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Selecting correct capacitor types? (EL Inverter Circuit)

I recently purchased a simple EL kit consisting of a 4x AAA battery inverter and 2metre length of wire. The inverter itself is very noisy but I expected this having read product reviews online etc. I'm not certain whether this happens because of the natural characteristics of the high frequency circuit or maybe a result of low quality components. I heard someone mention that poor quality coil windings on the transformer only make things worse...

In any case, I am now looking to build my own inverter circuit with higher quality components.

From what I understand, a 555 timer IC simulates an alternating current and then a 1:10 output transformer produces the high voltage that the EL wire requires.
- The higher the voltage, the brighter the wire but a too powerful inverter decreases lifetime.
- The frequency determines colour (noticeable in natural phosphor blue/green EL wire).

I'm looking at replicating this circuit from makezine:
http://makezine.com/21/primer/

Schematic:
http://cdn.makezine.com/make/21/drglo-elwire-schematic.jpg

I was wondering if anyone could point me in the right direction regarding which type of capacitors I should use? e.g electrolytic / ceramic etc.

Picture of Selecting correct capacitor types? (EL Inverter Circuit)
The big ones, the 100 and 47uF are going to have to be electrolytics, there's no point using anythng else. I might use low ESR ones though. The 1uF could be ceramics (XPO) or polyesters.
Alexdlp (author)  steveastrouk4 years ago
Great, thanks for the help! Do you know of any general guidelines for choosing capacitor types for particular circuits? I mean, are the characteristics of some capacitors better suited to some circuits than others? Just so that I know for the future ;)
Phew, well a lot comes with experience, a lot comes with training (thanks Dad) and a lot comes from reading books by the Gods of Analog(ue) design, like Robert Pease of National Semiconductor,

Bear in mind immediately that there is NO SUCH THING as a capacitor !

Electrically, any real component is some assembly of other parasitic elements, with a major characteristic that you buy and is on the label. For instance, an electrolyitc capacitor has capacitance, but it ALSO had a greater or lesser amount of series resistance (ESR) depending on construction - which affects price BTW. The ESR isn't written on the label. In demanding, hi ripple applications in swicthing PSU's (and this IS one) your caps should be low ESR electrolytics AND I would add a 0.01uF ceramic IN PARALLEL with the 100uF too.

Why ? Because electrolytics have a huge inductance, and, once the frequency gets high enough they BLOCK AC....soo we have to add a ceramic BYpass cap, which allows the HF to pass.

The realisation that electrolytics are CRAP is a key one....If you can get away with a ceramic use that. BUT ceramics can be "microphonic", and vary with atmospheric pressure, and sound,  so they tend to be less stable than some plastic capacitor types, like polyester or PPS. And for the best precision of capacitance with temperature, use a polycarbonate, but that can suffer from charge recovery in precision timing circuit, so use Teflon, only they are as rare as hen's teeth and VERY expensive. But not as expensive as precision glass ones....

Not a topic for a quick note, more for a book.

Steve
Alexdlp (author)  steveastrouk4 years ago
Excellent, Thank you very much! I'm glad I asked about this because I probably would have gone out and bought any old random caps if I hadn't have known!
+1. 555 timer circuits are quite resilient. The fat caps are used for power conditioning, the small ones (ceramics are just fine for most purposes) for timing. Use metal-film resistors to keep things stable. Don't use wire wound or carbon. total crap for the use.
He broke the rules and drew schematics you could actually read!