DTV decoder repair, capacitor gets too hot

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

Up the cap's voltage rating too, if a bigger cap will fit in the box.

Grant.

Unless this is a flyback type supply, a typical forward switching power supply design does not have a large capacitor on the output of the rectifier. The rectifier diodes go directly to an inductor with the main filter capacitor connected to the output of the inductor. If it a flyback design, you could try putting a 1 uf or so ceramic capacitor across the larger capacitor. Using a high voltage replacement is not a good idea since the higher the voltage rating, the higher the ESR.

David

Since when? Quick check for 1000/25 and 1000/35 Panasonic caps show ESR goes from .033 down to .025 for higher volt rating.

Grant.

might be worth checking the current drawn, out of interest. i've repaired dozens of these sort of boxes, many of which are the same inside (MX Onda, AXIL, T Boston, Engel....) and using caps rated at 25v in place of the 16v fitted usually keeps them working.

-B

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..

Agreed, and the chart on the front of my trusty Bob Parker Mk1 agrees too. I also repair a lot of this low cost crap, and I always put a 25v cap in them.

Arfa

On May 30, 4:45=A0am, Jeroni Paul wrote: > I'm repairing a DTV decoder Saivod DT2012 (chip Cheertek and tuner LG) > that had a swollen capacitor in the power supply. 6 months ago I > already replaced this capacitor with a low ESR high quality 105 deg > replacement but it just lasted 6 months and is swollen again. >

Where I used to work they had Samsung DTV tuners that had a similar issue. In that case we mounted the capacitor on its side to get it farther away from the diode next to it that was the source of the heat. You might look into the Nichicon HN series caps or possibly an organic polymer cap. AFAIK the HN caps are the lowest (?) ESR of the 'lytics but the polymers are supposed to be even lower. Would a larger value cap help here? What is the actual load current and what is the frequency of the ripple?

G=B2

I am always dubious about advice offered by someone who cannot comprehend a simple text description. From the original post:

"Typical design, small transformer, rectifier diode, first capacitor, inductor and second capacitor with TNY265 as primary controller chip. The second capacitor remains cold and checks fine on the ESR meter."

Looking up the datasheet for the TNY265 controller I see that it does indeed suggest the circuit description described above.

Now the question is, are you a troll, or simply uneducated?

PlainBill

On Mon, 31 May 2010 10:12:44 +1000, Grant put finger to keyboard and composed:

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

On Sun, 30 May 2010 04:45:45 -0700 (PDT), Jeroni Paul put finger to keyboard and composed:

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

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.

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.

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

Thank you to all.

I tried this taking only the fall ramp: dV =3D 0,781 V dt =3D 4,72 us C =3D 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.

"Jeroni Paul"

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.

An **ideal** 1000uF cap has an impedance of under 1 milliohm at 200kHz - in reality no such beast exists so the simple formula above fails hopelessly.

A standard grade 16 volt, 1000uF cap has an ESR of around 200 milliohms ( at high frequencies) while a low ESR type has about 20 - 30 milliohms and both have series inductance that amounts to 20 milliohms at 200kHz.

Your 1000uF cap is getting hot cos it has way too much ESR !!!

Get a low ESR type, rated to handle 2 amps or more of ripple.

The Panasonic FM series 16V, 1000uF cap has a rated ESR of 19 milliohms and a ripple current rating of 2.2 amps.

Note:

The ESR of electros goes DOWN when the cap gets hot - by as much as a factor of 5.

Means cheap, high ESR electros can be made to do the job in a SMPS at the expense of having a very short life.

.... Phil