drlegendre at gmail.com
Mon Aug 1 03:03:29 CDT 2016
It's pretty simple, really.
Non-polar caps are used in locations where they must pass AC. Film caps (of
various constructions) are practical until the values exceed a few uF. Once
you need more than a few uF of non-polar capacitance, it's time to look at
NP electrolytics, as they are the cheapest route to a large-ish value
without the polarity issue.
Film caps (in the 63-350V range) are available in values as large as 10uF,
22uF, 47,uF and even 100uF - or more. But those are usually high-priced
specialty audio parts, for things like speaker crossovers and output
coupling caps in some audio preamp circuits.
A non-polar electrolytic is nothing more than a pair of normal (polar)
electrolytics, with the negative (-) leads tied together. There is no
difference between a (nominal) 10uF non-polar and a pair of 22uF polars
tied neg to neg.
Pos to pos works just as well, but it's just not the standard for a very
good reason.. think about how a standard electrolytic cap case is built &
On Mon, Aug 1, 2016 at 2:30 AM, Jim Brain <brain at jbrain.com> wrote:
> On 7/31/2016 12:37 AM, Ian McLaughlin wrote:
>> These are non-polarized (or bi-polar) electrolytics. An example is the
>> following digikey part number: 493-12697-3-ND
>> You can always whip one up out of your junk box - just put 2 normal
>> (polarized) electrolytic in series with the polarities alternating (for
>> example, connect the two positives together). Each capacitor has to be
>> twice the value of the result - so for example, to replace a 1uF 50v
>> non-polarized, you can put two 2uF in series.
>> Hope this helps.
> Thank you (and Chuck) for the response.
> I guess it brings up more questions for me:
> * Why would Tandy spec such a cap in the design?
> * As I look at the specifications of the design, it looks like the
> rest of the design assume mac .250W of power on the audio line
> (given the selection of resistor networks and other parts) . I
> noticed the Digikey unit you referenced has 17mA of ripple current
> capability. I realize audio is not truly a ripple current, but the
> disparity between 17mA and 250mW seems like a problem... (Again, my
> analog is severely rusty, so maybe I am looking at this wrong. I
> spent all night trying to find a way to determine the power of a R2R
> ladder given 5V logic and 8mA output drive of the 74ls374 IC, to no
> avail, so maybe I am looking at this wrong)
> o you'd think with the prevalence of R2R ladder logic, someone
> would have whipped up a power calculator, if nothing else, to
> determine the wattage of the resistors in the R2R, but I came up
> empty... I'll have to either calculate it longhand by summing
> all of the currents or sim it; not sure I can assume that since
> the effective impedence is 10K, the amperage would be .5mA @ 5V
> and thus the power would be 2.5mW
> Sometimes, I wish I'd cared more about analog in college...
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