estimating ripple current rating

[earl of sodbury]

Hi, I'm experimenting with capacitor-based DC-offset blocking filters for use on 50Hz 240VAC mains supply.

I'm currently looking at replacing low volt electrolytics with motor run films so I can delete the bypass diode required by the 'lytics.

Problem is selecting suitable motor runs given that none of the usual suspects publish ESR and ripple current characteristics.

Any notion how I can calculate/estimate current-handling at 50Hz 240VAC?

thanks, Sod

[AndrewT]

Hi,
The diodes only conduct on start up or overload.
The caps normally carry all the operational current.

If you delete the diodes and uprate the voltage of the caps, then the operational currents are exactly the same.

However, what would be the voltage drop during overload condition or at start up?
Can this be simulated?
Or would you have to build it and measure it?

[earl of sodbury]

Hi Andrew, thanks. What I'm trying to do is build a stand-alone filter so hopelessly over-specified that it can be used by halfwits who will try to run 2 x 3kW electric fires and a compressor off it! I'm exaggerating for effect, but you get the picture - it's impossible to predict load conditions (to say the least!).

Hence caps rated well over operational AC voltage are appealingly foolproof and robust, but that leaves current handling to consider.

I'm aiming for something that can handle =/>13 Amps after that the fuses should do their job!

cheers, Paul

[AndrewT]

No!!!!
have you calculated the voltage drop for a 500W amplifier yet?
What size of cap is required for that?

How will you build a box big enough to take a 400Vac 5mF cap and who's going to pay for it?

[earl of sodbury]

Originally posted by AndrewT
No!!!!
have you calculated the voltage drop for a 500W amplifier yet?
What size of cap is required for that?

How will you build a box big enough to take a 400Vac 5mF cap and who's going to pay for it?

Dunno about a box, prolly need a house extension!

As I say, I'm exaggerating for effect.

I've run 1.1kW/4.5A through electrolytics good for 6.5A @ 120Hz with no detectable problem, nonetheless I'd like to increase the safety margin and current handling capacity, plus drop the diodes... Doesn't seem unreasonable...

[AndrewT]

I'm gone

[earl of sodbury]

I do love the friendly, helpful nature of this forum

If I had all the answers I wouldn't be asking the question...

[AndrewT]

Not completely gone.
5 answers given and 7questions asked of you.
Have you tried to answer any of the questions posed?
If you did you would start to appreciate why

Doesn't seem unreasonable...

is totally nonsensical.
Go and do some homework, then try criticising.

[richie00boy]

What Andrew is trying to say is that upon fault conditions and start-up, the cap(s) will see the full mains potential across them. This dictates that they must be rated to at the absolute minimum 325V DC, preferably double.

Now, a typical capacitance needed to avoid huge voltage drop across the cap(s) even just in normal use is 5000uF or more.

A 5000uF 400V cap if there is such a thing will be about the size of a large cola bottle and cost about as much as a good hifi amplifier. So before you even get to start thinking about ripple current ratings there are impassible walls in the way.

And exactly what is wrong with using electrolytics and diodes?

[earl of sodbury]

Originally posted by AndrewT
Not completely gone.
5 answers given and 7questions asked of you.
Have you tried to answer any of the questions posed?
If you did you would start to appreciate why is totally nonsensical.
Go and do some homework, then try criticising.

Andrew, you lost me long ago - all I want to know is if/how I can estimate the ripple handling capacity of a given capacitor at a given AC voltage/frequency.

As I hoped "it's impossible to predict load conditions" and my explanation of the planned usage would illustrate, I've no idea what startup/usage conditions - including voltage drops will be, they're as variable as the countless possible systems at the receiving end.

Perhaps if you could offer some simple direct advice, rather than this elliptical pedagogic solipsism, some actual light might be shone into my old and avowedly feeble mind.

Thanks very much, Paul