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Old 3rd February 2011, 12:42 PM   #21
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by MJL21193 View Post
Does it make a difference for normal operation if your soft start is on the hot or neutral? No, strictly speaking it does not.
Indeed you are correct.
The Hot/Live to Cold/Neutral flows around a circuit.
What flows in must flow out.
A resistance placed in that circuit will limit the peak current. The resistance can be located anywhere around the loop (circuit) and still have exactly the same limiting effectiveness.
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Old 3rd February 2011, 03:16 PM   #22
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Tnx for input all
perhaps these questions are mundane to all
but 2 me, better safe than sorry

i will use dp switch
connect ss and fuse to hot

1 question remains unanswered:
value of three5 w resistor for 110v main
and 150va trans
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Old 3rd February 2011, 03:40 PM   #23
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Use a thermister - it's safer.

http://search.digikey.com/scripts/Dk...me=570-1055-ND
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Old 3rd February 2011, 04:09 PM   #24
AndrewT is offline AndrewT  Scotland
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A properly selected Thermistor/s does a better job of soft starting a transformer. It also has the advantage that if the relay fails to close the Thermistor does not go up in flames.

The downside to using Thermistors is the added cost.
A small transformer will need more resistance in the primary circuit to reduce the peak current by a fixed factor (let's choose reduction by half) than a larger transformer.

That extra resistance has to be bought and has to suit the voltage and current of the equipment.
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Old 3rd February 2011, 04:45 PM   #25
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Default salvage NCT thermistors

Being on the internet, I have a large & shrinking collection of PC-AT power supplies that don't work anymore. Each has at least one soft start NTC thermistor. They are wrapped in black wrap to keep them hot, look like a disc capacitor, and the chinese ones are green. I unwrapped the first one, it is marked "2r5" and reads 2.5 ohms cold and about 1 ohm hot in front of a hair dryer. It came out of a 400 W power supply, on the main input to rectifier, and I'm putting it in series with a transformer of about 15 VA with a cold resistance of about 2.5 ohms on the primary. I can't read the inductance without too much setup and calculation. In the future, I will measure them with the ohmmeter and hair dryer, and leave the shrink wrap on, as you only want it to cool down when the unit is turned off, not when the CD or other input has to be changed.
These power supplies also have useful power click turnoff capacitors, UL rated at .47 uf 250 VAC, and UL rated lightning/motor surge arrestors that are blue discs, also useful across the mains inputs. Unless struck by lightning, the input parts of the power supply is usually not blown up.
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Old 8th November 2012, 01:26 AM   #26
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Way back, long time ago, Dynaco had the C-100 outboard capacitor bank for the Dynaco ST400 And ST416 amplifier. This capacitor bank had a soft start relay circuit and was attached after the power transformer. Do any members of this group have the schematics for this circuit ? It would be nice to be able to clone one of these cap banks.
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Old 8th November 2012, 08:44 AM   #27
AndrewT is offline AndrewT  Scotland
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If AFTER the transformer then it cannot be a soft start to limit start up current of the transformer.

I suspect it was/is a slow charge circuit to limit the capacitor charging current.
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Old 8th November 2012, 03:33 PM   #28
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Wouldn't the affect be the same? If not explain.
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Old 8th November 2012, 03:53 PM   #29
AndrewT is offline AndrewT  Scotland
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No, it's not the same.
I have already explained many times.
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Old 24th January 2013, 05:57 PM   #30
prezden is offline prezden  Canada
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Where is the best place to put a thermistor to reduce inrush
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