where is soft start relay inserted in PSU circuit

[AndrewT]

Originally Posted by MJL21193

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.

[necplusultra]

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

[MJL21193]

Use a thermister - it's safer.

http://search.digikey.com/scripts/Dk...me=570-1055-ND

[AndrewT]

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.

[indianajo]

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.

[Don Nebel]

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.

[AndrewT]

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.

[Don Nebel]

Wouldn't the affect be the same? If not explain.

[AndrewT]

No, it's not the same.
I have already explained many times.

[prezden]

Where is the best place to put a thermistor to reduce inrush