There are lots of poly film caps for motor run use available out in the surplus world. These look like they'd be good for tube circuits, since they're available in fairly high capacitances and they have maximum AC voltage ratings of 220VAC, 440VAC, etc. My circuits usually call for DC working voltages of 400VDC or sometimes 450VDC.
Let's say I have a 10uF film cap that's rated at 220VAC max. There is no DC working voltage stated.
Can we infer the max DC working voltage from the max 220VAC voltage printed on the capacitor? Is there a formula that can be used, or is this a complex question that requires taking into account a number of considerations?
Thanks for any insight on this.
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PS - I found this PDF from Panasonic entitled "Permissible AC voltage corresponding to DC rated voltage":
https://industrial.panasonic.com/cdbs/www-data/pdf/RDI0000/DMI0000COL75.pdf
It suggests that the maximum AC voltage will be somewhat lower than the rated DC working voltage of a capacitor. It looks like the rated DC working voltage will be somewhere between 1.5 and 2.5 times the permissible AC voltage.
Therefore, is it safe to assume that the working DC voltage of a poly film motor run cap rated at 220VAC will be somewhere around 350VDC?
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Let's say I have a 10uF film cap that's rated at 220VAC max. There is no DC working voltage stated.
Can we infer the max DC working voltage from the max 220VAC voltage printed on the capacitor? Is there a formula that can be used, or is this a complex question that requires taking into account a number of considerations?
Thanks for any insight on this.
-
PS - I found this PDF from Panasonic entitled "Permissible AC voltage corresponding to DC rated voltage":
https://industrial.panasonic.com/cdbs/www-data/pdf/RDI0000/DMI0000COL75.pdf
It suggests that the maximum AC voltage will be somewhat lower than the rated DC working voltage of a capacitor. It looks like the rated DC working voltage will be somewhere between 1.5 and 2.5 times the permissible AC voltage.
Therefore, is it safe to assume that the working DC voltage of a poly film motor run cap rated at 220VAC will be somewhere around 350VDC?
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From a long term wear out standpoint, AC RMS and DC are pretty close. As far as dielectric withstanding, peak AC voltage (X1.41) is applicable. I have no problem using a 440VAC cap where a 450VDC one is called for, but I doubt I’d use a 220VAC cap at 275VDC. I
Yes, the standard rule of thumb you see around the innerwebs is AC voltage rating * 1.414 for DC withstanding voltage rating.
For that 220VAC rated cap, 220VAC(1.414) = 311VDC withstanding voltage.
I'd figure that since 250VDC is 80% of that estimated 311VDC maximum, a 250VDC 'rating' should be OK. (20% less than the estimated withstanding voltage.)
275VDC would be 88.4% of 311VDC. So I can see why you'd want to not go that high.
For that 220VAC rated cap, 220VAC(1.414) = 311VDC withstanding voltage.
I'd figure that since 250VDC is 80% of that estimated 311VDC maximum, a 250VDC 'rating' should be OK. (20% less than the estimated withstanding voltage.)
275VDC would be 88.4% of 311VDC. So I can see why you'd want to not go that high.
I do not consider using a capacitor manufactured to be used for AC is a good idea considering they are usually costlier and bulkier compared with electrolytics of the same voltage rating.
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I often find AC rated motor run MKPs surplus for cheap. Most of the time they find their way into speaker crossovers. Other times they end up as bypass or blocking caps, which might be with tubes.
Would I go out of my way to buy one at full price? Hell no.
Would I go out of my way to buy one at full price? Hell no.
Taking just pure maths into account, the translation factor should be 1.414 in both directions, but things aren't that simple.
First, there is the AC current effect: in AC, the current can become significant but at 50 or 60Hz, it is rarely an issue.
Another problem is ionization of gas pockets trapped inside the caps: it appears when the peak voltage exceeds some tens of volts in AC (I don't remember the exact threshold, but it should be easy to find).
This causes a progressive degradation of the dielectric, ultimately leading to catastrophic failure.
With DC applied, the effect is only caused when the cap is charged or discharged completely, and the degradation is much slower than when the polarity inversion occurs 50 or 60 times/second.
This means that a cap rated at 141VDC will generally not be able to withstand 100VAC, but a cap rated at 100VAC will not necessarily withstand more than 141VDC.
If you want to play safe, you shouldn't use a DC-only rated cap for AC, and you shouldn't use an AC-rated cap at more than 1.414 times DC.
These are conservative, but safe guidelines
First, there is the AC current effect: in AC, the current can become significant but at 50 or 60Hz, it is rarely an issue.
Another problem is ionization of gas pockets trapped inside the caps: it appears when the peak voltage exceeds some tens of volts in AC (I don't remember the exact threshold, but it should be easy to find).
This causes a progressive degradation of the dielectric, ultimately leading to catastrophic failure.
With DC applied, the effect is only caused when the cap is charged or discharged completely, and the degradation is much slower than when the polarity inversion occurs 50 or 60 times/second.
This means that a cap rated at 141VDC will generally not be able to withstand 100VAC, but a cap rated at 100VAC will not necessarily withstand more than 141VDC.
If you want to play safe, you shouldn't use a DC-only rated cap for AC, and you shouldn't use an AC-rated cap at more than 1.414 times DC.
These are conservative, but safe guidelines
275VAC caps are good to 400VDC in my experience. X2 safety types preferred. I use these for coupling tube stages all the time: Polypropylene Capacitors DIP 275VAC 0.0022uF 0.022uF 0.22uF 15 Pcs | eBay
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Capacitors intended for mains AC are designed to withstand far higher transient voltage than 1.414x the AC voltage, DC capacitors do not have this safety margin
Indeed - ever noticed that opto-couplers are rated for several kV? They need to be for mains use, similar for capacitors(*). Find the datasheet for an X or Y class mains capacitor and you'll see values like 275V ac rated voltage, 630V DC permissable voltage.
(*) Capacitors absorb much of the transient voltage spikes due to their capacitance, so the multi-kV rating is not needed for a capacitor of substantial capacitance such as a typical X-class cap.
(*) Capacitors absorb much of the transient voltage spikes due to their capacitance, so the multi-kV rating is not needed for a capacitor of substantial capacitance such as a typical X-class cap.
I would like to add too this.
I have found a source for Paktron Capacitors. Surplus Gismos near me has caps for 10Cents a piece. And the 0.1UF and tons of other values aswell. All are marked with 150VAC.
This threw me off so much, these capacitors voltage ratings have confused me for almost a year.
These are very large, Navy blue color. A modern Mylar metalized film capacitors. Extremely densely compressed core.
They are marked with ""150VAC"" only. Not DC ratings. I only managed to find a couple reference to these capacitors. One of which was a few Ebay listings for these same capacitors. The seller marked in his description that they are marked 150VAC, But good for 500-600Volts. WHICH JUST MADE ME WANT TO INVESTIGATE SOME MORE.
I decided too take the capacitor apart, and unraveled the Film, essentially to understand the insulation, and to compare directly to DC rated Mylar film capacitors of many different types and constructions.
I ran 2000KV through the cap for a short period, and I could not get them to fail, or even change characteristics at these incredibly high voltages. I'ts the maximum voltage I have access to, so I could not go higher.
They are the same exact size, and same construction as Mallory Brown drops for example, rated at 600VDC.
Based on the insanely OVERKILL Mylar metalized Construction.
I used my own judgement and decided they would be safe for 600VDC as did the Ebay seller aswell. I marked them with 600V for my own sanity, and I have never had a single momentary issue. No Leakage either 🙂
I am assuming these where some sort of "safety capacitor" or maybe a specifically manufactured marked part with such a incredibly high "safety margin",
These see 600VDC all day long in many of my Tubed stuff. Never had an issue.
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I have found a source for Paktron Capacitors. Surplus Gismos near me has caps for 10Cents a piece. And the 0.1UF and tons of other values aswell. All are marked with 150VAC.
This threw me off so much, these capacitors voltage ratings have confused me for almost a year.
These are very large, Navy blue color. A modern Mylar metalized film capacitors. Extremely densely compressed core.
They are marked with ""150VAC"" only. Not DC ratings. I only managed to find a couple reference to these capacitors. One of which was a few Ebay listings for these same capacitors. The seller marked in his description that they are marked 150VAC, But good for 500-600Volts. WHICH JUST MADE ME WANT TO INVESTIGATE SOME MORE.
I decided too take the capacitor apart, and unraveled the Film, essentially to understand the insulation, and to compare directly to DC rated Mylar film capacitors of many different types and constructions.
I ran 2000KV through the cap for a short period, and I could not get them to fail, or even change characteristics at these incredibly high voltages. I'ts the maximum voltage I have access to, so I could not go higher.
They are the same exact size, and same construction as Mallory Brown drops for example, rated at 600VDC.
Based on the insanely OVERKILL Mylar metalized Construction.
I used my own judgement and decided they would be safe for 600VDC as did the Ebay seller aswell. I marked them with 600V for my own sanity, and I have never had a single momentary issue. No Leakage either 🙂
I am assuming these where some sort of "safety capacitor" or maybe a specifically manufactured marked part with such a incredibly high "safety margin",
These see 600VDC all day long in many of my Tubed stuff. Never had an issue.
Some decades ago on the request of a valued customer I replaced several oil and PP motor run type caps in a power conditioner with hundreds of dollars worth of Hovland Musicap film caps which only had a DC rating, and they buzzed...