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Soft start-up circuit for high power load need advice

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May 21, 2003
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It's an on-going high power amplifier project. There is a 3KW transformer and some large capscitors be used. The problem is: Start-up shock current, it broke the power switch when turn on. :cry:

I'd design a soft start-up circuit show as below. Is it a good way to slove the problem? Any advice? [/img]

A different way is to use a NTC as a inrush current limiter.
They have one drawback that the NTC require a cool-down time after power is removed.

Brands: Vishay, Ametherm...
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Thank you guys!

Had already done a inrush poor circuit with NTC & relay delay circuit. :eek:

TD Relay

For high power applications I've used a time delay relay switching in some power resistors in series with each side of the line. Say 10 Ohm 50Watt resistors in each leg. This will limit line current draw making it easier on the bridge rectifier also. The time delay should be greater than the (R say10ohm) and C(capacitance of bank) time constant to ensure the bank is at full voltage DC before swithing out the resistors. Of course you know how to hook it up that the resistors are always in circuit shorted out by the relay contacts ie: relay NC
Good luck :wink:

The way we did it on high power variable speed drives was to put a power resistor in series with the caps on the main voltge bus coming from the rectifiers. At switch on the resistors will limit the current and a contactor closes when a comparator sensed that the caps were charged about 90%



hi ...

i am using circuit similar to this **broken link removed**
we are using 5kV, abt 2A 3 phase transformer with abt 10uF capacitor
and our step start is simple circuit where we start transformer with abt
25Ohms/10W resistors in series and after abt 1 sec we shortcut these resistor so you have full voltage on transformer primary. Works fine and resitor also protect you from shortcut on HV part. They will blow in case of hv failure during startup. pm me for more infos.



using resistors in series is a cheap solution, commonly found in industry. start the motor through the resistor to limit the current, then by-pass the resistor when at full speed.

if however you have any sort of speed control, electronic, you can have your motor start up on a trapezoidal wave form. i.e. the output voltage to a DC motor could be ramped up slowly.


common industrial method

One method used is to have taps on the transformer. The taps are automatically selected based on curent level being below a threshold before the next higher tap is selected.

Thank you guys

After some test, I decide to use my original design which was show above. It's a single button on/off circuit, with inrush current poor. Simply diagram show below.

I need too,
one scheme was published in 305 Circuits (Elektor) page 115
the project "Mains power - on -delay" for power amplifier.
If someone can post it I would be very thankful.

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