Transformer noise - Page 1
Forum section: electronics / beginners

author: IvoS
author_rank: Frequent Contributor
timestamp: July 14, 2015, 01:18:05 am
content: I am using BTA24-800 triac and MOC3041 optocoupler to turn toroidal transformer ON/OFF. When unloaded, there is really loud noise coming from the transformer. When loaded, the noise goes partially away. If I power the transformer directly from 120V AC there is no noise. So the triac is the problem. Is it a DC that is being generated by the triac a problem? Or what is going on?

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: July 14, 2015, 01:27:11 am
content: Probably the triac is driving the transformer with a square(er) wave. Mains will be a near-perfect sine wave. A square wave will create more noise due to the much shorter rise time.You can attempt to mitigate noise by damping the transformer, eg heatshrink, epoxy, hot snot, etc. (keeping operating temps in mind)

author: Mr.B
author_rank: Unknown
timestamp: July 14, 2015, 01:50:33 am
content: You don't mention if you are using the snubberless version - BTA24...WI thought triacs switching inductive loads needed snubbers.

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 14, 2015, 06:59:47 am
content: Use a relay to switch the transformer. Triacs will struggle with the varying phase angle of the load and the holding current and the possibility of flux walking due to possible asymmetry of excitation voltage. Toroidals are sensitive to this as they have tape wound cores that have no airgap at all resulting in a high remanent flux.

author: IanB
author_rank: Super Contributor
timestamp: July 14, 2015, 08:19:13 am
content: Quote from: IvoS on July 14, 2015, 01:18:05 amAre you trying to run the transformer from a phase angle dimming circuit? Noise is usual from such circuits at low power settings. Even incandescent bulbs make a humming noise when you do that.But "dimming" an inductive load like a transformer is probably not a good idea.

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: July 14, 2015, 09:24:08 am
content: Oh, you mean turning on the transformer as in like a switch to the mains? For some reason I had in my head that you were "switching" the transformer as in switchmode PSU type switchingIn that case (or many really), triacs aren't ideal for switching on inductive loads. As above, try a relay instead. You've already proven this will solve the problem.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 14, 2015, 09:49:54 am
content: Quote from: IvoS on July 14, 2015, 01:18:05 amI think DC is the problem. I think the zero-crossing opto is compounding the problem because (my best guess): When the transformer is unloaded it presents a largely inductive load for the triac. Say that triac is conducting for the positive half-cycle. When the supply voltage crosses into the negative half-cycle the triac is still conducting - because the current is maintained due to the inductance of the transformer. At some point in the negative cycle the current decays to zero and the triac turns off - if the voltage at this point is greater than the detection voltage of the opto, it (correctly) holds off triggering until the next zero-crossing at the beginning of the positive cycle. And repeat. So the transformer is effectively being feed with only positive half-cycles.

author: IvoS
author_rank: Frequent Contributor
timestamp: July 14, 2015, 08:42:39 pm
content: Yes, I forgot to mention that it is a snubberless triac. Although, I don't think it makes any difference. I think this triac is just more robust and can handle inductive load. So, is it the optocoupler being fooled by the inductive load that screwes up the correct "zero-cross" detection and therefore it doesn't turn on-off for positive and negative cycle properly....??No phase shifting circuitry is used for dimming or anything like that, straight rock solid ON and OFF only.There is a suggestion for snubber in MOC3041 datasheet on page 7http://www.mouser.com/ds/2/149/MOC3041M-195885.pdfwould that help?Thanks.

author: IvoS
author_rank: Frequent Contributor
timestamp: July 14, 2015, 08:51:23 pm
content: OK, so maybe non-zero cross opto isolator would solve the problem.