to a mcu, you also need to add two caps. The caps are typically in the range of 10-20pF. The traces and solder are typically around 2pF. That's no insignificant capacitance!

The crystal is high frequency AC, running in the MHz range . . .

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