Input Cap

[kinser]

Hi all,

Long time since iv'e posted here, anyways:

Do i HAVE to have a Non-Polar cap at the input of an OPA or LM chip?
main reason why im asking is beacsue i have none left and i have to travle alot to get them(30km away from me).

Thanks,
Kinser

[Christer]

The best choice is to use a film cap, which is non-polar. However, I assume from your question that you don't have any large enough film cap at home, but only polar electrolytic ones. Using a polar cap is not advisable, since the cap is there for protection and could see a large reverse voltage if there is some fault in the equipment preceeding it. The standard solution is to use two polar caps in series, but in opposite direction. It doesn't matter if you tie the plus ends together or the minus ends. However, you ger only half the capacitance so you must use caps twice as big as the value you would have used for a single cap. For instance, if the schematic says to use a 2.2 uF non polar cap, you can use two 4.7 uF polar caps instead, like this.

......+....+.....
---||----||---

[kinser]

Christer,

Thank you so much! i though of the idea of connecting the caps in series, but didn't know if it would work, i gusse i will do that!

thank you once again,
Kinser

[Christer]

That's the standard solution, which can also find in many amp schematics. Just remember they must be in opposite direction.

[kinser]

Christer,

I didnt know that its done like that in some amp, i learnt something new :-)

i will start building my OPA549 subwoofer amp now!

thanks!
Kinser