Soft start question

[a.wayne]

Originally Posted by peranders

Mr. Wayne, how low impedance do you have really?

The waveform of the current is short spikes which creates over tones the mains and they may interfere with other apparatus connected to the mains. Your rectifier bridge will have a harder life and also the transformer. The VA rating becomes half the value if you have a huge capacitor battery. A 500VA transformer can only take 250 VA if you have a PSU like this.

Hello ,

Thanks for the explanation , i was not aware of losing 50% of my transformer rating , why would this be so ? The bridge rectifier is rated @ 30 amps.

[Michael Chua]

Originally posted by a.wayne

You suggest increasing the resistor value to as much as 33 ohm from 4.7?
I had thought about upping it to a 30 watt , so i will now consider a higher imp by using a 33 ohm do you think the rest of the circuit will be sufficient ?

I do not have a clamp meter , so I'm not able to adjust as you suggest and was going to increase the resistor to 12ohm/30 watt .. do you still suggest 33 ohm ?

Yes, 33 ohms/30W should do. As long as the delay is about 1/2sec, you'll be fine.

I've just finished a Dual Mono a month ago. Power Transformers are external with their associated Inrush circuitry. Each channel is powered by a 800VA transformer with 88,000uF/100V of filter caps. Electrically, the 2 channels are isolated.

I used a small transformer for the Inrush. It also provides 12V regulated DC for the 12V Panel switch and powers a tiny extractor fan at the back panel. The 12Vdc front panel switch activates a High Current (30A) Relay which is wired to the incoming Mains fuse. This relay acts as the Main power switch. The front panel switch is a trigger. Safer (low voltage) and much Prettier (with illuminated LED).

You can see the metal clad resistor on the Inrush PCB. It is a 39 ohms/30W.

Mike

[peranders]

Originally Posted by a.wayne

Hello ,

Thanks for the explanation , i was not aware of losing 50% of my transformer rating , why would this be so ? The bridge rectifier is rated @ 30 amps.

The losses is I*I*R and if the peaks go really high then you'll get high peak power in losses but the thing is that the VA rating is only valid for a resistive load and when you add so much capacitance you will also generate lots of reactive power distributed in a wide spectrum.

[mightydub]

Originally Posted by a.wayne

Your simply sim. predicts there will be an improvement in transient response..

The point is that with rails of +/- X, you shouldn't expect the output to swing to +/- X. For real music, X-7 is probably the limit. For full power bench testing, X-15 or X-20.

More capacitance will get you to maybe X-5 for real music, that it isn't an effective way to get a few extra volts of output swing. It won't help at all for sine wave bench testing.

The diodes are only forward biased for a very small part of the AC cycle, and the current pulses to top up the caps are at 120Hz. You have to consider whether there is any upstream current limitation (such as the transformer) which would mean that the additional capacitance may not do any good at all for an extended high power load.

Here's another simulation result where the load applied is real music, full power into 4 ohms. Top trace (blue) is the music waveform, bottom trace (green) is the PS rail with 70000uF.

Basically, if you're playing real music at realistic levels, the PS capacitance you have now should be more than enough to handle any transients. You could argue that the additional capacitance would result in a lower ESR and therefore higher current delivery for transients, but it would likely be masked by the effective output impedance of the output transistors and the emitter resistors.

If you want to do the experiment, please let us know the results. But as I and others have said, you need to look at all the upstream components to make sure they can handle the increased inrush current.

[lgreen]

Rather than a resistor how about to CL-60 thermisters? These start at 10 ohms and dcrease as the current flowing through them heats them up. They can take a few amps. I use 2 in series with my Krell clone and it works well. And you wont blow them up if you use a variac because they can handle the current.

[MJL21193]

Originally Posted by lgreen

Rather than a resistor how about to CL-60 thermisters?

That's what I use.
Complete soft start:

361.JPG

[peranders]

Warning: R6 very close to the chassis!

[MJL21193]

Originally Posted by peranders

Warning: R6 very close to the chassis!

Not as close as it seems. Besides, it is just 12V there - power for the relay.

[peranders]

Aha, I thought it was at the primary side which is the usual approach.

[Upupa Epops]

Here is all, what you need - inrush resistors ( and PTC ), bypassed by 16A relay, controlling switch on/off logic, mains filters, remote control for cascade switching of next equipments connected in chain...unit is usable for 100-2000 VA transformers ( by 230 V AC )....