Understanding the significance of a few passive components

Thread Starter

Veraborn64

Joined Apr 15, 2019
8
Hello all,

I'm currently doing some work on a piece of equipment which has been in my work's storage cupboard for about 8 years. On said piece of kit was this PCB that has seen better days (picture attached). I can't find any paperwork for it and it doesn't appear on any of my electrical drawings although I am told it controls a heater. Studying the components and its application leads me to believe its a triac/diac dimmer circuit with hysterisis elimination much like the one I have attached.

My question is what is the significance of the three passive components I've highlighted; they are two capacitors and a resistor and lead me to believe they are part of a feedback loop of some sort but how does that integrate into this circuit, what is the significance of it and is it very important to the circuit as a whole?
 

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MrChips

Joined Oct 2, 2009
21,642
It is called a snubber circuit.
When triacs switch on in the dimmer circuit they do that sharply during an active phase of the AC cycle. This throws out huge amounts of RF noise which can interfere with household electronics, radios, modems, etc.
The snubber circuit is meant to reduce the amount of RF noise going back into the AC mains supply. It does this very poorly.
A better design uses zero-crossing detection to control when to switch the triac.
 

Thread Starter

Veraborn64

Joined Apr 15, 2019
8
It is called a snubber circuit.
When triacs switch on in the dimmer circuit they do that sharply during an active phase of the AC cycle. This throws out huge amounts of RF noise which can interfere with household electronics, radios, modems, etc.
The snubber circuit is meant to reduce the amount of RF noise going back into the AC mains supply. It does this very poorly.
A better design uses zero-crossing detection to control when to switch the triac.
Ah brilliant, I was only vaguely aware of the concept of a snubber circuit, but that makes perfect sense, and the snubber circuit would sit in parallel with the Triac I presume? I will look into zero-crossing detection as it may be the case that I design a more efficient version of this board for manufacture. At least that way we can have the paperwork knocking around.

Thanks very much for your help! Greatly appreciated.
 

MrChips

Joined Oct 2, 2009
21,642
Don't get me wrong. Manufacturers are always looking for the lowest cost solution. A snubber circuit adds pennies to the cost of the board. With today's high volume manufacturing, advanced high-tech solutions are only a fraction of the total manufacturing cost.

Zero-crossing detection is used in solid-state relays. Here, the function of the rely is to simply turn the power on and off. With zero-crossing detection, the power is switched only when the current and voltage are zero. This minimizes the production of sharp power spikes which cause the RF noise interference.

A dimmer circuit is a bit more complex. Not only do you want to switch the power at zero crossing, you also want to select which AC cycles will be controlled, on or off. You can do this with digital technology by counting the AC cycles. You can also do this with a PWM signal (pulse-width modulation) plus the zero-crossing detection.
 
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