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Old 25th January 2005, 04:46 PM   #1
Mlaen is offline Mlaen  Croatia
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Default Which NTC to limit inrush current

where can I find info to calcutale and pick the right NTC resistor to limit inrush current. I would put it before transformer (primary coil)
It's for amp.
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Old 25th January 2005, 05:12 PM   #2
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Here's a link to General Electric's GESENSING site for their CL Series of Current Inrush Limiters:

http://www.gesensing.com/products/re...asheets/cl.pdf

they have some of the formulas on this PDF
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Old 28th January 2005, 11:43 AM   #3
K-amps is offline K-amps  United States
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Digikey has these. Nelson used the CL-101 in the Adcom 5802.
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Old 28th January 2005, 12:45 PM   #4
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Digikey is my source, they don't have a link to the GE website, however.
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Old 28th January 2005, 04:07 PM   #5
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Here's a link to the ones I use...

Thermometrics (GE)

-Casey Walsh
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Old 28th January 2005, 04:30 PM   #6
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Ok, so I have two 33kuF caps in my power supply with 58Vrails. My transformer is a 620VA torroid.

If I use the CL101 (best match), I am way over the recommended max Capacitance (4000uF). I realize it says these are just recommendations, and that exceeding them would simply shorten the life of the NTC, but how much would I be shortening that life?

Is this the best solution?

I am also planning on a turn-on delay with a relay, but would like to not smoke my torroid when the caps are low. I was thinking that, because these caps are so big, I would want drain resisitors on them for safety, but it seems they would also limit the lifespan of the other power supply components due to the need to more fully charge the caps every time I power up.

Am I worrying about something I don't really need to?

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b
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Old 28th January 2005, 04:58 PM   #7
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The load of the bleed resistors should be negligible, if you spec them properly.

Also, a thermistor in the primary is only any good if your amp is class-a and therefore fairly constant load.
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Old 28th January 2005, 04:59 PM   #8
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If you pre-charge the capacitors using a series resistance or current source to within a few volts of 100% then use a relay to direct-connect (through a fuse of course) the AC line, then you don’t need an inrush current limiter.

If you plan on just using the CL-101 to deal with the inrush, then it is way undersized for the application. It wants to pass about 64 joules of energy before it self-heats enough to become low resistance. The 64 joules comes from Q = (C*V^2)/2 where C = 4mF and V = 180VDC. The best you could use the CL-101 for is to top off the last 44VDC of your caps, which gets you back to needing to pre-charge them.

I haven’t seen any consumer grade current-limiting NTC thermistors bigger than the CL-101, and I wouldn’t recommend paralleling them just like one doesn’t parallel diodes.

Hope this helps…

-Casey Walsh
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Old 28th January 2005, 05:15 PM   #9
Mlaen is offline Mlaen  Croatia
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So which should I choose (spec: )
input 220V/output 2*25V AC, 250VA
In ps im using totaly 40000 uF. 10000 per rail for each channel
I don't know wich NTC to choose
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Old 28th January 2005, 05:37 PM   #10
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Hey Guys,

Any help on this. My parts box has a pair of devices that sure look like a CL-30 thermistor except their label is C30-18. What have I got? My ohmmeter shows about 2 ohms resistance when cold out of circuit.

Are these CL-30?

Thanks
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