A Collection of Soft Start Circuits

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#7
Indeed - with a soft start the intial inrush current is limited anyway, and once the transformer is running there is no sudden change when the soft-start resistor is bypassed, just a modest increase in voltage seen by the primary.

Without a soft-start circuit then switching timing will make a difference (and zero crossing is the worst point to switch for an inductive load). With some transformers hard switching is a lottery anyway as the sign and magnitude of the remnant magnetization of the core is determined by the point it was last switched off...
 
#15
:cop:

Thread split from:
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Thread 'Yet Another Soft Start Circuit'

After looking at a bunch of soft start circuits I decided to design one according to my beliefs as to what is the best way to do it. It may be completely different from your beliefs but there it is.

I wanted a separate power supply for the soft start circuit so it could be used to turn the amp on and off and the powering up of the amps large transformer would not interact with the circuit. The transformer has a dual primary so it can be used for 115 and 230 VAC. There is a surge suppressor, R1, across the transformer input as well as a small high voltage capacitor to filter out RFI on the...
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@Jenyok if you want a different title then just report this post and state what you want it replaced with 🙂
 
#16
Here's the inrush current limiter circuit ("soft start") from Texas Instruments app note snaa057c . It has three 5 watt resistors in parallel, limiting current to the transformer primary. When the rectified and filtered DC supply named "Vcc" reaches a certain voltage threshold, the relay coil pulls-in and shorts out these resistors; they are "bypassed" and current limiting ends. Notice how few parts they use for inrush limiting.

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#17
And here is the inrush current limiter sold by Antek Transformers, the same folks who make nice toroidal power transformers. It is their part number SS-12 and its schematic appears on Antek's website page for the SS-12. It has two power resistors in series, limiting current to the transformer primary. A little timer circuit, composed of C1, R3, and C2, provides a delay. After the delay interval, the relay pulls-in and shorts out these resistors; they are "bypassed" and current limiting ends. Notice how few components are included in this complete product.

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#19
I had above circuit built and connected to a 1KVA transformer years ago. It starts softly just fine. But sometimes if I made mistake, power was turned off and tuned on again within a few seconds, it will blow fuse. This is due to C2//C3 do not discharge quickly, the relay still conducts for a few seconds when powered off.
 
#20
Try this circuit out. It features asymmetric turn-on and turn-off times, the former being approx 16 seconds, and the latter being ~40 ms. If power is removed, even temporarily, the circuit will still generate a 16 second turn-on delay. Component values are not critical, with 5% resistors and 10% capacitors being OK. It does require access to the transformer secondary, and the relay coil voltage should be slightly less that the rail voltage. R2 is selected to drop the voltage to the desired relay coil voltage. The circuit shown utilizes an FWB across the transformer split secondary; however, the circuit will also work with a single-ended rail
 

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