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Old 14th November 2014, 12:16 AM   #1
SSDIY is offline SSDIY  Hong Kong
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Default The Use of Thermistor NTC

Hello

I am looking for a simple slow start up current inrush limiter for my up coming class a power amp project.

Attached image is the power supply section of Nelson Pass's SA-12e power amp. It is very simple and easy to build so I want to use it.

Would anyone please tell me what resistance of the NTC thermistor I should use if the secondary voltage rating of my transformer is 25v x 2, power rating is 500va and the capacitors 120,000uF x 2. Thanks in advance.
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Old 14th November 2014, 02:07 AM   #2
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At 220 VAC 500 va is about 2.3 amps. So you could try a GE CL-80 with a 3 amp steady state current rating and a cold resistance of 47 ohms.
At 120 vac a 4 amp CL--70 would be more like it with a cold resistance of 16 ohms. These are in series with the primary of the transformer.
This are trial values, verify with your prototype.
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Old 14th November 2014, 12:23 PM   #3
AndrewT is offline AndrewT  Scotland
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Indian is referring to a primary circuit soft start resistance.

Your diagram shows secondary circuit slow charge resistances.
You decide what current limit you want to apply to your capacitors and rectifiers.
Then simply divide the peak secondary voltage by the peak current you have chosen to determine the required resistance.

You MUST add a time delayed relay (either mechanical or SS) to bypass the Thermistors if you want/need a low source impedance to feed your capacitor bank.

If you want ~10Apk through each rectifier (20Apk into the capacitor bank), then a resistance of around 3r5 to 4r gets you to near that target.

Note that the two rectifier bridges are in parallel and thus the secondaries are also in parallel.
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Last edited by AndrewT; 14th November 2014 at 12:29 PM.
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Old 18th November 2014, 12:28 AM   #4
SSDIY is offline SSDIY  Hong Kong
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Hi Indian & Andrew

Thank you very much for your advises. I am thinking to buy a Soft Start for Toroidal Transformers from Sjostrom Audio. It looks very good to my requirement:

http://sjostromaudio.com/pages/index...s#.VGqRnCKUdvA
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Old 18th November 2014, 02:01 AM   #5
hongrn is offline hongrn  United States
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This is the soft start I use for all my amp projects. Works really well, and only about US$8.00 in parts.

LM4780 Chipamp Redux - Page 2
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Old 18th November 2014, 04:04 PM   #6
SSDIY is offline SSDIY  Hong Kong
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Quote:
Originally Posted by hongrn View Post
This is the soft start I use for all my amp projects. Works really well, and only about US$8.00 in parts.

LM4780 Chipamp Redux - Page 2

Hi hong

Thank you for sending the soft start circuit.

The soft start you sent is very interesting to build but I have the following questions:

1. Is VCC the operation voltage of the soft start? If so what is the voltage?
2. What is the mute control? Where should it be connected to?

Please clarify as I would like to build this simple soft start circuit for my power amp.
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Old 19th November 2014, 03:02 PM   #7
AndrewT is offline AndrewT  Scotland
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That is the same idea as I have roundly condemned on many other posts.

Vcc is LOW when the current limiting resistor is "in circuit".
If mains is also LOW during start up, then the combined effect can be that the relay does not see sufficient voltage to pull in cleanly. AND when it does pull in it draws more current dropping the voltage available and drops out again. This can become chatter.

During this time the resistors are getting hotter and hotter and hotter.

The basic problem is that an RC used as a timer is not a good timer. The Zener helps, but poor selection of component values and inexperience in "what to test" can leave the resistor permanently in circuit without the Builder knowing what is happening and when the resistor will burn out.

Use a proper timer (eg a 555) and preferably use a separate low VA transformer that is direct on line when the OFF/ON switch goes to ON.
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Old 19th November 2014, 05:33 PM   #8
hongrn is offline hongrn  United States
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Quote:
Originally Posted by SSDIY View Post
Hi hong

Thank you for sending the soft start circuit.

The soft start you sent is very interesting to build but I have the following questions:

1. Is VCC the operation voltage of the soft start? If so what is the voltage?
2. What is the mute control? Where should it be connected to?

Please clarify as I would like to build this simple soft start circuit for my power amp.
Here's the link for the soft start from the TI web site that describes the mute function as well. The module has always performed reliably for me, so up to you to consider Andrew's concerns with this circuit.

www.ti.com/lit/an/snaa057b/snaa057b.pdf

Hong
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Old 19th November 2014, 06:57 PM   #9
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Hi,
Why you do not use an Arduino micro and program it to read the Vcc voltage and then close the relay. It is a simple program and you do not have to depend on the Vcc voltage for the timing or change components to adjust the timing. You change the timing values in the program. You can buy a board chip in Ebay.
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