Soft-start circuit with no relays and no aux. transformers - diyAudio
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Old 3rd November 2006, 01:02 PM   #1
Eva is offline Eva  Spain
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Default Soft-start circuit with no relays and no aux. transformers

My current high-power off-line SMPS work requires big toroid transformers and variacs to be employed in order to achieve proper mains isolation and voltage adjustment. That is currently involving three 1KVA toroids, another 750VA one, a smaller 470VA one, and a 5KA variac is hopefully coming next week. Furthermore, the AC output of the transformers has to be usually rectified and fed to high value capacitors in order to fed the SMPS test circuits with reasonably steady DC.

Well, after blowing two 35A diode bridges and another 50A one in a week and tripping circuit breakers several dozen times, I got really fed up this morning and figured out a simple soft-start circuit.

Click the image to open in full size.

The triac shunts the 10 ohm ballast resistor as soon as the ~13V undervoltage lockout (leftmost comparator) allows the trigger oscillator to run (rightmost comparator). That takes approx 70ms to happen (it depends on mains voltage), which is just enough time to tame the initial inrush current transient.

Since 50mA of gate current are required in order to guarantee proper triggering of my BTA16-600B triac, a 10Khz oscillator is employed, which feeds the gate with 5us 50mA pulses every 100us. Trigger quadrants are I and III. As a result circuit current consumption is reduced to approx. 3.75mA, being 2.5mA (average) employed to trigger the triac and 1.25mA used by the rest of the stuff. The led came for free.

Due to the very low supply current requirements, the circuit is easily powered from the mains line through a ballast capacitor as shown in the schematic. After the first tests everything seems to work just fine.

Enjoy...

Obvious disclaimer: Working with mains live circuits is dangerous and can result in fire, injuries, death, etc... So if you decide to use the information contained in this post you must do it at your very own risk.
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Old 4th November 2006, 03:25 AM   #2
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Thanks Eva. Seriously, your contributions here are truly amazing.
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Old 4th November 2006, 03:47 AM   #3
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One thing is missing for safety. In case of catastrophic failure of the control circuit, always a risk when running directly from main voltage, the power resistor can become dangerously hot and burn. A risk of fire is even present. Elliott, on is web site (http://sound.westhost.com/project39.htm) has an excellent discussion on the subject.

I would recommend as a safety to install a thermal fuse in serie with the power resistor. This fuse is mounted side by side with the power resistor and coupled using a metal bracket or clip. This way if the circuit fails, the power resistor will heat and trig the fuse that will kill the power and protect the equipment.

Just my two cents
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Old 4th November 2006, 05:12 AM   #4
Eva is offline Eva  Spain
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Oh, but every traditional relay-based circuit has the very same pitfall.

Actually I have an unfinished version of that circuit that does not use a ballast resistor and is based in a trigger phase sweep, but it's controlled by a PIC12C508 and the code is not finished yet. I was in a hurry and needed something that I could get working in a couple hours.
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Old 4th November 2006, 03:02 PM   #5
luka is offline luka  Slovenia
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Looks great, maybe some sort of fuse would be a good thing to have before whole circuit to stop its functions just in case something does go horribly wrong.

Nice work...
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Old 4th November 2006, 03:21 PM   #6
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I'd send it to AudioXpress, EDN or Elektor --
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Old 4th November 2006, 03:45 PM   #7
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Nice circuit. But I might have found something to whine about.

If the circuit is switched on when the mains is at its peak, the 1N4148 would have to pass 3 amperes for a brief moment. Isn't that a bit close to its peak rating? (1 amp for 1ms, 4 amps for 1us). 1N4001s aren't that much more expensive are they? The 100 ohm resistor should be rated to take the full voltage too I guess.


Must seem like I have no life, pointing out something like this
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Old 4th November 2006, 03:54 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
I'd send it to AudioXpress, EDN or Elektor --
and I'll claim the author's fee three times over.

nice one!

any info on selecting triac parameters? (just in case we have to buy something different and have a lower load)
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Old 4th November 2006, 03:54 PM   #9
Eva is offline Eva  Spain
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Quote:
Originally posted by luka
Looks great, maybe some sort of fuse would be a good thing to have before whole circuit to stop its functions just in case something does go horribly wrong.

Nice work...
I thought that it was too obvious to mention... Yes, it does require a fuse, and indeed there are a couple of them, and more...

Click the image to open in full size.

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Old 6th November 2006, 12:36 AM   #10
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HI,

The cct design is great, but no feed back element like opti- coupler, the AC input with no protection like fuse, surge diode and filter coil that prevents EMI interference.




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