Is there any reason not to use a 20-uF 900-volt film capacitor as a power supply filter? I'm updating a vintage Heathkit capacitor checker that I use regularly for re-forming electrolytics, and its power supply filter is currently two 40-uF 450-V in series, without balancing resistors; applied voltage is nominally 600 V DC. This hasn't given any trouble, but I'm uneasy about its future. Now that film capacitors are available with suitable ratings, is there any good reason not to use one? (P.S. This may be one of the few instruments in active use that have both a tuning eye tube and an LED. It's already an anachronism.)
Film capacitor as power-supply filter
Started by ●October 7, 2019
Reply by ●October 7, 20192019-10-07
On Sunday, October 6, 2019 at 9:19:57 PM UTC-7, m...@uga.edu wrote:> Is there any reason not to use a 20-uF 900-volt film capacitor as a power supply filter? > > I'm updating a vintage Heathkit capacitor checker that I use regularly for re-forming electrolytics, and its power supply filter is currently two 40-uF 450-V in series, without balancing resistors; applied voltage is nominally 600 V DC. > > This hasn't given any trouble, but I'm uneasy about its future. Now that film capacitors are available with suitable ratings, is there any good reason not to use one?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).
Reply by ●October 7, 20192019-10-07
Reply by ●October 7, 20192019-10-07
On Sun, 6 Oct 2019 21:19:52 -0700 (PDT), mc@uga.edu wrote:>Is there any reason not to use a 20-uF 900-volt film capacitor as a power supply filter? > >I'm updating a vintage Heathkit capacitor checker that I use regularly for re-forming electrolytics, and its power supply filter is currently two 40-uF 450-V in series, without balancing resistors; applied voltage is nominally 600 V DC. > >This hasn't given any trouble, but I'm uneasy about its future. Now that film capacitors are available with suitable ratings, is there any good reason not to use one? > >(P.S. This may be one of the few instruments in active use that have both a tuning eye tube and an LED. It's already an anachronism.)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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●October 7, 20192019-10-07
mc@uga.edu wrote...> > 20-uF 900-volt film capacitor ...Did you have a specific part in mind? -- Thanks, - Win
Reply by ●October 7, 20192019-10-07
On Monday, October 7, 2019 at 11:47:58 AM UTC-4, Winfield Hill wrote:> mc@uga.edu wrote... > > > > 20-uF 900-volt film capacitor ... > > Did you have a specific part in mind? >This, maybe? https://www.mouser.com/ProductDetail/EPCOS-TDK/B32776Z0206K000?qs=sGAEpiMZZMsh%252B1woXyUXj1%2FT6OBaNpJ8kU%2Fi46kWMVw%3D
Reply by ●October 7, 20192019-10-07
> > This, maybe? > https://www.mouser.com/ProductDetail/EPCOS-TDK/B32776Z0206K000?qs=sGAEpiMZZMsh%252B1woXyUXj1%2FT6OBaNpJ8kU%2Fi46kWMVw%3DReading 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.
Reply by ●October 7, 20192019-10-07
mc@uga.edu wrote...> > On October 7, 2019Winfield Hill wrote: >> mc@uga.edu wrote... >>> >>> 20-uF 900-volt film capacitor ... >> >> Did you have a specific part in mind? > > This, maybe? >https://www.mouser.com/ProductDetail/EPCOS-TDK/B32776Z0206K000?qs=sGAEpiMZZMsh%252B1woXyUXj1%2FT6OBaNpJ8kU%2Fi46kWMVw%3DThat's a good one. -- Thanks, - Win
Reply by ●October 7, 20192019-10-07
jla...@highlandsniptechnology.com wrote: ----------------------------------------> > > 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. > >** It's highly amusing that while electros generally self protect when used in series, film caps do not. 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
Reply by ●October 7, 20192019-10-07
Phil Allison wrote...> >jla...@highlandsniptechnology.com wrote: > >> 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. >> > > ** It's highly amusing that while electros generally self > protect when used in series, film caps do not. > > 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.I'm not comfortable with self-balancing for any type. Electrolytic leakage currents must create small carbon pathways, that's not good. -- Thanks, - Win