Discussion in 'General Electronics Chat' started by s214519, Dec 13, 2014.

1. ### s214519 Thread Starter New Member

Dec 10, 2014
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If a circuit diagram calls for a certain capacitor you could use a capacitor with a slightly higher farad rating and it would work the same right?

Jul 18, 2013
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Depends entirely on the application, some are very value specific.
Max.

3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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If the capacitor sets the frequency of oscillation or a time delay, then the capacitor directly influences that parameter.

If the capacitor is used as filter, bypass, or coupling cap, then it is not so critical.

4. ### s214519 Thread Starter New Member

Dec 10, 2014
23
0
A circuit I'm using calls for a couple capacitors that I can't seem to find. Since most capacitors have a tolerance of +-20%, as long as the capacitor is within a couple pF it should be fine right?

5. ### MrChips Moderator

Oct 2, 2009
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A 10pF capacitor with 20% tolerance maybe.

Show us the circuit with the capacitor in question and then we can discuss the tolerance required.

6. ### s214519 Thread Starter New Member

Dec 10, 2014
23
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The ones I can't find are the 203pF, 151pF, 103pF, 102pF, 222pF, 104pF, and 501pF

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7. ### MrChips Moderator

Oct 2, 2009
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Whoever chose those component values must be nuts. Just round the last digit to zero.

Let's do one at a time. First the transmitter:

 Item Value Substitute Comment C1 203pF 200-100,000pF non-critical filter cap C2 47μF 10μF-100μF non-critical supply filter cap C3 47 47 non-critical C4 68 68 non-critical C5 203 200-100,000 non-critical filter C6 68 68 non-critical C7 203 200-1000 non-critical HF filter C8 151 150 non-critical C9 151 150 tuned tank circuit will depend on L3 in any case C10 103 100 tuned tank circuit will depend on L3 in any case

Last edited: Dec 14, 2014
8. ### s214519 Thread Starter New Member

Dec 10, 2014
23
0
Thanks Mr chips! That's pretty much what I thought. The only ones that are important to have really close capacitance to what's written are the ones from L3 to ground.

9. ### MrChips Moderator

Oct 2, 2009
12,624
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There is a strong possibility that someone transcribed the values incorrectly from another schematic:

 Item Marking Value Comment C1 203 20nF non-critical filter cap C2 47μF 10μF-100μF non-critical supply filter cap C3 47 47pF non-critical C4 68 68pF non-critical C5 203 20nF non-critical filter C6 68 68pF non-critical C7 203 20nF non-critical HF filter C8 151 150pF non-critical C9 151 150pF tuned tank circuit will depend on L3 in any case C10 103 10nF this one does not seem right

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10. ### MrChips Moderator

Oct 2, 2009
12,624
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BTW, for a 27MHz crystal you want to choose one of the following frequencies used for 27MHz RC band:
• 26.995 MHz -- Channel 1 (Brown)
• 27.045 MHz -- Channel 2 (Red)
• 27.095 MHz -- Channel 3 (Orange)
• 27.145 MHz -- Channel 4 (Yellow)
• 27.195 MHz -- Channel 5 (Green)
• 27.255 MHz -- Channel 6 (Blue)

Dec 10, 2014
23
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12. ### MrChips Moderator

Oct 2, 2009
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As far as I know 27.000MHz is not a legal frequency.
Operating at this frequency will interfere with CB Channel 4 operating at 27.005MHz.
There is a 10kHz separation between legal alloted channels in the 27MHz band which has been allocated for CB and RC.

13. ### s214519 Thread Starter New Member

Dec 10, 2014
23
0
Well it's +-20ppm.

14. ### MrChips Moderator

Oct 2, 2009
12,624
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That doesn't matter. You need to understand frequency spread caused by modulation.

15. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,436
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Read the bill of materials again. I do believe the third numeral is a power of 10.

16. ### s214519 Thread Starter New Member

Dec 10, 2014
23
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Ok MrChips, sorry. I understand what you're saying now.

17. ### s214519 Thread Starter New Member

Dec 10, 2014
23
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Shouldn't there be a receiver crystal in the receiver circuit? Or does this circuit not need one?

18. ### s214519 Thread Starter New Member

Dec 10, 2014
23
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If anyone has any suggestions on a completely different transmitter/receiver feel free to tell me. I just need at least 4 channels.

19. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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Get yourself some of these. Do not get the ones without the encoder/decoder chips.

I am using 4 of the 315MHz transmitters and one of the receivers. I mounted the receiver inside a weatherproof fullsize 1/2 wavelength vertically-polarized Dipole antenna which is mounted on the outside front of my metal hangar building. I am able to open/close a swing gate, two garage doors, and the aircraft hangar door from a distance of about 100m from the inside of of a car.

It is all about how good the receive antenna is, and where it is mounted....

Last edited: Dec 15, 2014
20. ### s214519 Thread Starter New Member

Dec 10, 2014
23
0
Both the transmitter and receiver should have a crystal to set them at the same frequency right?
And is this a better circuit for what I'm trying to do?