NTC questions for Class A

NYCOne · 15th January 2011, 04:49 PM

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?

DF96 · 15th January 2011, 05:03 PM

mf? Is this old American microfarads or misspelt mF = SI millifarads (=1000uF)?

NYCOne · 15th January 2011, 05:25 PM

To be clear, the PS has 108000uf.

Quote:

Originally Posted by DF96

mf? Is this old American microfarads or misspelt mF = SI millifarads (=1000uF)?

DF96 · 15th January 2011, 05:48 PM

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.

metalsculptor · 16th January 2011, 05:35 AM

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.

AndrewT · 16th January 2011, 12:09 PM

Quote:

Originally Posted by NYCOne

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.

nicoch58 · 27th January 2011, 03:00 PM

Quote:

Originally Posted by AndrewT

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?

AndrewT · 28th January 2011, 04:53 PM

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.

15th January 2011, 04:49 PM	#1
NYCOne diyAudio Member Join Date: Jul 2010 Blog Entries: 1	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?

28th January 2011, 04:53 PM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

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15th January 2011, 05:03 PM	#2
DF96 diyAudio Member Join Date: May 2007	mf? Is this old American microfarads or misspelt mF = SI millifarads (=1000uF)?

15th January 2011, 05:48 PM	#4
DF96 diyAudio Member Join Date: May 2007	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.

16th January 2011, 05:35 AM	#5
metalsculptor diyAudio Member Join Date: Sep 2010 Location: Melbourne	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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