0.1 uf electrolytic and 0.1 pf capacitors

[Gekko022]

I need a 0.1 µf electrolytic capacitor and a 0.1 pf ceramic capacitor for a project I am working on but I have never seen such low farad capacitor before and I do not know where to find them.

Does anyone have an idea where I might buy them or what I could scavenge them from?

thnaks!

 

[kchriste]

Just use a ceramic, poly, mylar, etc cap for the 0.1 µf electrolytic capacitor. Should work fine in most circuits.
0.1pF is very small. I doubt you'll have less board capacitance than this. Are you sure this is the correct value? You could make a "gimmick capacitor" for this value, but it would require some experimentation.

 

[Sceadwian]

PCB or wire traces will have a higher capacitance than .1pf that is a really strange value. Are you sure you're not looking at equivalent values for parasitic capacitance? Posting the schematic for the values you're reading might help.

 

[MikeMl]

0.1pF is 100fF, which is the magnitude of the parasitic capacitance encountered between traces in Integrated Circuit Design. What are you doing?

 

[Sceadwian]

Yeah, sounds like he's looking at a schematic and interpreting every element literally, even the parasitic ones. .1u isn't that weird so that might be real, but .1p most definitely is not. Gekko needs to post a schematic, or he could be refering to the PDF for a chip he's using that has detailed parasitics listed?

 

[Gekko022]

Just use a ceramic, poly, mylar, etc cap for the 0.1 µf electrolytic capacitor. Should work fine in most circuits.
0.1pF is very small. I doubt you'll have less board capacitance than this. Are you sure this is the correct value? You could make a "gimmick capacitor" for this value, but it would require some experimentation.

Are you sure I can use any capacitor for that value? Shouldn't I use an electrolytic capacitor?

As for the 0.1 pf cap I will post the schematic so you can see. It is an ultra sensitive FM transmitter.

I thought that sounded a bit too small... but its on the schematic..

 

[MikeMl]

The circled part is just a bypass on the power input to the stage. It should be 1nF to 10nF.

 

[Sceadwian]

Which one Mike, there are 5 circled parts.

 

[kchriste]

The 0.1pf cap in the diagram is obviously an error. Use something between 0.001uF and 0.1uF or so. For the 0.1uF cap on the mic use a poly or mylar for lower distortion. A ceramic would add some distortion, but I doubt you'd notice it in this circuit anyway. There is no need for it to be an electrolytic capacitor. If you did use a electrolytic capacitor, they marked the polarity wrong on the diagram to boot.

 

[colin55]

What sort of MIC is that? If electret, the circuit will not work.

The supply decoupling should be 10n to 22n

What freq are you transmitting? The 10p should be 39p to 47p at 88MHz to 100MHz for best "Q"

Are you using F29 material for the core?
It should be 3t - 4t on 2.6mm dia F29 ferrite for 88MHz to 100MHz.

 

[Gekko022]

The 0.1pf cap in the diagram is obviously an error. Use something between 0.001uF and 0.1uF or so. For the 0.1uF cap on the mic use a poly or mylar for lower distortion. A ceramic would add some distortion, but I doubt you'd notice it in this circuit anyway. There is no need for it to be an electrolytic capacitor. If you did use a electrolytic capacitor, they marked the polarity wrong on the diagram to boot.

I think you are correct about the mistake because I took a look at the previous project(see attachment) which was derived from the one I am working on and it looks like it was supposed to be 0.1 µf.Do you think I could make that assumption? The 0.001 µf was circled by accident...

Poly or mylar? Would you mind explaining what those are? I don't think I've heard of them.

 

[Gekko022]

What sort of MIC is that?

The supply decoupling should be 10n to 22n

What freq are you transmitting? The 10p should be 39p to 47p at 88MHz to 100MHz for best "Q"

Are you using F29 material for the core?
It should be 3t - 4t on 2.6mm dia F29 ferrite for 88MHz to 100MHz.

It is electret and why won't it work?

What is supply decoupling? What is "n". What is "p". (Picofarad maybe??). And what is F29 material? I am using a coil of copper wire around a ferrite core.

PS This is the first FM transmitter I have made haha.

 

[kchriste]

Poly is short for polystyrene. It is a type of capacitor dielectric which is better than ceramic especially for audio applications. Mylar is another type of capacitor dielectric.

it looks like it was supposed to be 0.1 µf.Do you think I could make that assumption?

Yes.

the previous project(see attachment) which was derived from the one I am working on

It looks like you got the mic bias resistor (10K) wrong in your "derived" diagram. That's probably why collin55 is asking about the mic type.

 

[colin55]

Use ceramic for all the capacitors. I have sold over 100,000 kits and the audio is so good that you cannot tell if someone is talking in the next room or the sound is coming from an FM radio.

<mod edit: self promotion deleted. If you have content to share, please post directly to ETO. Do not link to your own personal website>

The purpose of the 22n to 100n across the power rails is to reduce the impedance of the rails when the battery gets weak.
You must connect the battery ( 3v) as close as possible to the circuit otherwise the power rails will become "radiators" and the circuit will not work at all. This is much more evident if the 22n is not included. This is called "supply decoupling" or more accurately "supply coupling" or "supply impedance redcuing." But it is generally given the term "supply decoupling" as we are effectively decoupling the 3v rail of the project from the battery and making the circuit "tight." A "tight" circuit allows the transistor to "push and pull" the signal "into and out of" the antenna and thus produce electromagnetic radiation.

 

[kchriste]

You could also build AudioGuru's design:

 

[Gekko022]

Poly is short for polystyrene. It is a type of capacitor dielectric which is better than ceramic especially for audio applications. Mylar is another type of capacitor dielectric.

Yes.

It looks like you got the mic bias resistor (10K) wrong in your "derived" diagram. That's probably why collin55 is asking about the mic type.

Well Im making the first one I posted not the "derived" one so it should be alright..

Thanks everyone for your help, I will build it on my breadboard and see how it goes..

 

[kchriste]

It won't work very well on a breadboard if at all. Too much stray capacitance and inductance.

 

[colin55]

There is no "stray capacitance" on a breadboard.
The fact that there is very little "ground plane" on a bread-board makes the circuit susceptible to "stray capacitance" of your hand when tuning the circuit.


The reason it will not work well on bread-board is due to "impedance" not inductance.
The oscillator section should have a low impedance so the voltage produced by the inductor can be passed to the capacitor. These two components make up a circuit known as a TANK CIRCUIT and they will pass energy back and forth (without the need for any other components) when a voltage is applied. The only problem is: the energy passed back and forth will gradually die away due to losses. This is where the transistor comes in. It constantly “tops up” the energy to them and this is how the circuit oscillates. If you have long tracks between the inductor and capacitor or the tracks are thin, more of the energy will be lost in the “track-work.”
Secondly, you want to have a good ground plane so the circuit can push the signal into and out of the antenna.
The positive rail is the “ground-plane” in this design as the inductor is connected to it. The positive rail is also connected to the negative rail via a 22n and thus both rails act as “ground-planes.”
If you use bread-board, the fine wires under the board have a high impedance and will not provide the “tight circuit” you require.

Two faults in the second circuit have been noticed:
The coil has 10 turns, making the capacitor about 20 - 25p for 90MHz. The ideal Q for this frequency is 5 – 6 turns and 39p.
Secondly, the air trimmer should have a 22p ceramic across it so the trimmer only adjusts a small amount of the capacitance. It makes for a more-stable operation and easier to fine-tune

 

[Gekko022]

I don't see why I would have so many problems when the guy in the book made it on a breadboard and tested it and it worked. Then he made it on PC board and only had to adjust the coil a bit.


Why are you telling me there will be so many problems when he had none.

 

[ericgibbs]

I don't see why I would have so many problems when the guy in the book made it on a breadboard and tested it and it worked. Then he made it on PC board and only had to adjust the coil a bit.


Why are you telling me there will be so many problems when he had none.

hi Gekko,
When you say 'breadboard' do you mean a project board where the components are pushed into sockets and not soldered.?

or do you mean stripboard [veroboard] where the components are soldered into copper tracks.?