Whats your application ?
You want to use a variac and thermistor ? Are you sure you don't mean a Varistor ? These are often used for surge limiting.
They are also referred to as MOV or Metal Oxide Varistor.
They go across the AC line and are rated for the working voltage (120-240 vac) and have a clamp voltage above this working voltage.
http://www.littelfuse.com/varistor.html
What are you trying to do ? Maybe I can point you in the right direction..
You want to use a variac and thermistor ? Are you sure you don't mean a Varistor ? These are often used for surge limiting.
They are also referred to as MOV or Metal Oxide Varistor.
They go across the AC line and are rated for the working voltage (120-240 vac) and have a clamp voltage above this working voltage.
http://www.littelfuse.com/varistor.html
What are you trying to do ? Maybe I can point you in the right direction..
Woops
Sorry, I just realized what your doing.. My bad.. See this post
http://www.diyaudio.com/forums/showthread.php?postid=1148590#post1148590
Sorry, I just realized what your doing.. My bad.. See this post
http://www.diyaudio.com/forums/showthread.php?postid=1148590#post1148590
Eva said:The own variac produces the surge when it is plugged, and it may be strong enough to trip the circuit breaker
I never noticed that on any of the variacs I used.
When the secondary is set for zero volts,
How much surge current have you noticed?
It must be a huge surge to trip a breaker.
The circuit is discussed in this thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=89576
Unfortunately, my old internet hosting is no longer working, so the pictures in that thread have just disappeared.
Schematic (replace 1N4148 by 1N4004):
http://www.diyaudio.com/forums/showthread.php?s=&threadid=89576
Unfortunately, my old internet hosting is no longer working, so the pictures in that thread have just disappeared.
Schematic (replace 1N4148 by 1N4004):
Attachments
You can check the datasheet of the NTC yourself:
http://www.thermometrics.com/assets/images/cl.pdf
It's rated at 5A so you will be limited to 600W or 1100W load depending on whether your mains voltage is 120V or 230V.
The datasheet states the inrush energy whitstanding capability of the NTC in terms of maximum capacitor bank size for off-line SMPS applications. 5000uF charged with rectified 120V AC will hold 72 joules, so that's the amount of inrush energy that this thermistor can whitstand. I can't determine whether this is enough or not for your variac, as in a worst case situation a transformer will appear as a dead short during every other mains half cycle for a short period of time after power up. Half a 230V 50Hz cycle on 10 ohms already makes 53 joules, but the real figure will be much higher because the resistance will decrease drastically during the half cycle due to self heating.
I think that NTC thermistors are intended to charge capacitor banks only. For transformer startup, a ballast resistor bypassed by a relay or a triac is probably going to be a more reliable solution. However, a higher resitance lower current NTC (or several NTCs in series) bypassed by a relay or a triac should be fine too.
http://www.thermometrics.com/assets/images/cl.pdf
It's rated at 5A so you will be limited to 600W or 1100W load depending on whether your mains voltage is 120V or 230V.
The datasheet states the inrush energy whitstanding capability of the NTC in terms of maximum capacitor bank size for off-line SMPS applications. 5000uF charged with rectified 120V AC will hold 72 joules, so that's the amount of inrush energy that this thermistor can whitstand. I can't determine whether this is enough or not for your variac, as in a worst case situation a transformer will appear as a dead short during every other mains half cycle for a short period of time after power up. Half a 230V 50Hz cycle on 10 ohms already makes 53 joules, but the real figure will be much higher because the resistance will decrease drastically during the half cycle due to self heating.
I think that NTC thermistors are intended to charge capacitor banks only. For transformer startup, a ballast resistor bypassed by a relay or a triac is probably going to be a more reliable solution. However, a higher resitance lower current NTC (or several NTCs in series) bypassed by a relay or a triac should be fine too.
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