Re: Say What?
Don't use motor starting caps, they aren't intended for continuous duty - motor run caps are.. There are plenty of threads here on this forum about the differences in motor run vs motor start capacitors.
I think Andrew T was suggesting you not connect caps in series at, but that you find a capacitor with a sufficiently high voltage rating to make this unnecessary. That's good advice in the real world because of things like ESR and ESL in the capacitor.
duderduderini said:
Don't use motor starting caps, they aren't intended for continuous duty - motor run caps are.. There are plenty of threads here on this forum about the differences in motor run vs motor start capacitors.
I think Andrew T was suggesting you not connect caps in series at, but that you find a capacitor with a sufficiently high voltage rating to make this unnecessary. That's good advice in the real world because of things like ESR and ESL in the capacitor.
Point taken
Well then thats that then. So many different ways to skin the cat as they say. So many series up the resistors yet it makes sense not to.
On another point what values in uF is a satisfactory clcrc filter wrt ripple etc. WHat do you recommend Andrew. I am asking to see if my thoughts on this are well grounded (which is probably not the case)
Thanks
Well then thats that then. So many different ways to skin the cat as they say. So many series up the resistors yet it makes sense not to.
On another point what values in uF is a satisfactory clcrc filter wrt ripple etc. WHat do you recommend Andrew. I am asking to see if my thoughts on this are well grounded (which is probably not the case)
Thanks
I think you have already used Psud2.
Compare C, CRC, CLC, CLCRC and note how low mains effect becomes as you add stages to the cascaded filters.
I would save your best cap for the last C maybe 40uF.
use 20uF for the first and second.
Then select the L and R to give the attenuation you need and still retain the B+.
This scheme also very effectively removes the HF rubbish as well as the hum and it's harmonics.
Compare C, CRC, CLC, CLCRC and note how low mains effect becomes as you add stages to the cascaded filters.
I would save your best cap for the last C maybe 40uF.
use 20uF for the first and second.
Then select the L and R to give the attenuation you need and still retain the B+.
This scheme also very effectively removes the HF rubbish as well as the hum and it's harmonics.
About 25 years ago I talked to a Nissei Denki/Arcotronics application engineer about DC ratings of AC-rated film capacitors. You can simply calculate this and apply a safety margin.
You absolutely cannot calculate an AC rating from DC-rated capacitors. Period. Remember this and you will be smarter about capacitors than a lot of folks. If you compare AC ratings for DC-rated capacitors, you will notice they vary by dielectric type and even within different series from the same manufacturer. AC is a tough environment for film capacitors, both sinusoidal and pulse. Power dissipation due to dielectric losses is actually another major failure mode. I did not know that (or think about it) until I worked in the electronic fluorescent ballast industry.
OK...an AC-rated film capacitor has been designed with corona and other factors in mind. That is probably the main reason you cannot determine AC rating from DC rating.
So if you have an rms sinusoidal voltage rating on an AC-rated film capacitor, you can calculate a peak-to-peak value from the rms sinusoidal rating. This is a math exercise so far. You will not use this value, so no arguments or disputes yet.
Pick an easy number like 100 VAC rms. Peak-to-peak is 2*(2^0.5)*rms =2.828*rms.
De-rate this 10% for some safety margin: 0.9*2.828*Vrms for the AC rating of the capacitor. A 100 Vrms AC-rated film capacitor can be used at up to 254 (call it 250 VDC).
Again you cannot do the reverse. Look at DC-rated film capacitors that have AC ratings (with limitations, like frequency). Many don't even give you AC ratings, but for those that do, you will see a much smaller ratio of DC/AC rating. The DC-rated capacitor is not designed for phenomena like corona. AC-rated film capacitors utilize construction techniques.
I have some Panasonic paper-in-oil capacitors in ceramic tubes with 450 Vrms AC ratings. With the 10% derating, 1145 VDC. I am comfortable saying that, having experienced and avoided capacitor failures, and having gotten advice from multiple capacitor engineers.
A DC application with 100 Hz or 120 Hz ripple is a vacation for an AC-rated film capacitor. You see a difference in size and cost.
I ended up at this thread today because I finally, after all these years, looked up the cost of 20 and 40 uF film capacitors as an alternative to electrolytics, and I am happy with options and prices available today.
Another factor I always tried to maximize with electrolytics is ripple current rating at an appropriate frequency (do you have 120 Hz ripple, or switchmode ripple?). I generally cannot control ripple current, but I can choose the highest ripple current rating I can find, among other factors on my list. Compare ripple current rating for film capacitors to electrolytics...amps vs mA...nothing to worry about there.
Now, you do not need AC-rated film capacitors for filtering ripple from DC rectified power. I just explored the DC rating of AC rated caps, following the preceding discussion.
You absolutely cannot calculate an AC rating from DC-rated capacitors. Period. Remember this and you will be smarter about capacitors than a lot of folks. If you compare AC ratings for DC-rated capacitors, you will notice they vary by dielectric type and even within different series from the same manufacturer. AC is a tough environment for film capacitors, both sinusoidal and pulse. Power dissipation due to dielectric losses is actually another major failure mode. I did not know that (or think about it) until I worked in the electronic fluorescent ballast industry.
OK...an AC-rated film capacitor has been designed with corona and other factors in mind. That is probably the main reason you cannot determine AC rating from DC rating.
So if you have an rms sinusoidal voltage rating on an AC-rated film capacitor, you can calculate a peak-to-peak value from the rms sinusoidal rating. This is a math exercise so far. You will not use this value, so no arguments or disputes yet.
Pick an easy number like 100 VAC rms. Peak-to-peak is 2*(2^0.5)*rms =2.828*rms.
De-rate this 10% for some safety margin: 0.9*2.828*Vrms for the AC rating of the capacitor. A 100 Vrms AC-rated film capacitor can be used at up to 254 (call it 250 VDC).
Again you cannot do the reverse. Look at DC-rated film capacitors that have AC ratings (with limitations, like frequency). Many don't even give you AC ratings, but for those that do, you will see a much smaller ratio of DC/AC rating. The DC-rated capacitor is not designed for phenomena like corona. AC-rated film capacitors utilize construction techniques.
I have some Panasonic paper-in-oil capacitors in ceramic tubes with 450 Vrms AC ratings. With the 10% derating, 1145 VDC. I am comfortable saying that, having experienced and avoided capacitor failures, and having gotten advice from multiple capacitor engineers.
A DC application with 100 Hz or 120 Hz ripple is a vacation for an AC-rated film capacitor. You see a difference in size and cost.
I ended up at this thread today because I finally, after all these years, looked up the cost of 20 and 40 uF film capacitors as an alternative to electrolytics, and I am happy with options and prices available today.
Another factor I always tried to maximize with electrolytics is ripple current rating at an appropriate frequency (do you have 120 Hz ripple, or switchmode ripple?). I generally cannot control ripple current, but I can choose the highest ripple current rating I can find, among other factors on my list. Compare ripple current rating for film capacitors to electrolytics...amps vs mA...nothing to worry about there.
Now, you do not need AC-rated film capacitors for filtering ripple from DC rectified power. I just explored the DC rating of AC rated caps, following the preceding discussion.
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But the peak value is the max value that can be across the cap at any one time, which in your example is + or -141V. So logically, that would lead one to conclude that you can charge it up to 141V (say 150V) but certainly not 250V.
Jan
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