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# Triac control system for soft start

#### anja

Nov 21, 2022
1
Triac control system for soft start

Hello,

I built a circuit in LTSpice in order to come up with a control circuit for the triac, which is designed to limit the current flowing to the receiver (which is the power supply), and without which there is an overvoltage that stops the voltage flow.
The plan is to design a triac control system that limits the current to 10 A but I don't know how to approach that.
The initial approach of applying a behavioral source of tension led to the analysis shown in the PDF document, which shows that this approach really has no effect.
What ideas do you have for the analysis circuit for LTSpice? I thought that the triac would control the zener diode, but I have too little experience to figure out how to work efficiently.
I am asking for help in inventing a system with a triac for soft start, so that a current of 10 A is constantly flowing to the receiver.

I am asking for help in inventing a system with a triac for soft start, so that a current of 10 A is constantly flowing to the receiver.
In the picture you can see a red rectangle and this is where you need to come up with a control system for a variable resistance load.

Best regards,
Anna

#### Attachments

• Triac control system for soft start.png
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• V1 source analysis with triac.pdf
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#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,891
TRIACs are semiconductor switches. As such, they have ZERO current-limiting ability and are not serious candidates for "soft-start" start-up circuits without sophisticated trigger circuits. To do this properly requires timing the trigger pulse to be a variable interval from the first zero-crossing of the power input waveform. Trigger timing must begin there (timing triggering synchronized to the power line frequency) and then it must be delayed without triggering until just prior to the next zero crossing, which will occur shortly before the input waveform reaches zero voltage. If the TRIAC is triggered then, and only then, (just before the power line voltage reaches zero) the conduction angle will initially be very small and a small current, limited by external impedances, will occur briefly until the power line voltage crosses zero (for a resistive load... reactance complicates things a lot).

The trigger delay from the zero crossing of each input cycle is then decreased to increase the conduction angle. Clearly this cannot be accurately accomplished digitally since it is impossible to predict when subsequent zero-crossings will occur, mainly because frequency variations of the (presumably) commercial power source are subject to arbitrary variations over which the end-user has no control. However, you could build a clock oscillator with, say, 100 times the power line frequency (600 or 500 Hz clock) and use that to drive a digital counter that would then determine how many clock pulses past the zero-crossing you should wait before generating a trigger pulse for the TRIAC. That should give sufficient accuracy to provide a soft-start from mains power. If you want to get "fancy" you can synchronize (phase-lock) the clock oscillator to the power line input. Just remember to always zero (or pre-load) the counter with the delay count after every zero-crossing of the power line input. A microprocessor may be helpful in implementing this approach.

Since this problem is posted in the ELECTRONICS HOMEWORK HELP forum, I will not describe exactly how I would solve it. One approach that you may find satisfactory is to enable a 555 timer at the first zero crossing of the power line and then use the delayed timer output to trigger the TRIAC. Do this repeatedly at each subsequent zero-crossing. Then execute a variable decreasing delay that you can control electronically to effect a soft start. This can be a purely analog design as compared with the digital approach that I outlined in the above paragraph.

The devil is in the details though. Good luck trying to figure out how to model such circuits with LTSpice. If this were my homework assignment, I would first gather together some parts and start "modeling" with a real circuit whose operation I could observe with a real oscilloscope. Simulations certainly have their place in electronics, but this is an electronics makers forum. Member here actually build stuff. Sure, they may learn something from simulations, but their goal is generally how to improve or modify existing circuits, not to discover new ways to build new circuits. That is a fun job in itself, but generally requires actually building something... whether it works or not is sort of anticlimactic.

Last edited:

#### Minder

Apr 24, 2015
3,484
For some Triac phase control info, look up Fairchild AN-3006 and AN-3003.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,891
Thanks for finding those two app notes, @Minder! Unfortunately, AN-3006 has a schematic that shows exactly how to implement line-synchronized phase control in analog fashion. All that is missing is the automatic, variable, time delay needed for a "soft-start". application. I didn't think we were allowed to show such specific information for a "homework" question. Perhaps @Harald Kapp could comment on this.

Is there anyone with a link to a schematic showing how to do this digitally without violating the posting guidelines for the ELECTRONICS HOMEWORK HELP forum?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,799
I didn't think we were allowed to show such specific information for a "homework" question.
Absolutely right, Hop.
Is there anyone with a link to a schematic showing how to do this digitally without violating the posting guidelines for the ELECTRONICS HOMEWORK HELP forum?
I guess that would be cheating, wouldn't it?
All that is missing is the automatic, variable, time delay needed for a "soft-start". application.
Looking at figure 3 in AN 3006, how about using pin 5 by applying a voltage ramp to this pin, thus changing the timing of the 555 based monostable multivibrator to achieve a soft-start?

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,891
I’m typing on my iPhone in a forum for the first time… what a PITA this is!

#### Martaine2005

May 12, 2015
4,968
I’m typing on my iPhone in a forum for the first time… what a PITA this is!
Completely agree. I don’t get much time to sit down in front of a computer/laptop these days.
Some web sites are a REAL pain. Especially trying to fill in forms, dragging the screen left, right, up and down!. I give up half the time.

Martin

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,891
Well, being "retired" (with a tiny income from Social Security and a small annuity from TIAA-CREF), I have plenty of time on my hands. So, I thought I would give it a go on my iPhone. BIG mistake! I have run into the same problem in trying to type username and password (and other text) for apps on our Sony Bravia OLED flat-screen "smart" TV.

This is an old problem that I tried "fix" with a battery-operated Bluetooth wireless keyboard when all we had were a couple of Samsung flat-screen "pretty dumb" TVs. That worked out okay, but I can never remember where the keyboard is or whether or not its battery is still charged.

So, from now on I will use either this laptop or my desktop PC for posting. My iPhone 8 is still good for reading online stuff (an iPad would be better!) and I don't need to upgrade to iPhone 14 like my wife did.

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