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Old 24th February 2001, 08:05 AM   #1
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I am planning to build a class A power amp. However I do not know how to use thermistors required by the power supply unit and the front end stage of the circuit.

Value of thermistor specified for power supply unit:
2.5 Ohm @ 25 deg. C8A

Value of thermistor specified for front end stage:
1k & 3k

My questions:

1. Which type of thermistor should be used. I know there are two different types of thermistor NTC AND PTC. What is the difference between these two.

2. Will there be any problem if I use regular resistors to replace the thermistors at the same value because it is hard to get thermisters in my country.

3. What is the difference between the thermistors on the power supply unit and the front end stage, the power supply unit specified 25 deg. C8A but the front end stage none.

Thank you in advance for replying these questions.
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Old 24th February 2001, 12:58 PM   #2
djk is offline djk
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Default Thermistors

The KC003L-nd is an inrush current limiter with a cold resistance of 2.5 ohms and .06 ohms at 8 amps, USD$2.58 from http://www.digikey.com The 1K NTC would be KC003G-nd, USD$2.66 and the 3K NTC is KC006G-nd USD$2.66 The manufacturer is Keystone Thermometrics (formerly Keystone Carbon,notice how the part numbers all start with KC) PTCs are used like circuit breakers.NTCs are used as inrush current limiters and for temperature measurement or compensation.Do not substitute.
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Old 24th February 2001, 06:36 PM   #3
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Hi - sounds like we are talking surge guard devices here - probably on the mains side of main transformers. I've got this on my Zen Revisited, have been given the following hints by supplier:

These devices have an energy rating too - in Joules - should be specified in the catalogue (or go to maker's www). Make sure any alternative is similary rated. Idea is that the device soaks up the surge until it gets to working temp (thereafter lo ohms). I think energy is mainly a function of the caps being charged in apps. such as this. A fairly simple calc - can look it up for you if need be.

In my case I have 3 in series to cope with (i.e. share) the calculated energy in my PSU.

So an ordinary resistor of the same cold ohms will probably be ok to control the surge, but it will run much hotter than the thermister as it won't change it's ohms. Poor solution. You'll also get excessive volts drop compared to the hot thermister. Work out if this is acceptable for you.

Hope this helps

Pete
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Old 14th October 2002, 07:07 PM   #4
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I'm going to build a Leach amp and have a question about thermistors for the PSU.

My transformer has four secondaries and I'm planning to use four bridge rectifiers and eight 10000uF caps. That way each secondary gets it's own rectifier and two caps in parallell for a total of 80mF. Each secondary is rated 42V/4.64A.

I'd like to install thermistors between the bridges and filter caps to limit the inrush current. But how do I know what the resistance should be and which power/current-ratings they need to have?

-truls
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Old 15th October 2002, 08:32 AM   #5
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Default thermistor questions

any suggestions on resistance or ratings?

-truls
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Old 16th October 2002, 03:31 PM   #6
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Default what to protect..

I know that the bridge rectifiers may need to be protected against too high inrush currents, but what about the transformer. My transformer is rated 4.64A for each secondary. I want to use one bridge per secondary. The bridge can handle 35A, so what happens to the transformer if I limit the current to say 20A?

-truls
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Old 16th October 2002, 03:56 PM   #7
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Truls,

If you put the thermistor on the primary of the transformer, everything will be protected, including your rectifiers, since the current into the transformer will be limited. I'm not sure about exact values.
Some people use a resistor in series with the primary, with a delayed relay to bypass the resistor once the caps have charged. There are many references to this in many threads - try a search.
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Old 17th October 2002, 08:47 AM   #8
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I guess what I want to know is if the inrush limiter have to make the current through each secondary become less than 4.64 amps (which is far less than what the rectifier can handle).

-truls
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Old 17th October 2002, 11:53 AM   #9
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Default Thermistor in primary side.

Even if you have nothing connected to the secondary you will often get a current surge when you turn the tranny on. If you happen to turn it on right at the zero crossing of the mains waveform this is the worst time for the transformer and it will usually saturate momentarily, making the house lights blink as it makes a "bong" sound. Paradoxially, the zero crossing is the best time to turn on for the sake of the capacitor charging current so you are stuck between a rock and a hard place. I say stick a thermistor in the primary side, say 12 ohms for 120v, 24 ohms for 240v etc. That will limit the surge to 10 amps rms. Also, put a bit of heatshrink tubing around the thermistor so it will run hotter and therefore stay at a lower resistance when running.

The transformer secondary current rating is a long term thermal one. You can exceed it by a LONG way for a moment or two at switch-on. Same with the diodes. In the databook they will have an Itsm rating - this is usually the surge current rating for one half cycle of mains power. It can be often 10 or 20 times the normal continuous rating.

GP.
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Old 17th October 2002, 12:43 PM   #10
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Thank you Circlotron, finally some good answers!

I had a look at the power supply example in Nelson Pass's docs about The Preultimate Zen (image below). The thermistor used there has a cold resistence of 10ohms and can tolerate 5A. I was quite confused about how the 3A fuse and 5A thermistor could handle 12 amps of current. On the secondary side of the transformer this would be 40A, which to me just seemed to be too much for the poor tranny.

Now I take it that all this will work as long as the high current lasts for a very short time. The duration of the inrush current must obviously depend on the size of the caps. Will I be ok with 12A inrush, using a total of 80mF for smoothing?

-truls
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