arcaarca wrote:Hi everyone,
here are my Eagle version of the schematic, should be correct to last version... hope so
http://www.mediafire.com/download/s666l ... _Eagle.rar
cheers and good luck!
DiscoVlad wrote:1) Use a single 2.2uF and connect the +ve lead to whichever side has the highest voltage.
DiscoVlad wrote:3) Just use non-polarised 2.2uF film caps because Mouser/Element14/RS Components/Digikey/etc. all have a wide selection of them and they are neither difficult to obtain nor expensive?
DiscoVlad wrote:2) Use 2x 4.7uF in series back to back (+ve leads together) to make a 2.35uF (close enough) non-polarised electrolytic cap.
induction wrote:You'll want to use 2u2 caps in series.
The math for caps in series breaks down for polarized electrolytics. Polarized caps only block DC in one direction and behave like jumpers in the other direction, so putting them back to back blocks DC in both directions. When a signal attempts to reverse-bias one of the caps, the other cap blocks the current flow, which prevents damage to the reversed cap. At either polarity, a given signal will forward bias one cap so it acts like a cap, and reverse bias the other one, so it acts like a jumper. In effect, there is only one cap being used at any given time, so the capacitance of the series pair is the same is that of the individual polarized caps.
DiscoVlad wrote:edit, because this just occurred to me: The "avoid using near 0V" thing which RG was saying I take to mean that your DC Bias voltage needs to be higher than the AC ripple voltage to avoid reverse biasing the capacitor, and running the risk of it failing (explosively usually)
R.G. wrote:In the series-NP connection, the capacitance value is funny. Normally caps in series are a smaller capacitance than either cap by itself. If you had two 3.3uF polyester caps, then the expected value for two of them in series is 1.65uF. However, electrolytic caps actually conduct in the reverse direction, so two 3.3uF polarized aluminum electrolytic caps act like they each have a diode in parallel with them that conducts when the voltage is backwards for that one cap. So two 3.3uF caps hooked up as series non polar (i.e. negative to negative) look like a single 3.3uF NP cap.
... except for tiny region near zero volts where they withstand a tiny reverse voltage, so they look like 1.65uF there
Tantalum will withstand a reverse voltage of a few volts before the leakage comes up on the reverse biased side. So with tantalum the "funny area in the middle" is much bigger, maybe as much as 4-5V depending on the cap. Also, most DMM testers test with a small voltage across the cap so they give better answers when the caps are tested in-circuit. You're probably getting both effects.
In your conditions, I would expect just the results you're getting.
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