NTC recommendation for 0.4F cap 51V transformer - diyAudio
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Old 14th May 2016, 08:39 AM   #1
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Default NTC recommendation for 0.4F cap 51V transformer

Which NTC to be used for the 0.4F cap with 2KVA transformer with constant current of 5 amps of current.

Want to build a 100W Class A amplifier so considering these specs:

I have a class A audio amplifier requirement where the Capacitor bank is about 0.4Farad in total for both the rails in the secondary. 1.2KVA transformer. Constant current consumption will be about 3 to 4 Amperes Can you please recommend the right NTC for the inrush current limiting.
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Old 14th May 2016, 01:43 PM   #2
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Did you read the ntc design info and work out the cvv joule requirement for charging your cap?
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Old 15th May 2016, 09:49 AM   #3
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you need two current limitiers.

The primary start up current to get the transformer working needs an added resistance in the primary circuit that gets shorted by a time delayed relay after about 200ms. What mains voltage are you using?

The secondary current builds rapidly after the transformer has started to charge the discharged capacitors.
This is exactly the duty that NTCs are used for and is detailed in the NTC datasheets.
This added resistance in the secondary circuit is best shorted out with a time delayed relay set to about 10s.
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Old 16th May 2016, 03:49 AM   #4
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Quote:
Originally Posted by AndrewT View Post
you need two current limitiers.

The primary start up current to get the transformer working needs an added resistance in the primary circuit that gets shorted by a time delayed relay after about 200ms. What mains voltage are you using?

The secondary current builds rapidly after the transformer has started to charge the discharged capacitors.
This is exactly the duty that NTCs are used for and is detailed in the NTC datasheets.
This added resistance in the secondary circuit is best shorted out with a time delayed relay set to about 10s.
Mains voltage:
For AB about 70 to 80V.
For 100Wclass A about 50V

working on a BA2000 class H as well so there it might be about 70V for one rail and 140 for another one.

but mostly a generic approach.

Regarding the transformers it would be about 2KVA in general so there will be an NTC in starting of the softstart ckt.

so the value of the cap is quite large so it would be in big Joules value. so do we need to use a an NTC with larger Joules value.

consider Ametherm NTC so acc to their site for 50V peak and 0.4F the total energy is about 500Joules so we need to take an NTC for 500Joules.

http://www.ametherm.com/inrush-current/

If you look at the bottom of the page

http://www.ametherm.com/inrush-curre...-limiters.html

you will find the Megasurge NTC with more than 500Joules capability but which one to pickup like take about an NTC with 750 joules would be sufficient?

Last edited by rhythmsandy; 16th May 2016 at 03:54 AM.
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Old 16th May 2016, 03:59 AM   #5
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Take this as an example NTC
Ametherm MS35 3R030 ---3 ohm / 30 Amp Inrush Current Limiter Data Sheet
what is that parameter mean?

Max Capacitance @ 120 VAC: 51600 F
Max Capacitance @ 240 VAC: 13024 F
Max Capacitance @ 480 VAC: 3010 F
Max Capacitance @ 680 VAC: 1500 F

so where does this capacitor comes in the circuit?
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Old 16th May 2016, 05:16 AM   #6
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The NTC design info relates to a direct mains VAC rectifier charging a capacitor. Calculate 0.5 x C x V x V for each of those examples and they give the same Joule rating that will not hurt the NTC during a mains turn-on.

You need to determine the Joule requirement of your DC cap - and then find a somewhat larger NTC.

The following link has some NTC advise that may help a bit:
http://dalmura.com.au/projects/Valve%20amp%20fusing.pdf
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Old 16th May 2016, 05:55 AM   #7
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considering 50V peak and 0.4F the total energy is about 500Joules.
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Old 16th May 2016, 10:19 AM   #8
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A GE CL-30 can charge 6,000uF with rectified 120VAC supply (ie. to 170VDC), so can cope with 87J of energy throughput.

NTC should only be connected in series, so 6 of those CL-30 would need to be placed in series on the secondary side to alleviate turn-on surge current.

You then need to determine what the max continuous rms current through the NTC devices would be, and relate that to a possibly derated 8Arms capability.

If you wanted, the NTC could be placed on the primary side to also manage the transformer in-rush. The energy throughput requirement then goes up - but probably not by a substantial level. Placing the thermistor on the primary has the advantage that the max continuous current may be lower.

There may be other NTC devices that have similar energy capability but with a lower resistance, if series resistance is aesthetically displeasing to you, but that does trade-off the level of in-rush.

And of course you could contemplate adding complexity and bypass the thermistor string, but they are designed for this activity so the KISS principal is worth considering.
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Old 19th May 2016, 06:02 AM   #9
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Quote:
Originally Posted by AndrewT View Post
you need two current limitiers.

The primary start up current to get the transformer working needs an added resistance in the primary circuit that gets shorted by a time delayed relay after about 200ms. What mains voltage are you using?

The secondary current builds rapidly after the transformer has started to charge the discharged capacitors.
This is exactly the duty that NTCs are used for and is detailed in the NTC datasheets.
This added resistance in the secondary circuit is best shorted out with a time delayed relay set to about 10s.
so one comes in primary and once comes in secondary.

So instead of having a 5 ohm resistor in the primary and short after 200ms can we use an NTC there too.

Actually what Im thinking to use is use a NTC right after the EMI filter and then use another NTC in series with the primary of the trafo and short after 0.5sec and then use another NTC in the secondary of the transformer before the Large caps and then short after 10 secs. Is that fine?
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Old 19th May 2016, 07:10 AM   #10
AndrewT is offline AndrewT  Scotland
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NTCs are good. Use them.
Select the correct resistance value to give effective current limiting.
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