Re: Big capacitors

Tim Shoppa wrote:
Nick wrote:
Thanks for the magic words 'memory backup'; I see even Maplin sell them.

I need them to keep an ADSL modem alive during mains brownout
conditions. Tried a cheap UPS but it didn't stop the brownouts reaching
the modem and latching the firmware/hardware.

Hmm, typically these will draw several hundred mA. A 0.1F capacitor
will keep the supply up for a few seconds, which may be enough or may
not.

It seems rather more straightforward to get some batteries
always-being-trickle-charged instead.

And you might want to stay away from the "memory backup caps" (which
are optimized for a discharge current way below a mA) and stick with a
conventional electrolytic. 100000uF at 16V is like $10 here in the US.

Tim.

The brownouts are typically 0.1 to 0.5 seconds, so I think a few tenths of a farad would work if the modem took say 9v at 0.5A. (I need to measure this obviously).

The only cheap farad range capacitors I can find in the UK are memory backup. I assume that the ESR would still be in the sub 1 ohm range.

I can get 0.5F at 10v for less than $3US...

http://www.bardwells.co.uk/detail.asp?stockid=MEM008&ref=subcats%2Easp%3Ft%3D%26qfcode%3D%26catid%3DMEM001%26manuid%3D%26keywords%3D%26minprice%3D%26maxprice%3D

....with 2 of these.

Conventional electrolytics would cost more like $80 for the same capacitance in the UK.

Nick
.

Relevant Pages

  • Re: Big capacitors
    ... I need them to keep an ADSL modem alive during mains brownout ... Tried a cheap UPS but it didn't stop the brownouts reaching ... The brownouts are typically 0.1 to 0.5 seconds, so I think a few tenths ... The only cheap farad range capacitors I can find in the UK are memory ...
    (sci.electronics.components)
  • Re: Big capacitors
    ... Tried a cheap UPS but it didn't stop the brownouts reaching ... the modem and latching the firmware/hardware. ... Depending on the modem's voltage and current requirements, ... find it easier and cheaper to use a lead-acid "gel cell" battery. ...
    (sci.electronics.components)
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