Big capacitors

Depends 'massively', on what other features you want it to have.. You can get such a capacitance, in a very small space, by series connecting (with suitable resistors to ensure voltage balance), some of the 'memory backup' capacitors (supercapacitors). 3*5.5v 1F capacitors, will give you 0.33F. If you want something with better ESR etc., then the cost starts to rise.

Best Wishes

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.

So rather than spend £400 on a true UPS, I thought I'd try a big capacitor and a relay (to stop the capacitor shorting the supply) on the low voltage side of the wall wart.

Thanks again Roger.

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.

Depending on the modem's voltage and current requirements, you might find it easier and cheaper to use a lead-acid "gel cell" battery. Use a wall wart which can be adjusted to "float" the battery at its preferred long-term standby voltage, while providing adequate power to the modem.

A lot of consumer-electronics devices of that sort will take DC voltages over a fairly wide range, and regulate down to 5 volts internally. A "12-volt" 2 amp-hour gel cell floating at 13.5 volts, running an ADSL modem drawing less than an ampere, would provide lots of ride-through time.

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...

...with 2 of these.

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

Nick

Agreed. I had thought about making a true UPS from scratch with a SLA or NiCd batteries that could trickle charge 24/7.

Don't think I have the time to develop it though, and to do so would be insane given that the modem only cost £20!

True UPS off the shelf are serious money - see above.

Nick

"Nick" skrev i meddelandet news:43e8ce89$0$64385$ snipped-for-privacy@authen.yellow.readfreenews.net...

You can find that in a "hifi car store". They use that kind of capcitor to "stabilize" the car battery for "big" power amps.

Danne

Danne,

They are hugely expensive here in the UK, but thanks for the suggestion.

Nick

I wouldn't make that assumption. Glancing randomly at some Panasonic spec sheets, I see ESR around 30 ohms, rising with age. As Tim says, they're optimized for running very tiny loads for a long time (memory backup).

The fact that some car stereo folks use them does not mean they work in that application. It just means that some car stereo folks are easily fooled by specs.

You won't get one.

Graham

A. Your modem is crap.

B. Have you considered a battery ?

Graham

Graham