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From: Franc Zabkar on 1 Jun 2010 06:44
On Sun, 30 May 2010 04:45:45 -0700 (PDT), Jeroni Paul
<JERONI.PAUL(a)terra.es> put finger to keyboard and composed:

>The waveform on this capacitor looks like a sawtooth, rises really
>fast and falls slowly until half cycle, then remains constant for the
>remaining half cycle. Amplitude around 0,4Vpp to 1Vpp depending on the
>capacitor used.

The current in an ideal capacitor is given by ...

I = C x dV/dt

Can you use this to estimate the current draw?

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
From: Cydrome Leader on 1 Jun 2010 11:52
Franc Zabkar <fzabkar(a)iinternode.on.net> wrote:
> On Mon, 31 May 2010 10:12:44 +1000, Grant <omg(a)grrr.id.au> put finger
> to keyboard and composed:
>
>>Quick check for 1000/25 and 1000/35 Panasonic caps
>>show ESR goes from .033 down to .025 for higher volt rating.
>
> I just tried a thought experiment, but it failed. :-(
>
> Let's say we have four identical capacitors, each with capacitance C,
> esr R, and voltage rating V.
>
> If we connect them as follows, we would expect the equivalent
> capacitor to have a capacitance C, esr R, and voltage rating 2V.
>
> || ||
> |---||--||---|
> | || || |
> o---| |---o
> | || || |
> |---||--||---|
> || ||
>
> - Franc Zabkar

that's how I read it, C and esr stay the same, and the voltage rating
could be upto 2V, assuming the C is really the same across all 4 caps.
From: Jeroni Paul on 1 Jun 2010 18:18
> What brand and series of  "low ESR high quality 105 deg replacement"
> did you use?  I would recommecd using a Panasonic FM series or
> equivalent ultra-low ESR high quality ca fron Nochicon, UCC, or
> Rubycon for something like this.  
>
> PlainBill

The failed capacitor is "Jun Fu". I have more of these here and they
have the lowest ESR amongst all I measured (KRG, Nichicon, TL). I
found Nichicon in a local shop but it has higher ESR and heats even
more. For Panasonics I will have to shop online. My ESR meter is of
the needle type so I will not post exact measurements but it lets me
compare quite precisely.

Thank you.
From: Jeroni Paul on 1 Jun 2010 18:25
On 31 Maig, 03:18, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote:
> Also, you may want to check or replace the UF "ULTRA Fast" diode in that
> circuit. I found some that developed a leak but not enough to short and
> pass some AC through from the switcher.. This can not only heat the
> diode but the Cap. A simple test is to perform a thermal test on the
> diode.. It should only be warm at best..

Good point, the diode gets very hot, too. Will try to replace it.

Thank you to all.
From: Jeroni Paul on 2 Jun 2010 16:20
> The current in an ideal capacitor is given by ...
>
>  I = C x dV/dt
>
> Can you use this to estimate the current draw?
>
> - Franc Zabkar

I tried this taking only the fall ramp:
dV = 0,781 V
dt = 4,72 us
C = 1000 uF

It results in 165 A. I may have done something wrong but I think the
capacitor ESR may play a role here as it causes an increase of dV not
reflected at I.

I also noticed the ramp has some ringing in it, not sure if that may
mean something.
The rectifier diode is SR360 and it gets hot very fast in a few
minutes, it could be leaky as suggested by Jamie.

The schematic shown in TNY265 datasheet for a 2A supply on page 9
figure 15 appears to match the circuit.

Thank you.
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