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NTC questions for Class A
NTC questions for Class A
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Old 15th January 2011, 04:49 PM   #1
NYCOne is offline NYCOne  United States
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Default NTC questions for Class A

I'm trying to select an NTC for a Class A PS. The filter Caps add up to 108mf in a CRC configuration (54mf-R-54mf). The output is expected to be 16v at 4A.

In looking at NTCs, the Cmax ratings are far lower than 108mf. I also noticed in Nelson Pass' F5 PS he uses CL-60s. Their Cmax rating is 5mf. I must be calcuating something wrong, because NP has more than 5mf in his PS design.

What Cmax does one use to chose the right NTC?
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Old 15th January 2011, 05:03 PM   #2
DF96 is offline DF96  England
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mf? Is this old American microfarads or misspelt mF = SI millifarads (=1000uF)?
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Old 15th January 2011, 05:25 PM   #3
NYCOne is offline NYCOne  United States
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To be clear, the PS has 108000uf.

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Originally Posted by DF96 View Post
mf? Is this old American microfarads or misspelt mF = SI millifarads (=1000uF)?
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Old 15th January 2011, 05:48 PM   #4
DF96 is offline DF96  England
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OK, I suspected that is what you meant, given the output of the PSU. It's just that the US was slow to adopt SI units, so m can still mean milli or micro when used by an American.
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Old 16th January 2011, 05:35 AM   #5
metalsculptor is offline metalsculptor  Australia
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A soft start system is rated in Joules, = 1/2 CV^2 or 13J which is not a lot. Look for an NTC which can handle 13 or more joules.
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Old 16th January 2011, 12:09 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by NYCOne View Post
I'm trying to select an NTC for a Class A PS.................
In looking at NTCs, the Cmax ratings are far lower than 108mf. I also noticed in Nelson Pass' F5 PS he uses CL-60s.
Pass soft start is in the primary of the transformer.

The manufacturer's data is for slow charging of capacitors where the NTC is in the secondary.

These two arrangements do different jobs.
Soft start is to allow the transformer to start up without blowing the close rated mains fuse.
Slow charging is to reduce the peak current through the capacitor during charging.
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Old 27th January 2011, 03:00 PM   #7
nicoch58 is offline nicoch58  Europe
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Quote:
Originally Posted by AndrewT View Post
The manufacturer's data is for slow charging of capacitors where the NTC is in the secondary.

These two arrangements do different jobs.
Soft start is to allow the transformer to start up without blowing the close rated mains fuse.
Slow charging is to reduce the peak current through the capacitor during charging.
HI andrew
just to be clear, putting on primary we have soft start & slow charge ,rigth?
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Old 28th January 2011, 04:53 PM   #8
AndrewT is offline AndrewT  Scotland
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I don't agree, but I seem to be a lone voice that prefers a slow charge circuit to be designed exclusively to do that job properly.
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