Technical Help needed !

[N1SQB]

Ok, I am trying to build a cirquit board for an LED signal strength meter project to be used on my Pro-2006 scanner. It uses an LM 3914 IC chip which will drive 10 LEds. Now, I am comfortable with this build as I have studied it fowards and backwards. One part of the schematics calls for a 2.2-uF capacitor. I cannot seem to find one nearby. So I toyed with the idea of using a 10 uF one that I do already have.
Can someone tell me what harm will be done if I use a capacitor with a larger value than the smaller one that is called for? It's been a while since I took to the mod books! LOL...

Manny

 

[gewecke]

Ok, I am trying to build a cirquit board for an LED signal strength meter project to be used on my Pro-2006 scanner. It uses an LM 3914 IC chip which will drive 10 LEds. Now, I am comfortable with this build as I have studied it fowards and backwards. One part of the schematics calls for a 2.2-uF capacitor. I cannot seem to find one nearby. So I toyed with the idea of using a 10 uF one that I do already have.
Can someone tell me what harm will be done if I use a capacitor with a larger value than the smaller one that is called for? It's been a while since I took to the mod books! LOL...

Manny

Without the schematic it's hard to say, but with led drivers like that one usually the component tolerance should be kept close. Maybe clip it in and give it a try?


73,
n9zas

 

[davidgcet]

can you find sizes that would either add up or divide down to 2.2uf if you series/parallel them? i would think using a larger cap would delay level change indications, depending on where it is in the circuit of course.

 

[gmclam]

2.2uF

It depends on where in the circuit the 2.2uF capacitor is used. If it is used for power supply filtering, then 10uF will be fine. If it is used for coupling, we'll need to know more about the circuit. If it is used for timing, then you'll want to get it as close to the value as you can, or otherwise perhaps change a resistor value to compensate.

Another factor is the voltage rating of the capacitor. You can certainly use virtually any higher voltage rating than it calls for. For example, if it says 10uF at 10V, then 12v, 16v or 25v ratings would be great.

I'd look for a 2.2uF at a higher voltage before using a different capacitance, unless it is power supply filtering, in which case higher capacitance is fine.

 

[N1SQB]

Well, the best that I have been able to do to keep things "close" to what the schematics call for is running two 1.0uF capacitors in series. So I will only be .02 off. The voltage is exactly what the schematics ask for. What do you guys think, pretty close no? I will see what I can do about getting a picture of the schematics.

Manny

 

[WA1ATA]

Well, the best that I have been able to do to keep things "close" to what the schematics call for is running two 1.0uF capacitors in series.

You need to put the two 1uF capacitors in parallel, not series.

In parallel, two 1uF capacitors are 2uF. In series, they are 0.5uF.

As another posted noted above, if the capacitor is just used for filtering the power supply then, within reasonable values, bigger is better.

The voltage rating doesn't matter, as long as the rating is higher than the voltage seen by the capacitor.

 

[N1SQB]

I did put them in parallel, I just said it wrong, my brain is fried potatoes from all the wiring. It is not that difficult a project, just time consuming! Here is a link to the schematics:
Schematics-1 pictures from entertainment photos on webshots
The capacitor in question is C-1 on the top left side of the diagram.

Manny

 

[WA1ATA]

Since pin 5 is the signal that is being monitored, the function of C1, in combination with R2 is to provide some filtering on the input signal. In other words, the time constant of C1 * R2 determines how fast the LEDs change in response to a change in the signal strength.

Since the impedance at pin 5 is high (draws 100nA max), you could have just used the 1uF cap and multiplied the value of R2 by a factor of 2.2.

 

[com501]

Since pin 5 is the signal that is being monitored, the function of C1, in combination with R2 is to provide some filtering on the input signal. In other words, the time constant of C1 * R2 determines how fast the LEDs change in response to a change in the signal strength.

Since the impedance at pin 5 is high (draws 100nA max), you could have just used the 1uF cap and multiplied the value of R2 by a factor of 2.2.

That would involve knowing basic electronics.

 

[N1SQB]

Since the impedance at pin 5 is high (draws 100nA max), you could have just used the 1uF cap and multiplied the value of R2 by a factor of 2.2.

I didn't think about that! I have not built the project yet so I may just do that. I have the resistor I would need too. Hmmm! Thanks for the suggestion!


Thanks to everyone who gave their good their advise in one way or another. I really do appreciate it! I will post pictures or maybe even a video link when all is said and done!

Manny/N1SQB