Capacitors

[Moe2255]

This may be a silly question, but I want to get my circuit right with little errors.

If a circuit calls for a 471 uf capacitor, and the only thing I can find available is a 470uf, would 1 uf make a difference, or would it be OK to use the 470? The circuit in question is the FM Telephone Bug.

Thanks

 

[Styx]

If the value was in pF then yes it would make a difference since it would probably be a timing cap

But in uF (and to an extent nF) it is more than likely tank capacitance on rails.

Also 470uF is the standard capacitance value, 471uF was probably a priting error.

However, that extra 1uF could just be parallel capacitance of difference di-electric for fast decoupling.

Use the 470uF BUT also find a 100nF (ceramic) and put in parallel

 

[Nigel Goodwin]

This may be a silly question, but I want to get my circuit right with little errors.

If a circuit calls for a 471 uf capacitor, and the only thing I can find available is a 470uf, would 1 uf make a difference, or would it be OK to use the 470? The circuit in question is the FM Telephone Bug.

Electrolytics are usually +/- 25%, so you wouldn't get one specified as 471uF. I suspect what it says is just '471' - this means '47' + one '0', which is 470pF. A 472 would be + two '0', 4700pF. It's a common method of labelling small capacitors.

No disrespect Styx! - but 470uF to 471uF is only a difference of just over 0.2% - they don't make components approaching that accuracy

 

[Moe2255]

Capacitor

Thanks for the speedy response!

I apologize for the mixup. Yes it says 471 pf and not uf. Can I use a 470 pf in parallel with a 1 pf to make 471 pf?

 

[Oznog]

Re: Capacitor

Thanks for the speedy response!

I apologize for the mixup. Yes it says 471 pf and not uf. Can I use a 470 pf in parallel with a 1 pf to make 471 pf?

Capacitance in parallel is additive. But the spec is impossible. The best ceramic caps made are +/- 2%, and you have to special order this kind of thing. It's already +/- 9.4pF, you're doing nothing useful to get closer to an exact value. Cheap, common caps can be as much as +/-20%.

There are trimmer capacitors used to tune a circuit for an exact result. Be aware you still have the thermal variances in the component values to deal with. A cap tuned with a trimmer cap to exactly 471pF will probably not be 471pF when the temp's 20 deg warmer.

 

[Dean Huster]

Even with the error of µF to pF, I would still doubt the validity of the "471" part. As mentioned, 470 is a standard value. 471 would be a nearly impossible value to attain, requiring a 1% or better tolerance. You might carefully recheck the parts list and schematic to be sure of that value. Errors like this, if not typos, are often the result of someone who designed a circuit for perfect values and never built the thing for a practical application and the circuit might well not work when constructed.

Dean

 

[Moe2255]

Re:471uf capacitor

I'm sure you're right about the value, because I'm not.

But here's the link that has the parts list: www.aaroncake.net/circuits/phonebug.htm

After you view it, do you think 470 will pass or get 470 uf plus 1 uf

You guys must live here

 

[Nigel Goodwin]

Re:471uf capacitor

I'm sure you're right about the value, because I'm not.

But here's the link that has the parts list: www.aaroncake.net/circuits/phonebug.htm

After you view it, do you think 470 will pass or get 470 uf plus 1 uf

It says 471pF - which is obviously a typing error!.

It should say 470pF, a standard prefered value!.

It's only an HF decoupler, intended to bypass RF and pass AF, the value isn't critical at all - anything from 100pF to 1000pF should be fine, but 470pF is a standard value anyway.

But be absolutely positive, it's 470pF not 470uF (which is a million times too large).

 

[Dean Huster]

Sounds like a good call, Nigel. I'm not sure why the subtle difference between C1 and C3, however.

Dean

 

[Styx]

This may be a silly question, but I want to get my circuit right with little errors.

If a circuit calls for a 471 uf capacitor, and the only thing I can find available is a 470uf, would 1 uf make a difference, or would it be OK to use the 470? The circuit in question is the FM Telephone Bug.

Electrolytics are usually +/- 25%, so you wouldn't get one specified as 471uF. I suspect what it says is just '471' - this means '47' + one '0', which is 470pF. A 472 would be + two '0', 4700pF. It's a common method of labelling small capacitors.

No disrespect Styx! - but 470uF to 471uF is only a difference of just over 0.2% - they don't make components approaching that accuracy

none taken I was just pointing out that you can only buy 470uF not a 470uF so it had to be a typo (as you stated later) or sloppy combining together two caps and not stating that

 

[bmcculla]

I think there are some caps that are labled with 2 digits and an order of magnitude. So

471pF -> 47 * 10^1 = 470pF.

472 -> 47 * 10^2 = 4700pF

edit: just realied that Nigel said this earlier. If you're worried just build your PCB with a space for an extra cap in parallel just in case you need to adjust.

 

[Nigel Goodwin]

Sounds like a good call, Nigel. I'm not sure why the subtle difference between C1 and C3, however.

Possibly they act as a potential divider to lower the amount of modulation?.

 

[Dean Huster]

"I think there are some caps that are labled with 2 digits and an order of magnitude. So

471pF -> 47 * 10^1 = 470pF.

472 -> 47 * 10^2 = 4700pF "

That's true, but I've never seen caps listed in a parts list or on a schematic in that fashion ... only on the parts themselves, and that because it's more compact and less prone to marking error at the factory.

It would be difficult to deal with the units of measure if listing in a parts list like that. "471" with no units of measure or "471F" which wouldn't be appropriate at all. "470pf" would be the only proper and understandable way to list the value. If "471" were listed next to the part on the schematic, that would be another issue altogether, for the schematic symbol would indicate the obvious unit of measure. However, as on Euro skems, "470p" would be the better way on the roadmap anyway.

Dean