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Old 16th August 2014, 10:06 AM   #121
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by BobEllis View Post
True, but that is more of an issue on the primary side, where the transformer may present essentially a short circuit if the magnetization is worst case. Then you wouldn't have time for the NTC to cool and present a significant resistance. But an amp with enough capacitance to require a slow charge (current discussion) will bleed off its rails very slowly even at fairly significant bias, so a short interruption isn't such a big deal

For example, my A75 with only 45K F per rail and 2A bias will play for at least 20 seconds after removing power to the output stage. (separate transformer for the front end) In that time the NTC will have cooled significantly and recovered most of its cold state resistance. Not to mention that you've also got the primary side soft start helping to limit inrush currents.
the NTC starts cold and has the high resistance required to limit the inital charging current.
After a short charging time the NTC is warming and the resistance is dropping.
after a further time, say 200 mains cycles, the NTC current has fallen considerably below the initial peak charging current and the NTC dissipation has fallen (Intc * Vdropntc) considerably. The NTC cools before the smoothing capacitance has fully charged.
The charging rate in the later period of charging is quite slow, the ntc may have come back up to 20% to 50% of the cold value.
Finally the smoothing caps reach ~ 99.9% of their full charge voltage and the NTC has come back up to 50% to 90% of it's cold resistance value. NOW is the time to trigger the NTC bypass.
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Old 16th August 2014, 10:10 AM   #122
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by BigE View Post
................I honestly don't see an idiot proof method except for using lots of resistors and applying the bias current to the outputs ONLY when the caps have been filled. ie, place an NO relay between the power supply and the amp, or even between the front end and the output boards (thinking F5T here).

Is there really no better way?
The NTC manufacturers make NTCs specifically to control the speed of charging capacitors.
They know their product.
They show you how to apply their product.
Do as they say and the slow charge is taken care of.

BUT !!!! then bypass the NTC to make the PSU ready to do it's job. To provide a low impedance source of current.
The PSU cannot do that job of "low impedance" if the NTC is still in circuit.
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Old 16th August 2014, 10:13 AM   #123
AndrewT is offline AndrewT  Scotland
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If cost is an issue then fixed resistors can be used as a second grade version of current limiting in a slow charge circuit. NTCs are best.
I use fixed resistors(lots of them) because, for me, cost is an issue.
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Old 16th August 2014, 10:11 PM   #124
BigE is offline BigE  Canada
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Just a note: Ametherm says that the cool down time is 120 seconds.
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Old 17th August 2014, 08:49 AM   #125
AndrewT is offline AndrewT  Scotland
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IF you fully heat them up and require them to coll down to the fully cold temperature.

If all you do is pass a current that subsides from ~90% of max rating to ~5% of max rating over a period of around 10 to 15 seconds, the device is not going to get anywhere near it's maximum temperature.

I have "seen" the effect of a Power NTC in the primary circuit of a Krell Klone PSU and what it did to the maximum output of the Klone. It all happened in less than 2seconds.
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Old 17th August 2014, 12:58 PM   #126
BigE is offline BigE  Canada
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Thanks AndrewT,

I am seeing in the sim that the NTC is in circuit for about 2.5 seconds before getting switched out.

I am using the epcos thermistor models. Ametherm models would be preferred, but I cannot locate any. Perhaps a quick email to Ametherm would help.
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Old 18th August 2014, 01:26 AM   #127
BigE is offline BigE  Canada
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Actually, there was an error in the sim.....

40 seconds before switchover. That ensures that the neither the KVA rating of the transformer nor the contact rating of the relays are not exceeded. I'm certain that by then, the NTC is good and hot.
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Old 18th August 2014, 08:00 AM   #128
AndrewT is offline AndrewT  Scotland
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No, you are not drawing max current into the smoothing caps for all that time (40s).
The caps are substantially charged in a few time constants.
15r and 10mF gives RC = 150ms.
5 time constants is 750ms.
If the NTC resistance is dropping below that start value of 15r, then 5*RC is less than 750ms

At the lower currents after the first couple of RC periods the NTC starts to cool.
It's resistance value to heading back towards it's cold value.

If you have a BIG ClassA bias current then the NTC will remain warm after start up. That warm resistance will not be the hot resistance. But the voltage drop of warm resistance* bias current can be fairly close to the voltage drop of max charging current * hot resistance.

i.e. you can be dropping a volt or so across the NTC. That is equivalent to having a high source impedance to your transformer/PSU.

High source impedance can significantly reduce amplifier performance !!!!!

You must switch out soft start NTC and switch out slow charge NTC, just as one would for fixed resistors. It's only the delay period that is different for fixed vs NTC.
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Last edited by AndrewT; 18th August 2014 at 08:02 AM.
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Old 18th August 2014, 04:53 PM   #129
BigE is offline BigE  Canada
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Please find attached the simulation I have been using. I do not pretend to be an LTspice guru, but this is what I have come up with. There are 4 CRCs, one per rail for each channel ( probably wrong terminology, but wth)

It uses the epcos thermistor libraries. The file ntc.txt is to be renamed ntc.asy , as diyaudio does not allow asy files to be attached.

The simulation runs for 40 seconds, then the thermistor bypasses are switched on and the sim continues to run. You can examine the current spikes in the NC switches and their bypass switches, as well as the power through one of the seconday side inductors that makes up the transformer.

Some switches are marked NC others NO. That shows their operation.

The internal resistance of the transformer consisting of the coupled inductors, was chosen to match the inrush of a similar transformer modelled on Duncan amp tools. The inductances were chosen to provide the correct secondary voltages that will be in my amp.

The switch delay time can be changed by modifying the delay param.

The load resistors are intended to model the amplifier bias a idle. Constant current sources were not used.

If there are any problems with this sim, please let me know.
Attached Files
File Type: asc slow charge thermistors.asc (11.8 KB, 7 views)
File Type: txt ntc_20130313.txt (104.4 KB, 5 views)
File Type: txt ntc.txt (173 Bytes, 3 views)

Last edited by BigE; 18th August 2014 at 04:57 PM.
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Old 19th August 2014, 06:50 AM   #130
AndrewT is offline AndrewT  Scotland
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could not find ntc.

How do I use the txt files?
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