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'[EE:] Measuring capacitor values'
2004\06\19@094612 by Anthony Toft

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Hi,

I recently got an assortment of capacitors, mostly unmarked :( What is
the best way to measure them without a capacitor range on my multimeter?

I have bread boarded an astable 555 timer circuit using my scope to
measure the period, but with known cap values this is not particularly
accurate...

Are there better ways?
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2004\06\19@095442 by David VanHorn

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>
>I have bread boarded an astable 555 timer circuit using my scope to
>measure the period, but with known cap values this is not particularly
>accurate...

It should be pretty accurate, as long as the resistor values are known.
Most caps are pretty sloppy though, -10 / +50% isn't uncommon in electrolytics.

You can measure their impedance at 1kHz, and work from that, but what the 555 does, effectively, is measure the R/C time constant. Since R is known, C is the variable. At room temperature, with a steady voltage, the results should be more accurate than the cap.

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2004\06\19@100104 by Dave Lag

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Remember electrolytics (assuming) are not particularly accurate anyway.
Measuring period versus a known value is probably not so bad.
D

At 09:44 AM 6/19/04, you wrote:
{Quote hidden}

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2004\06\20@055051 by Anand Dhuru

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How about builing a simple one chip instrument that does it, and has *15*
more functions as well?

http://members.cox.net/berniekm/super.html

Regards,

Anand Dhuru

{Original Message removed}

2004\06\20@100343 by Anthony Toft

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> Remember electrolytics (assuming) are not particularly accurate anyway.
> Measuring period versus a known value is probably not so bad.

With this in mind, then what is the prescribed function for figuring
caps?

Is it "right around this figure" (like resistors) or is it "at least
this much"?

The direct drive of this question is that in my grab bag I have some
700nF that I'd like to use instead of some 100nF, these are for
smoothing caps on a 7805.
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2004\06\20@110047 by David VanHorn

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At 10:00 AM 6/20/2004 -0400, Anthony Toft wrote:

>> Remember electrolytics (assuming) are not particularly accurate anyway.
>> Measuring period versus a known value is probably not so bad.
>
>With this in mind, then what is the prescribed function for figuring
>caps?

The RC time constant, if you know the R side, tells you what C is, within the accuracy of the R.  Measure the R with a good meter, and hand trim to <1%.

>Is it "right around this figure" (like resistors) or is it "at least
>this much"?

For power supply filter work, it's normally "at least this much".
For frequency dependent filters or timing, it's "right around this".

>The direct drive of this question is that in my grab bag I have some
>700nF that I'd like to use instead of some 100nF, these are for
>smoothing caps on a 7805.

For high frequency bypass, your 0.7 uF should be fine.

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2004\06\20@110048 by Peter Moreton

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700nF? - I guess these are actually 0.68uF which is an industry standard
value?

Peter Moreton


> {Original Message removed}

2004\06\20@112645 by David VanHorn

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At 03:54 PM 6/20/2004 +0100, Peter Moreton wrote:

>700nF? - I guess these are actually 0.68uF which is an industry standard
>value?

Probably. Aren't caps fun? :)

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2004\06\20@131518 by Anthony Toft

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> 700nF? - I guess these are actually 0.68uF which is an industry standard
> value?

My oscillator said between 700nF and 800nF, I guess I should label them
with the nearest standard size instead of actuals...

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2004\06\23@082345 by At van Wijk

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Anand,  you've presented a very nice design with your super pen,
including your assembler as well.
Thanks,
At.

> How about builing a simple one chip instrument that does it, and
> has *15*  more functions as well?
> http://members.cox.net/berniekm/super.html
> Regards,  Anand Dhuru

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2004\06\24@003340 by Anand Dhuru

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Hi At.,

I'm glad you liked the design; however, its not mine. That credit goes to
Luhan, a very helpful guy. I just stumbled accross it myself, and built the
circuit which is now become an indespensible part of my workbench.

Regards,

Anand


{Original Message removed}

2004\06\24@011017 by Bhargava

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Thanks Anand,

Very good design. I have added it to my "to build" list.

Thanks.



On Thu, 24 Jun 2004 10:03:07 +0530, Anand Dhuru <spamBeGoneardhuruspamBeGonespamvsnl.com> wrote:
{Quote hidden}

> {Original Message removed}

2004\06\28@172827 by hilip Stortz

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i'd label them both ways, and sort them by actual value.  this way, you
can find "matched" caps if you need them or actually want a very
specific value, and when it isn't critical you can just grab one (i'd
definitely group matched sets though for later use, matched capacitors
can be useful in rf and other work to simplify tuning circuits that use
several caps etc.)  this way you don't throw out data you already have
and may actually want/need later (you may also run into a situation
where you want 2 caps that are close in value but where one is
definitely slightly more/less than the other, like to generate 2 delays
of about the same length but with one definitely longer than the other
slightly without having to use a trimpot or variable inductor).

Anthony Toft wrote:
{Quote hidden}

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