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Old 17th April 2015, 10:47 AM   #1
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Default inrush current limiter

hello,
for a few years i am using a CL60 thermistor in series with primary winding (230V) of my 600VA toroidal transformer. The thermistor is shorted using relay after a few seconds. After the transformer i have a bridge and 75000uF capacitors. Everything is working fine.
Now i will have 3 same toroidal transformers and capacitors using the same power-up circuit. Can i use the same configuration 230V - CL60 - 3 transformers and after each transformer a bridge and 75000uF or it will be better to use two CL60 in parallel ? Or use one CL30?
thank you
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Old 17th April 2015, 08:14 PM   #2
AndrewT is offline AndrewT  Scotland
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Use separate current limiters, one to each transformer.

Do not parallel Power Thermistors.
It is permissible to series connect Power Thermistors.

I am surprised you get effective current limiting using a single CL60 on a 230Vac transformer.

What fuse rating do you use?
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Old 18th April 2015, 08:43 AM   #3
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Hi,
I cannot use seperate limiters because i need to use the pcb i already have. This pcb have one cl60 and one relay whitch shorts the cl60 after about 5 sec. There is space to use one bigger thermistor or more than one but i cannot add another relay to this board.
Now i have 5a fuse and is working fine.
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Old 18th April 2015, 10:27 AM   #4
AJT is offline AJT  Philippines
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5 secs seems too long, if using 60hz mains, one cycle is 17msecs, so even if you use 100 periods,
that is just 1.7 secs, surely you need less than 100 periods to normalize current..

one board is good enough if you ask me, but instead of cl60 i would use a dale 100 ohm 50 watter resistor...
this is my soft starter, used one traffo but i see no reason it can not be used for 3 or more traffos...

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Old 18th April 2015, 12:05 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by ioannidis View Post
Hi,
I cannot use seperate limiters because i need to use the pcb i already have. This pcb have one cl60 and one relay whitch shorts the cl60 after about 5 sec. There is space to use one bigger thermistor or more than one but i cannot add another relay to this board.
Now i have 5a fuse and is working fine.
you can use one PCB to trigger more than one relay.
What current does your relay draw @ what voltage?
Three relays will draw three times as much current.
Just ensure the switching transistor can supply that much trigger current.
That means the switching transistor base current must be Irelay*3/~10

But, if you are using a direct to mains powered circuit you are in dangerous territory.
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Old 18th April 2015, 12:11 PM   #6
AJT is offline AJT  Philippines
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well, what is the continuous current expected? knowing this will let you make
the correct size relay contacts in amperes....

in my case is used a relay with 6A contacts, with 230 volts mains
this is around 1200 watts for an amp that will deliver 200 watts total
at full load....and with music playing, much less than 200 watts...
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Old 18th April 2015, 08:53 PM   #7
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hello,
the main relay which is used as a 230v main switch for the transformer have 2 switches 16A/250vac and i have connected those switches in parallel (32A). I suppose this relay will be ok for 3 transformers 600a each (total 1,8kva=8Amax).
The delay of 5 sec i have made to short the CL60 is not critical because the current is already steady and the thermistor have a very small resistance. If i had used resistor instead of the CL60 then yes, the shorting time will be critical and it should be as short as possible. Even without the short-relay the circuit will be working fine, doesn'it?
I don't like the resistor solution because what will happen if the short relay don't close?
So, as it seems the best solution is to add 2 more relays to short another 2 CL60 for the extra 2 transformers. The relay i already have is driven by a 2n3904 small transistor. Of course i will replace this transistor with a bigger one so it can drive 3 relays.
but why i cannot use a for example one bigger thermistor? or maybe one Cl60 for those 3 transformers? and if i would not use short relay at all will it be ok (i mean each transformer its own CL60 all the time)?
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Old 18th April 2015, 10:44 PM   #8
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that should work...

in situations where i use NTC's i do not bother with any relays...
i just use NTC's and that is alll...
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Old 18th April 2015, 11:02 PM   #9
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I designed my own current limiter myself using a PIC microcontroller and a triac.
Works great although I used to large a heat sink so its a little bulky.
The triac has the advantage it has no mechanical parts to wear out or arc.
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Old 18th April 2015, 11:54 PM   #10
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Quote:
he main relay which is used as a 230v main switch for the transformer have 2 switches 16A/250vac and i have connected those switches in parallel (32A).
you may not need soft starting at all with this large switches...
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