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Old 20th April 2005, 07:52 PM   #101
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Quote:
Originally posted by GregD
What about using an AC rated metalized polypropylene capacitor for DC blocking. Specifically, I'm looking at the Cornell Dublier SF series.
http://www.cornell-dubilier.com/catalogs/SF.pdf

It seems that this should do the job, handle lots of current, be relatively compact, and not require any bypass diodes for protection. Unless I'm missing something.
Yes, you are missing something, I'm afraid. Do you know how big capacitor you'll need? = several 1000's of uF, like 2200-10000 uF depending of situation.
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Old 20th April 2005, 08:14 PM   #102
GregD is offline GregD  United States
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Quote:
Originally posted by peranders

Yes, you are missing something, I'm afraid. Do you know how big capacitor you'll need? = several 1000's of uF, like 2200-10000 uF depending of situation.
Oops! I kind of forgot about the impedance of the capacitor at 60hz. So much for that idea.
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Old 20th April 2005, 08:20 PM   #103
GregD is offline GregD  United States
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Ok, here's my next thought. (Hopefully better than the last)

According to the application guide on Cornell Dublier's site, it sounds like if the capacitors are connected back to back, i.e., the two minuses together or the two pluses together, bypass diodes shouldn't be required as long as the ripple current rating of the capacitors is higher than the line fuse.
Quote:
If two, same-value, aluminum electrolytic capacitors
are connected in series, back-to-back with the positive
terminals or the negative terminals connected, the
resulting single capacitor is a non-polar capacitor with
half the capacitance. The two capacitors rectify the
applied voltage and act as if they had been bypassed
by diodes. When voltage is applied, the correct-polarity
capacitor gets the full voltage. In non-polar aluminum
electrolytic capacitors and motor-start aluminum
electrolytic capacitors a second anode foil substitutes
for the cathode foil to achieve a non-polar
capacitor in a single case.
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Old 23rd April 2005, 01:33 PM   #104
AndrewT is offline AndrewT  Scotland
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Hi,
the parallel diode/diodes limit the capacitor voltage to 0.7v or 1.4v (for 1 or 2 in series).
In start up or fault condition, the diodes will pass the bulk of the short term excess current. eg. till magnetic flux builds in a toroid or a fuse blows.
Thus allowing fairly low voltage capacitors to be used e.g. 16Vdc instead of 250Vac
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Old 23rd April 2005, 01:41 PM   #105
GregD is offline GregD  United States
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Quote:
Originally posted by AndrewT
Hi,
the parallel diode/diodes limit the capacitor voltage to 0.7v or 1.4v (for 1 or 2 in series).
In start up or fault condition, the diodes will pass the bulk of the short term excess current. eg. till magnetic flux builds in a toroid or a fuse blows.
Thus allowing fairly low voltage capacitors to be used e.g. 16Vdc instead of 250Vac
The AC voltage shouldn't matter since the AC will just pass through the capacitors. The only real issue is what the maximum DC voltage might be. I not coming up with any likely scenario where the DC would be more than a few volts.
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Old 23rd April 2005, 02:05 PM   #106
AndrewT is offline AndrewT  Scotland
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Hi,
you missed my point. I'll try again.
the AC voltage will not just pass through the series caps. A voltage will be lost across the caps. As the current rises the voltage rises in proportion.
At high peak current the voltage loss could be significant. The diodes are there to ensure that this voltage never exceeds your preset (diode number) limit.
BTW. I think you are right about the level of DC likely to appear on the line. So again a high voltage cap is not required.

Another thought, since the caps are operating at line voltage both terminals and the case could be at line voltage. I suggested in another thread that it might be better to locate the DC block on the neutral line for safety. No one came back to refute the idea.
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Old 23rd April 2005, 08:43 PM   #107
GregD is offline GregD  United States
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Quote:
Originally posted by AndrewT
Hi,
you missed my point. I'll try again.
the AC voltage will not just pass through the series caps. A voltage will be lost across the caps. As the current rises the voltage rises in proportion.
At high peak current the voltage loss could be significant. The diodes are there to ensure that this voltage never exceeds your preset (diode number) limit.
BTW. I think you are right about the level of DC likely to appear on the line. So again a high voltage cap is not required.

Another thought, since the caps are operating at line voltage both terminals and the case could be at line voltage. I suggested in another thread that it might be better to locate the DC block on the neutral line for safety. No one came back to refute the idea.
You just have to make the capacitance large enough to keep the series impedance down to a reasonable level.

The problem is you need 4 times the total capacitance when you put two capacitors in series versus a single or paralleled capacitors. After looking at the numbers, a couple of diodes are much cheaper than the capacitors necessary to achieve this, so while I like the simple elegance of the capacitor only solution, I'll be using the protection diodes.

Maybe most people agreed with you about installing the circuit in the neutral line. I certainly do.
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Old 24th April 2005, 08:32 AM   #108
AndrewT is offline AndrewT  Scotland
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Hi,
yes, keep increasing the uF to achieve your desired amperage capacity. multiple // caps, say 3 * 3.3mF + 3 * 3.3mF all at 16Vdc are pretty cheap and give an effective 5000uF with a current capacity of about 1.1Apk @ 0.7Vpk or 2.2Apk (400w) @1.4Vpk.
I'm appreciative of your confirmation on Neutral line blocking.
Now that I have your ear let's tell you why some may say NEVER interfere with the Neutral return line. If due to a fault in the caps and/or diodes that causes the DC block to fail open circuit, the whole mains side of the system becomes live. Your amp seems to be dead but everything around the transformer, fuses, switch, DCblock, softstart is now at 240Vac just waiting to KILL any fool that pokes a finger in!!!!
Comments please.
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Old 21st July 2005, 05:30 PM   #109
sam9 is offline sam9  United States
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Default Resurecting a thread

This thread has been inactive for a while, but suddenly I feel compelled to ask a question. Probably a dumb one.

An inductor introduces a 90 deg current lag. Adding a series cap offsets the lag so that current and voltage are once again sychronized. (Resistive elements make this approximate.) So my question is: how much, if any, of the a DC blocker's action with respect to reducing hum is actually due to reducing current lag? Particularly if there are solid state relays (triacs?) being used for soft start.
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Old 25th August 2005, 12:02 AM   #110
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Default DC blocking filter for toroid Xfmr

My apologies in resurrecting a long dead thread, but I have a question I'm hoping to get an intelligent answer for.

I'm calculating much larger values for capacitors than the examples listed in the thread show, and I'd like to understand how my assumptions are different (or wrong).

I have a 2kVA toroid which will be connected as an isolation transformer. Max average current for 240 VAC in will be 8.33 amps.

My understanding is that the impedance at 60Hz of the capacitor must be such that the voltage drop will be less than the forward voltage drop across the diodes.

If I assume 2.8 Vdc (4 diodes) to be blocked (high, but this relaxes requirements on the cap size), then the target Z is .336 ohms (2.8/8.33). This impedance requires ~8000uF @ 60Hz. Which seems really large relative to the other examples in the thread.

Are my assumptions incorrect, or perhaps overly conservative?

Thanks and regards,

Rob W.
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