That's easy......money

600 volt electrolytics are sometimes fabricated as two lower voltage types in series. This could aid to a higher ESR reading.

The physics behind the electrolytic usually limit them to about 450-475 volts. Just as tantalums are no higher than 50 volts.

-Steve

Back to the original question, which was about replacements for 4uF and 8uF filter caps for an old radio repair.

For this application, modern film caps are an attractive replacement for either paper or electrolytic originals. The film caps usually cost more than suitable electrolytics, but can be expected to offer longer life. This is assuming that parts are chosen that are conservatively rated for the particular use.

The film caps may be larger in size, at the same value.

It is possible to spend plenty of money on electrolytics, if that is the main point. Appropriate film caps need only cost a little more than the cheapest suitable electrolytics.

If you must shave costs at every point, electrolytics are probably the cheapest option. If you need more that a 450VDC rating, two in series will probably be the lowest cost. At these low capacitance values, the ripple current rating can matter, even in an old radio. Note that there is significant ripple only on the first filter capacitor; the one nearest the rectifier.

The ESR of a high voltage 4uF or 8 uf electrolytic will be much higher than you see for low voltage caps. Traditionally the electrolytics for old radios had no ratings for ESR. If the capacitor P/N has an ESR rating, and the part in hand has a much higher ESR than the rating, that could mean trouble. Otherwise, not to worry.

When you go to buy film caps, again you can spend as much as you want. There are boutique vendors just waiting to take your money for special audio components. The volume market at the moment appears to be motor run and lamp ballast applications. So such parts are easy to find and are good value. The film caps sold into these markets are in fact also suitable for DC filter applications, although their ratings emphasize use on AC.

A motor start electrolytic is not the thing to seek out for a DC filter application. Don't go and spend money for one. However if it is Sunday afternoon, and that's all you have on hand, it might work OK in some cases. At least for a while. It might surprise you. At least, you are not going to exceed the AC current rating.

Ted

Steve, I just recently purchased a 4uf 600v and 8uf 600v miec aluminum electrolytic capacitors from justradios.com.
But they have esr of 6-12 ohm depending on the frequency you measure at. I used 100hz
Mike

Good advice Ted, I will consider that "film capacitor s "next time I purchase electrolytics.for power supply filters

Mike

The size of the caps are also much smaller today. Is that a consideration for choosing capacitors ?

Mike

Here is a good choice for high value film caps at 630 VDC

https://www.tubesandmore.com/products/c ... ylene-fast

Values up to 47 MFD

You can also find decent prices on film caps at Digi-Key and Mouser such as these.

https://www.mouser.com/ProductDetail/Vi ... 0iUw%3D%3D
• High ripple current capability, low ESR, low ESL

-Steve

Thanks Steve, excellent. My sprague to-4 could use some of these but it requires 2-12uf 's in series. I wonder if the 15uf film caps sold by tubes and more could be used in place of the 12uf

I did buy some Nicholson 12uf 450v aluminum electrolytics from mousers for the sprague to-4. Would the film caps be better?

Mike

Yes, 15 is close enough!
-Steve

If you already have electrolytics for your TO4, no real reason not to use them.

For a film replacement, no need to use two lower voltage caps in series, since higher voltage parts are no problem with film caps.

Here is one possibility:

https://www.mouser.com/ProductDetail/KE ... trsA%3D%3D

6uF 1500VDC $3.66 / 1.

The TO4 schematic shows two 12uF 450V electrolytics in series to get a higher voltage cap.

http://bama.edebris.com/download/sprague/to4/to4.pdf

Ted

Great idea Ted!
I didn't look for a higher voltage cap and I knew they were out there

-Steve

Ted ,I was looking at those film caps at mousers.
They had them in 2 and 4 wire. What is the a 4 wire used for?

Mike

On these there are just two reasons.

1. Rigidity in mounting and able to deal with a high vibration environmnt (Automotive Grade)
2. Lower ESR and able to handle greater ripple currents.

-Steve

I followed the bread crumbs and found myself reading about capacitors for "DC Link":
https://www.google.com/search?q=dc+link ... e&ie=UTF-8

Not enough neurons right now to try and understand this.....

There are some 4 leaded capacitors designed for safety aspects.

They have a pass through interlink (in a way like those interlinks in a regulator tube to not allow a power supply to operate without the tube inserted) which disables the circuit if the capacitor ever fails.

This was especially used in many color televisions (notably Zenith) where the capacitor was a functioning element of HV regulation.
Zenith had models where multiple special capacitors were used, so if one failed, the others would still be in the circuit, but it wasn't enough and HV would soar as one by one the caps failed. (Zenith part number 22-5001 Special caps)
So the 4 leaded design came to be to disable the set if the cap failed.

Looking at the spec sheet for the one linked by Ted, it doesn't appear that it is one of the interlink safety capacitors. At least it made no mention that I could readily find.

-Steve

Ted, I went ahead and used the Nichicon aluminum electrolytics. But they look freakishly small compared to the original paper spragues .About an inch long. They worked great through. They are so small, it's difficult to understand
How they could work.

Mile

A transistor is waaaay smaller than a tube, and it works (some will say better than a tube.... )

With electronic components, the physical size is often a secondary parameter---meaning that it is determined by the design and material selection in response the the basic requirements. To be sure, the maximum size (or weight or some other physical attribute) can also be a primary requirement.

Pick a capacitance value and then survey how many types are available for that value. Each option that you find was designed for different requirements and constraints. In the case of electrolytics, the main drivers were---and still are---cost and physical size. Advances in technology enable both to be reduced.

There will always of course be tradeoffs. For example, what is the relative lifetime...or maybe maximum ripple...as a function of the physical size of an electrolytic?

Here are the specs for the nichicon 12uf 450v electrolytic that I used. They have a life of 10000 - 12000 hours at 105c.
However, it says they are recommended for ballast applications. I measured 4ohms esr at 100hz. With de5000

Mike