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Old 16th August 2014, 10:11 PM   #21
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   #22
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   #23
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   #24
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   #25
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   #26
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, 12 views)
File Type: txt ntc_20130313.txt (104.4 KB, 8 views)
File Type: txt ntc.txt (173 Bytes, 6 views)

Last edited by BigE; 18th August 2014 at 04:57 PM.
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Old 19th August 2014, 06:50 AM   #27
AndrewT is offline AndrewT  Scotland
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could not find ntc.

How do I use the txt files?
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Old 19th August 2014, 07:09 AM   #28
SGK is offline SGK
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you need to have them in the same folder as the .asc file

edit the .inc statement to direct LTspice to the right directory where they are stored. If in the same folder as the .asc file you don't need the file path just .inc [filename]

Last edited by SGK; 19th August 2014 at 07:12 AM.
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Old 19th August 2014, 08:12 AM   #29
AndrewT is offline AndrewT  Scotland
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where do I find .inc statement?
What do I change it to?
and does it need to be changed back after examining your asc?
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Old 19th August 2014, 08:17 AM   #30
SGK is offline SGK
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open the .asc file in LTspice (I assume you have it installed)

it is on the schematic

right-click on the .inc statement to edit it - remove the file path

.inc ntc_20130313.lib

also make sure ntc.txt (renamed to ntc.asy) is in the same folder (this is the Spice symbol for the ntc)

(it's "bad form" to share such work with dedicated file paths that only work on the originator's computer but not a biggie to fix - better to share a folder and have statements such as .inc without file paths and the .lib .txt files in the folder)

Last edited by SGK; 19th August 2014 at 08:21 AM.
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