Soft start using triac - Page 1
Forum section: electronics / projects

author: anjarosz
author_rank: Newbie
timestamp: December 02, 2022, 12:00:16 pm
content: Hi everyone,I want to develop a soft start system for the receiver. In real system, it is a power supply in the laboratory with a variable resistance value - hence programming the resistance R3 as a variable value determined by the voltage source.The system to perform a soft start for the receiver, which causes a large current peak when switching on the voltage, is based on a triac, i.e. when the triac sees that the current flowing to R3 is 12A, it cuts off the sine wave at this point, which makes that the intensity will never reach a greater value.Please help me develop an algorithm for setting the changeable angle. I thought about defining angle alpha using the arcus function, e.g. atan(I(R3))=12, but that was beyond my skills.Best regards,Anna

author: inse
author_rank: Super Contributor
timestamp: December 02, 2022, 01:27:15 pm
content: Sorry for asking, but do you know the working principle of a Triac or Thyristor?Those components can only be turned on, they will conduct until the load current crosses zero.Depending on your type of load (inductive or capacitive) leading or trailing edge control could be used to limit inrush current.Trailing edge control is more complicated and cannot be achieved with a Triac.But up to now a robust and universal approach to this issue is to put a resistor (may be PTC) in series with the load and bypass it after a predetermined time with a relay (or Triac, if you insist).

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 02, 2022, 01:53:14 pm
content: Simple series NTC will work great, there should already be one in the PSU if it's at least half decent. Of which, have you considered replacing the PSU with a better quality one?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 02, 2022, 02:18:31 pm
content: Hi,Here is a model that illustrates phase-controlled soft-start:The model works by generating the line voltage as a function of time. Time is a special parameter in SPICE meaning the current simulation time.A cosine ramp is also generated using a modulo function. The cosine ramp needs to represent 0-180 degrees or Pi radians.The signal alpha represents the firing angle.The switch is turned on if the alpha is greater than the cosine ramp.I have attached the LTspice model.Hint: find and read the GE SCR manual which was first published over 50 years ago. This is the best resource for this type of circuit.In modern times this could be implement with a phase-control solid state relay and small micro.Regards,Jay_Diddy_B[ Specified attachment is not available ]

author: inse
author_rank: Super Contributor
timestamp: December 02, 2022, 02:35:34 pm
content: NTC works great if it has sufficient time to cool off.The dimmer circuit works fine for resistive or inductive loads (transformer) but not so well for capacitive loads (SMPS).In my company they are developing a kind of huge power supply (22kW) and guess how the inrush current is handled there - right, PTC resistor and relay.You don’t need no friggin uC for such a thing, kiss.

author: Marco
author_rank: Super Contributor
timestamp: December 03, 2022, 05:53:05 am
content: Quote from: anjarosz on December 02, 2022, 12:00:16 pmThis is substantially the same as a trailing edge dimmer, trailing edge dimmers aren't made with triacs they are made with transistors. A google shows Elliot Sound Systemshas an example circuit, it's a little more complex than a triac.To avoid that complexity use PTC/NTC + relay bypass. Or try to predict the inrush current instead like Jay_Diddy_B, instead of trying to switch based on measurement.

author: MikeK
author_rank: Super Contributor
timestamp: December 03, 2022, 12:33:22 pm
content: FYI: I received an unsolicited private message from the OP requesting help and suggesting I send an email so she could send me some file. I concluded she is a bot or other nefarious thing and notified the admins.

author: inse
author_rank: Super Contributor
timestamp: December 03, 2022, 12:59:29 pm
content: Then let’s all wish her good luck turning off the triac.