Hi Everyone

I was wondering if someone could help me understand this circuit which is the power control board from an electric boiler.

The manufactures technical support team reported that their triac board exhibits often high levels of earth leakage but that this is normal.This happens to be an issues for this particular boiler which I am not too happy about ( 100ma). On looking at the board I cannot see where there would be a leakage to earth from. I suspect they may have been talking about leakage through the triac from live to neutral which I understand is normal for this type of semiconductor.

Anyway I would like to understand the circuit better and get some opinions on how this specific board works.

My current understanding as follows:




Three triac circuit each with a rc snubber circuit and connected to two opto-couplers each. One which is driving the gate and the other one picking up the start of the phase to aid the micro-controller in correctly timing the gate signal for throttling?

I cannot see any ground connections in the circuit as I would expect all power is going between live and neutral. As the control side of the board is also opto-isolated, The only ground connection I could think of would be through the body of the triac which is bolted to a heat-sink.

Is the above understanding of the circuit correct of have I misunderstood how traics work and if so could someone explain how the manufacturers claims can be correct ?

Any thoughts of the above would be much appreciated and thanks for your time.

Regards
Nick

 

Hi,
It is possible that the technicians did not mean current leakage from the PCB, but from the heating element to ground

 

Hi Pyrex. Thanks for your response. Yes they directly talked about their triac PCB rather than the heating element. The element has been dead tested and has over 500Mohms at 500v from the elements input to earth so it appears sound.

I am aware that some Triacs have insulated tabs and some have non insulated however I can't get a proper view of the triac to look it up online.I do have a picture which someone might see if they could confirm which type of triac it might be. I thought maybe in the assembly of the PCB into the heatsink something may have happened that could have caused this interface to be compromised?

Does anyone know if it is possible to test triacs in situ. I have tested them with the circuit off and the MT1 and MT2 to ground has high resistance reading however I am aware that triacs have quite a few semi conductors in them so dead testing might not encompass this all?


All opinions appreciated

Nick

 

From left to right MT1, MT2, Gate on middle case.
Test Gate to MT1 like a diode in both directions or Vbe.
Test MT1 to MT2 as open circuit that closes when Gate is conducting 15 mA or so in either direction.

 

It is unlikely to be the triacs themselves, rather than their effects when switching in a phase controlled mode.
That produces fast transients.
The heater winding capacitance to earth, then causes the high leakage current.

 

The element has been dead tested and has over 500Mohms at 500v from the elements input to earth so it appears sound.

Possibly the heating element has a leakage to ground when hot?

 

Resistance tests using a Megger or similar instrument, are done with a DC voltage. The heater winding capacitance does not affect the final reading measured in this way.
Current 'leakage ' can be caused when triacs switch on, particularly if phase controlling. That creates a fast rising voltage edge in the heater windings and a current flows to earth via the winding capacitance.
You could try to measure the capacitance to establish a value.
But it's impossible to determine what frequencies the fast edges are, caused by the triac switching.

 

So there is nothing wrong with it.
Just have to accept it has a high leakage current.

 

IF the issue is off-state leakage, I see messy soldered power connections. AND there is no need for a mains common on that PCB. Heating elements are often different at the operating temperature than when they are cold.
What is the complaint that brings up the discussion of "leakage." Is there a problem??