Film caps always better than lytics?

[SemperFi]

For powersupplies, are film caps always prefferable to lytics? Panasonic has some cheap motor run caps and I'm thinking of getting them instead of the usual lytics.

What is meant by 'build in safety feature'? Is there some current limiting device in there?

datasheet:

http://media.digikey.com/pdf/Data%20...client_id=5042

Also, I've got a bunch of film caps from lighting armatures. Anyone tried such caps?

[TheGimp]

Not familiar with lighting armatures.

However, if you read up on metal film caps you will find they are self healing (shorts blow open) and this in adddition to the vent system is probably the safety feature you are refering to. I've never heard of a metal film cap blowing up, but there are plenty of examples of electrolytics exploding in all form of spectular manner. I'm sure if one tries hard enough one could blow a film cap as well

George (Tubelab) have you ever blown up a film cap?

The ESR (effective Series Resistance) of film caps is lower than electrolytics (as a general rule), making them a better choice for power supply operation.

The downside of film caps is they are generally physically larger and more expensive.

So, preference becomes a personal decision. If I could find film caps cheap enough, I'd probably try using them more often, but it would require larger chassis for some amps to accomidate the large size.

An example is the FireFly amps I just finished. They are on 10X6X2" chassis. They are stereo SE amps with two OPTs, a main Transformer, three tubes and the power supply board. The power supply board is mounted under the chassis and contains a 220uF400V, two 100uf 385V, and two 47uF 450V caps on a 1.2 x 5.6" board. No way I could get film caps in that space.

[indianajo]

I have replaced the electrolytics once each in my ST70 and PAS2, and now they need it again. Just did the PAS2. Bleah, life is too short. If you have time and room, use film. Unless you're planning a heart attack at 50 like one of my college friends. (He had one, too.) I put some 50 uf (1000VDC) 600 VAC genelec brand motor caps from Newark.com in my H182 organ on the power supply chassis, right after the two 5AR4 rectifiers. They were $15 each. To cover the H.V. terminals, I had to install a separate stainless chassis up on the organ side, and run 20 ga shielded twinax up and back. The shield of the twinax is connected to chassis at one end. I put a shielded screw terminal strip (480 VAC rated) in place of the tall can caps on top of the power chassis. Use 600V rated wire. I did run a ground (green) 10 ga wire up to the separate chassis for safety. Somebody warned me to install resistance between the rectifiers and the cap because motor cap ESR is so low, but I didn't and my rectifiers have lasted 9 months now. My organ now has thunderous bass and enough volume to knock the tubes out of the sockets; the reviews on Harmony Central say an H182 is thin and lifeless. I did put a 1/2" whisker of wire on a phenolic header between transformer and the rectifier tube to keep from burning out the transformer if the rectifier shorts out. An organ repairman on organforums.com says modern tube rectifiers are prone to shorting out. I teased one strand out of a 19 strand wire salvaged from a PC power supply, maybe 20-22 gauge for all 19 together.
Metalux lighting caps for 480 VAC are usually about 6 or 17 uf, not big enough for audio usually, but you could gang them up if you have enough free ones. Make sure to make a good crimp on the 1/4" flag, I find Klein and Ideal tools make the best crimps and T&B, Ideal, and Panduit are the best crimp terminals.

[planet10]

After buidling a number of amps with solen poly caps (just the cheap entry level ones), unless it is specifically a junk box amp (or some other consideration), we won't use anything but. It made a big difference. Relative to the labor involved they weren't all that expensive. Big they are thou.

dave

[SemperFi]

Thanks for replies. 15uF versions are about 5-6dollars each, or 4.50 each for 10 pieces. Not very expencive. Solens aren't that much more, but i think I will try these Panasonics for a change. I like the mounting hole on the cap.

[Miles Prower]

Originally Posted by SemperFi

What is meant by 'build in safety feature'? Is there some current limiting device in there?

Internal fusible link. This is meant to burn out if the capacitor ever goes short circuit. Motor start duty especially is rather severe, and failures are likely. Last summer, had a compressor start capacitor short out, and it also took out the fan motor start/run capacitor (duals in same case).

The fusible links are also a reason to always soft discharge with a 500R / 10W resistor instead of simply shorting the terminals with a screw driver. If the stored charge is enough, a dead short across the terminals will poof the internal fuse.

[glennb]

Originally Posted by SemperFi

Thanks for replies. 15uF versions are about 5-6dollars each, or 4.50 each for 10 pieces. Not very expencive. Solens aren't that much more, but i think I will try these Panasonics for a change. I like the mounting hole on the cap.

These are rated for 210 VAC, which equates to 297 VDC peak. I wouldn't stress them on any more than about 300 VDC.

[TheGimp]

I talked with a buddy of mine at work today (Heating and cooling part time business) and he told me he has seen quite a few motor run caps fail. They puff up and go open most of the time. He stated that this usually happens in the fall after a really hot summer for some reason.

Next time I'm working on my heat pump I'll measure the voltage across the motor run cap so I can calculate the current and power dissipation in the one in my system. It might be interesting to see what the numbers are.