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A safe and elegant way for soft-start
A safe and elegant way for soft-start
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Old 27th November 2007, 07:51 PM   #1
Bakmeel is offline Bakmeel  Netherlands
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Default A safe and elegant way for soft-start

For my own amp build (Aleph2+), I needed some soft start circuitry. First of all, the need was there because of the 1000VA toroid transformer backed with 360mF and 3uH LC filtering. Secondly because I had this nifty looking switch with blue led illumination, which could handle only few volts and milliamps.

Of course, standard procedure is to put some CL60 NTC or something similar in series, but these components can get really hot and they will always leave some residual resistance in the line.

The attached schematic uses several RC timers to switch two relays in sequence: 1) Relay 1 closes, and connects the transformer to the mains via two 25 ohm NTC's in series. 2) after about two seconds, when the NTC's will be heated up relay 2 closes, shorting the NTC's and connecting the transformer to the mains directly. 3) because the relay resistance is probably lower than the NTC resistance, current will shift from relay contacts 1 to 2, and the NTC's will cool down again. After another 3 seconds, relay 1 opens again, leaving only relay 2 connecting transformer to mains.

A double led (red+green in a single housing) will indicate the circuit status: red=off, yellow=sequence running, green=on. Yellow is made by turning the green and red led on together

Further on the operation:
The push button is first debounced with R22, C9 and schmitt trigger U2E. I used CD4000 series because they were ready to use for me, but any standard logic will do for this purpose. The flip-flop will toggle on-and off on every push. Using the Q output in the AND gates later on, it will be possible to switch off the power supply in every moment of the sequence.
R2-C4 and R3-C5 are the two main RC circuits. I used solid tantalum for the capacitors for their size, and exact timing is not an issue here. Schmitt Triggers will provide fast switching when the threshold voltages have been reached.
To ensure proper startup after every switch-off, the capacitors are bypassed with transistors to discharge them quicly at switch-off.
Any generic BJT will do for the cause... just make sure they are at least robust enough to handle some peak discharge current from the RC caps.
R2-C4 also drive a second transistor, that in it's turn drives the led in the button. because it is located with its collector on VDD, you will see LED3 slowly light up as C4 charges. The potentiometer is there to have some adjustment capability for this led. Blue leds can be very bright, but with 2k in series it can be properly dimmed.
Finally, there is the power-on reset and under-voltage lockout. Because the initial status of a flip-flop is undetermined at start-up, there is a need to reset it when the power is applied. This wil ensure that your power supply will not spring to life unexpectedly when the power is applied. You need to push the button first. Undervoltage is there to ensure enough voltage is present to power the relays properly. The UVLO will reset the flipflop when the supply falls below 10.6V, and will release the reset only when voltage is above 11.2V again.

The circuit is powered with 12 to 15V, which should be independently supplied from the main -big- power circuits. The mains are not shown in the circuit diagram.

There is of course plenty of room to vary on this circuit. There are a few logic gates left to play with. I have built the circuit for myself twice (I'm building 2 monoblocks) and tested them thoroughly.

Have fun building and using it! Questions and remarks are welcome.

Attached Files
File Type: pdf softstart.pdf (24.7 KB, 710 views)
More Power Igor! More Power!
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Old 30th November 2007, 08:14 PM   #2
GrahamIT is offline GrahamIT  United Kingdom
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Nice circuit with some really sweet features, thanks for the effort.

I'm not an audio bod, but am in the process of designing a CNC router for aircraft propeller production and have an 800VA toroid in my driver PSU that needs turning on slowly.

I particularly like the gradually increasing brightness of LED3, though I think I'll add a bar graph driver to it, something like a LM3916N (I like flashing LEDs).

I'm presuming it's OK to save a few 's and use IRF610's as relay drivers and any TTL Schmitt-trigger chip and other TTL logic as long as I add a 5V regulator for them?

I've got a lot of 74 series chips laying round after being an analogue computer and printer engineer for the last 20 years

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Old 6th December 2007, 10:33 AM   #3
Bakmeel is offline Bakmeel  Netherlands
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Yes, you can run the lot on TTL with 5V, but I would be careful for the mosfet driver in this case. If your 5V line drops just a bit below 4V, you run the risk of getting in the linear region for the mosfets.

Either make the UVLO more critical, or use a darlington NPN instead of the relay mosfet and remove the driver.

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Old 15th September 2010, 08:45 AM   #4
bowdown is offline bowdown  Australia
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Looks like a good circuit..Any updates on it? Did you have any dramas with it or has anyone else built it?

Keep up the DIY spirit!!
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Old 15th September 2010, 10:53 AM   #5
AndrewT is offline AndrewT  Scotland
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a complicated circuit to send control signals to a pair of relays.
The essential current limiting section of the soft start has been omitted.
regards Andrew T.
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Old 15th September 2010, 07:47 PM   #6
Sch3mat1c is offline Sch3mat1c  United States
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The obvious conclusion: nature is punishing you for using an excessive, unnecessary and harmful amount of capacitance.

The solution is, remove about 90% of that capacitance. Then use a small NTC for what little surge remains. Yes, it will work. Yes, it WILL sound good.

Unless of course your amplifier has low PSRR. But that's another sign from the gods that something is amiss.

My typical precharge circuit consists of an LM311 and two transistors, and runs on anything from 3 to 30V (with proper resistor selection). What's up with the three chips?

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