Zero current / zero volatge switching using Triac for inductive loads - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 13, 2018, 09:57:58 am
content: Is there a way to minimize / eliminate EMI/ RFI emissions while using Triac while switching inductive loads.When using inductive loads what is best zero voltage switching or zero current switching and is it possible to achiev this with a Triac and a Random phase optotriac or even a Zero Crossing optotriac.If I switch triac at zero current the voltage might me close to max peak and same goes for zero voltage the current might be at peak.So what is best to do here?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 16, 2018, 06:55:57 pm
content: So, is this kind of behaviour not possible with a triac? Anybody?

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: August 16, 2018, 07:05:15 pm
content: Triac's have a leakage voltage. I think no matter how fast you switch the triac on and off you will always have some current.

author: iMo
author_rank: Super Contributor
timestamp: August 16, 2018, 07:29:16 pm
content: This app note says you have to switch an inductive load at mains peak voltagehttps://www.st.com/resource/en/application_note/cd00003867.pdfInteresting.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2018, 08:44:09 pm
content: Use a dI/dt snubber and filter.The most commonly seen snubber is an R+C snubber, which helps with dV/dt, but more to the point, damping. An R+C is connected in parallel with the switch or load. An R||L is complementary to this -- and connected in series instead.Fundamentally, a TRIAC is going from high resistance to low resistance within a microsecond or so. This is a very aggressive phenomenon. It's different from a signal generator delivering a high voltage step, because a signal generator has a constant source resistance; this doesn't. It also cannot be slowed down, or shrunk in size* -- your only option is filtering.*You could hide the TRIAC behind a transformer and get a different V/I ratio, but the same V*I product must still be switched. Power conservation and all that.So, you might add an LC lowpass filter to attenuate the switching edges. An LC filter has a characteristic impedance, but the TRIAC does not, and the load and mains probably don't have very consistent impedances either, so you must provide termination for the filter.The most likely choice, then, is a constant-resistance filter: a conventional LC filter, with RC and RLC elements added to its input and output, to dampen the filter's response even when the input or output impedances are very mismatched.There aren't many design tools around for these types of filters, but they are out there. It may help to know they are a special case of a more general type of filter: a diplexer. Specifically, one with the high-pass port terminated. Those may be more commonly seen, or easier to design. For example, an easy way to synthesize one, is to connect two Butterworth type filters together, one lowpass, one highpass, both of the one-port-open type (a filter needsat leastone port terminated; the most commonly seen filter tables are for both ports terminated, though).More likely -- since exact frequency response doesn't matter here -- you'll take an LCL filter, add R+C in parallel with, and L||R in series with, the input and output, and adjust values until the response (for one port shorted, and the same port open) is satisfactory.Tim