Simple amplifier soft start for inrush protection?

[m3vuv]

Don't know if this has been mentioned, but a simple method would be to place a light bulb in series with the mains, then bypass the bulb with a proper sized relay that is driven by the supply output voltage. The inrush current won't be any higher than the bulb by itself, and the large amount of energy to charge the supply caps will get spread over the bulb since the bulb resistance increases as it heats up.

Best,

what a great simple solution,hats off.

[Per Hansson]

CORRECTION: Forget my first proposed zener+resistor solution.  Go with the last (triac) circuit.

Thanks for your updated schematic, I will try to breadboard it this weekend and compare them.
I did try to simulate the comparator circuit I posted in reply #4 on falstad but I failed to get it working.
(I could get it working with an op-amp but that has the wrong characteristics).
I still think the comparator is the nicest solution because it is easy to set the hysteresis also.
(Since we are using the capacitors to hold the relay it is important that it does not pull out if the voltage sags)...

Don't know if this has been mentioned, but a simple method would be to place a light bulb in series with the mains, then bypass the bulb with a proper sized relay that is driven by the supply output voltage. The inrush current won't be any higher than the bulb by itself, and the large amount of energy to charge the supply caps will get spread over the bulb since the bulb resistance increases as it heats up.

I actually used this exact method when testing the amplifier:
I brought it up first on a 60w then 300w incandescent light bulb and it worked fine.
Then when I bypassed the lamp I instantly blew the 10A fuses on the transformers secondary!
The reason was as I stated in my OP that one of the capacitors was shorted above ca 50v.
Though as crammed as the unit is fitting a light bulb inside it would not really be feasible.
Unless a really low wattage one would work, I may experiment because it is a fun way to do it!
And it has the added bonus of the thing not burning up if the relay fails to activate!

I've used this circuit (from Elektor 7-8/97) for many years for a subwoofer amplifier with a 330 VA toroid transformer. For some reason the R3 resistor has burned at some point. This resistor limits inrush current at switch-on, whereafter C1 limits current (C1 should preferably be X2 rated or similar). I changed it to a 2W resistor now to be on the safe side. Otherwise the circuit has worked fine. The circuit is meant for 230V and the relay coil is 24V (1200 ohm).

Thanks, this circuit is similar to SST-01 that I posted in my OP, the reason I prefer to measure the voltage on the output capacitors instead is to get rid of the time-delay logic.
And instead rely on the actual charge buildup of the capacitors to act as a timer, it may be overkill, but it is my goal with this circuit

[Per Hansson]

Numbers time!
So I did first a control shot, it came out at 2.5ms, it is just the relay driven directly with 24VDC.
The scope is hooked up with channel A on the normally open contact and channel B the normally closed contact.
There is no actual load on the relay: I'm just sending a small signal voltage to have something to measure...

First is a test with the schematic in my OP (original post) in this thread, I'm calling it resistive divider + MOSFET.
The time to switch was recorded as 16ms but this really is not the worst it can do:
I can make it take up to 50ms if I increase the voltage really slowly!


Next up is pqass VBE multiplier from reply #39, I had high hopes for this one and it takes 3.6ms.
The reason for this can be understood by watching:
w2aew video #198: Basics of a Vbe Multiplier: what it is, how it works & where it is used


Finally is the comparator circuit in my post #4
It takes 1.6ms and this is really weird because this is quicker than my control test with direct 24VDC!
I think the reason is that I was not actually using the same power supply for the control, so we can probably ignore that.
But all else being said this is a very impressive switchover time! The reason is that there is no linear region: the MOSFET gets a sharp signal.

[Per Hansson]

It's been a few months but finally I had this thing finished and thought I'd post the final schematic.
There are some minor changes since the schematic in post #4:
The inrush resistors should always be in circuit like the circuit MarkL showed in post #11
I decreased the resistance of the bias resistors for the voltage dividers as Benta recommended in post #6
I also had to adjust the divider ratio slightly because the inrush resistors I used would not reach the target voltage.
So now I switch the relay at 60v instead of 70v like in that schematic.
I also decreased the dropper resistor for the relay to 2.2K, I then measure 25v across the relay coil.
I have also for clarity drawn the snubber diode for the coil, and included the fuses that are part of the original unit.
It does seem to work well: the unit has a "protect" board and it remains on till after my relay energizes.
So there is no pop in the speakers, and the inrush resistors only increase in temperature by about 5 to 10°C.
I attached oscilloscope shots, red shows the voltage on the big capacitors, and blue is the relay.
Thanks for all the advice given! This was a great learning experience in so many ways