Film capacitor as power-supply filter

Sounds like a good idea, to me. If there's no limit resistor, it might help to put ten ohms in series (old electrolytics weren't low-ESR, and you don't want to find out that the inrush to a good capacitor blows your rectifier).

Thanks. The rectifier is a vacuum tube. If I change it to a diode, I'll add resistance.

The film cap should be fine.

I don't think that electrolytic caps in series need balancing resistors. They take care of themselves. If you need a bleeder, you may as well split it in two, but otherwise don't bother.

Did you have a specific part in mind?

This, maybe?

Reading the spec sheet, I see that it is self-healing, which sounds great...

That would not be the only anachronism in this vintage instrument. It also has a power-on LED that I added a few years ago.

That's a good one.

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It is a near certainty that poly caps in series will fail from overvoltage if no balancing Rs are used.

Direct opposite of what so many assume.

... Phil

I'm not comfortable with self-balancing for any type. Electrolytic leakage currents must create small carbon pathways, that's not good.

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Leakage current with high voltage electros is normal, adding " balancing " Rs to a series pair only increase it by forcing one cap to a higher voltage.

BTW Win :

Got any more silly crap to post about noise in BJTs used or mic-pres?

Or about pretending Toyota Hybrids and full EVs are the same ?

Or the loopy notion that what you chose to do and believe is normal ?

..... Phil

supply filter?

r re-forming electrolytics, and its power supply filter is currently two 40

-uF 450-V in series, without balancing resistors; applied voltage is nomina lly 600 V DC.

film capacitors are available with suitable ratings, is there any good rea son not to use one?

a tuning eye tube and an LED. It's already an anachronism.)

Look at the ripple voltage and verify that your film can take the ripple cu rrent. Films don't last long when you run them up to rated. Then there is t he turn-on time of the circuit. Does this Heath Kit have a lot of tubes in it, or what kind of electronic does it have. The old tube circuit DC suppli es would overshoot at turn-on until the tube filaments warmed up and the tu bes started conducting, drawing down the rail voltage. Until then, the appl ied DC voltage rail would be way high, like 150% nominal. And this could la st for tens of seconds. The electrolytics are designed to handle this tempo rary overvoltage, usually spec'd for 30 seconds, the film definitely are no t.

Capacitor forming in electrolytic capacitors causes a thin dielectric layer to form on the anode. This is simple electrolysis and the film is aluminum oxide, Al2O3, which is non-conductive. The leakage current is through the dielectric. There are no carbon paths.

When the capacitor is left with no supplied voltage, the film tends to diss ociate and become thinner. Applying a voltage to the cap restores the film.

An excellent description is given in

"Leakage current properties of modern electrolytic capacitors"

Neither some polymer aluminum caps. Some have upward-curving leakage which seems to auto-balance and does no harm, but I have seen some that behave like film caps, linear ohmic up to some point of instant failure.

Excuse me! Where do you think those currents through an insulator are coming from? Any forming processes aside, high-voltage leakage,** over-voltage, or over anodization, which we're talking about, involves series current pathways, carbon formation, corrosion, oxygen loss, and eventually, failure. The currents increase exponentially with modest over-voltage. Proper usage is spelled out in the Guideline for use of aluminium electrolytic capacitors, DIN 45811, and elsewhere.

Thank you! I have ordered it.

Winfield Hill Posts another F****it OT reply :

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There was another poster who disputed Win's hypothesis re "carbon pathways".

Hmmmmm.....

Might be another one of those AKW / CO2 paranoid, fear mongering stories.

Greta would be amused.

..... Phil

er supply filter?

for re-forming electrolytics, and its power supply filter is currently two

at film capacitors are available with suitable ratings, is there any good r eason not to use one?

th a tuning eye tube and an LED. It's already an anachronism.)

current. Films don't last long when you run them up to rated. Then there is the turn-on time of the circuit. Does this Heath Kit have a lot of tubes i n it, or what kind of electronic does it have. The old tube circuit DC supp lies would overshoot at turn-on until the tube filaments warmed up and the tubes started conducting, drawing down the rail voltage. Until then, the ap plied DC voltage rail would be way high, like 150% nominal. And this could last for tens of seconds. The electrolytics are designed to handle this tem porary overvoltage, usually spec'd for 30 seconds, the film definitely are not.

The film capacitor is rated for many amperes of ripple current (vs about 1 A for the competing electrolytic); 1 kV working voltage; 150% overvoltage f or 10 seconds; and it's going to be carrying a nominal 600 volts. So I thi nk it's OK.

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Electros can do nothing of the kind.

Good HV electros are expensive, so your single poly cap is probably more affordable too.

.... Phil

That is precisely the charm of wet aluminum caps. The series string optimizes itself for minimum possible leakage current.