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Old 14th March 2012, 12:24 PM   #1
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Talking Thermistor rating

Hello,

I am interested in finding out what the appropriate calculations for adding a thermistor in series with the primary on a 500va torroid.

I have read that using a resistor it is a good idea to use around 60ohm to limit inrush to 5 amps or so on a 240v supply.

I had previously tried this and found that the voltage remained low enough so that the relay would not engage, 90v or so.

I had found an unknown thermistor on some old electronics which i felt looked the right size, and to my surprise the toroid now turns on without problem.

Upon connecting the rectifiers and caps, then turning on the thermistor fizzles and the fuse blows.

What i need to do is work out which thermistor will get this on. I use some nice 30a (from what i can remeber now) 250v rectifiers and a 10000 80v on each line.

I am confused as say a cl-70 is 16 ohm and i think about 5 amp current rating. but if you do the math it will allow 15a at turn on. Now this is where i get confused as many sources have stated that the inrush can be many many times the rated current at turn on.

Now 500va is about 2a so i guess the thermistor rating if for full running amps. Now the cl-70 will limit to 15a max if im right but it this the value i need. The softstart came originally with a 3.6 ohm 20w resistor. Why is this value so low as this would allow quite a large current to flow. ?

If anyone could clear this up for me as well 60 ohm doesnt work and 3.6 ohm seems too low, i would like to use a thermistor to replace the resistors which were shorted by a relay, keeping the thermistor relay combination.

Thanks
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Old 14th March 2012, 02:31 PM   #2
DF96 is offline DF96  England
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It is not the size of the thermistor which matters but its characteristics. Have you seen the datasheet?

If you use the wrong thermistor for your application then it will either make little or no difference, or it may overheat.
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Old 14th March 2012, 02:58 PM   #3
AndrewT is offline AndrewT  Scotland
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The trigger for pulling in the bypass relay must be a timer, not a voltage.

I think the current rating is the maximum current that the device will have to pass.

Further, if the cold resistance of the device is too low to limit the start up current pulse then you need to find a different device or add devices in series so that the total of the group can absorb/dissipate the start up energy.

eg,
two CL60 will start up a 115 dual primary transformer if one thermistor is placed in each 115Vac feed.

When that same transformer is fed from 230Vac then both thermistors are still used. One in each 115Vac feed. It's just that the feeds (and the thermistors) are now in series. Many builders miss the significance of this requirement.
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Old 14th March 2012, 03:12 PM   #4
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i had a blowing fuse problem with my softstart too. the softstart had 2 33ohm 5W resistors in series. and bypassed by relè after 10ms or so.
what happend then was, the 16A relè welded when it kicked in.
so i got rid of the resistors and placed 2 CL-60 thermistors in series insted.
now the relè works fine. and is powering up 1000VA and 408.000uF cap bank
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Old 14th March 2012, 03:14 PM   #5
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Old 14th March 2012, 03:18 PM   #6
AndrewT is offline AndrewT  Scotland
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10ms is too short a period to allow the start up current to get the transformer upto operating flux.
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Old 14th March 2012, 03:20 PM   #7
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maybe 100ms. i dont know. but it is short.
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Old 14th March 2012, 03:28 PM   #8
DF96 is offline DF96  England
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66 ohms is probably far too much resistance for that load. Why don't people design soft starts, instead of throwing random components together?
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Old 14th March 2012, 03:32 PM   #9
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yes it was. it can handle 1000VA and 60.000uf bank. but not 408.000uF.
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Old 14th March 2012, 03:33 PM   #10
AndrewT is offline AndrewT  Scotland
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Some Members like to guess.
These same Members are usually not able to design.

These same Members should be advised to copy exactly a known, working, proved project.

They should never proceed beyond that very limited endeavour until they have learned the appropriate electrical skills.

To do otherwise is irresponsible in that these Members are putting users at risk if they incorporate a mistake due to lack of training and/or knowledge. Ignorance is not a defense for killing someone.
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Last edited by AndrewT; 14th March 2012 at 03:40 PM.
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