Triac circuit line polarity dependent?

According to the example circuit in the manufacturer's datasheet of the LS7232 (touch control dimmer), the following circuit is independent of the AC line polarity:



I made some changes to the input of the LS7232, resulting in this circuit:



I used all advised component (values) except for the triac. The datasheet mentions a Q4006L4 but I used a BT137-600E instead because I could not get a Q4006L4. Problem is that my dimmer is now dependent on how you plug in the lines plug. It only functions when the neutral wire is on the MT2 side of the triac.

Could it be that I use the wrong (type of) triac?

Your triac has different triggering current for different quadrants (see the datasheet: **broken link removed**). It has a max 5mA for quadrant 1 but more than 30mA for quadrant 4.

First, check to see if you didn't switch over the MT1 and MT2 pins of the triac. Second, check the zenner diode. It should be able to supply the required triac gate triggering current. If the zenner is low current rated, the regulated zener voltage could fall on high sinking currents.

Anyway, if the circuit works if you switch the live/neutral poles then you might try to switch MT1/MT2 pins to force the triac to work on different quadrants. Or you could try a more sensitive gate triac.

Re-checked: MT1 and MT2 are not switched over. To check if the trigger current in the 4th quadrant is an issue, I used a 470uF capacitor over the zener instead of the 47uF. But same results.
Strange thing is that no current is specified for the 4th quadrant for the Q4006L4 so I guess it is a 3 quadrant triac. Maybe the triac must be a 3 quadrant one?

Q4006L4 has a _typical_ current of 50mA in the 4th quadrant (according to its datasheet: https://www.mouser.com/ds/2/240/E2Triac-18233.pdf).

Anyway, with your actual circuit (and the one from datasheet) you could supply up to 100mA (R3 resistor) to triac gate so it shouldn't be an issue (if the zenner voltage it's steady).

It doesn't work at all if you switch live/neutral or it trigger the triac only on one half of sinewave?

The sensitivity of the BT-137 is higher, 2.5 - 11 mA typical over the 4 quadrants. So I guess it is not a problem with the sensitivity of the triac.

In wrong polarity the dimmer is not working at all, dead as a dodo.

Maybe the problem is in the touch sensing part of the LS7232 because I copied that from this schema where the polarity is important:

On "note 4", they say to use connection A (before the lamp) when neutral is not available. That should make it immune to neutral position.

I changed the circuit accordingly:



but unfortunately: no change. I still have the perfect live/neutral detector circuit :-(

You may try to change the R4-R5-R6 resistors, to adjust touch sensitivity.

According to datasheet, R5 and R6 should be around 2M7 (but you have used 4M7 and 3M9?) and R4 between 1M and 5M (you have used the smallest value).

Those three resistors are a voltage divider for the parasitic voltage (charge) of the finger touch. The pin 5 voltage is Vtouch * R4 / (R4 + R5 + R6).

For a greater pin 5 voltage, R4 has to be larger and R5 + R6 smaller. Thus you could try R4 = 4M7 and R5 + R6 = 2M7 + 2M7 (to follow the datasheet recomandations).

It is very simple, and it is right there on page 8 of the datasheet. When using a push button switch, then and only then the operation is independent of line polarity.
However, if you plan on using a touch plate, the line and neutral should be connected as shown in page 6.

When you think about it, it makes sense. Touching the touch plate causes a miniscule current (100 nA, from the datasheet) to flow from the Live thru your finger and thru your body all the way to "earth".
If you reverse the L-N polarity, the couple of volts that the Neutral is floating above earth does not have sufficient potential difference to drive that current.

Touching the touch plate causes a miniscule current (100 nA, from the datasheet) to flow from the Live thru your finger and thru your body all the way to "earth".

Click to expand...

I've seen many battery operated touch dimmers (or simple touch switches) which surely don't use that "earth" leaking current.
They use a noise amplifier (the base of a bipolar transistor or a FET gate) to generate a pulse.

If you reverse the L-N polarity, the couple of volts that the Neutral is floating above earth does not have sufficient potential difference to drive that current.

Click to expand...

It's not the potential difference that's driving the triac gate. You just need micro/milivolts to generate a pulse that further activate the dimmer counter and triac driver modules.

red_alert said:

It's not the potential difference that's driving the triac gate. You just need micro/milivolts to generate a pulse that further activate the dimmer counter and triac driver modules.

Click to expand...

Read my post. The miniscule (100nA) current I'm talking about is the one that flows to the IC's input, which in turn drives the milliamps of current required to trigger the triac.

I'm also aware of the noise-amplifier based dimmers. I built a few using those old Telefunken ICs.

Even if my explanation is not fully accurate, the answer to your problem is still there, in black and white on the datasheet, on the pages I mentioned.

First, it's not my problem (I'm not the OP).

Second, it surely could be done (despite the datasheet paragraph you've pointed to). At anytime, one could use that electronic extension mentioned in the datasheet (the bipolar transistor we're talking about) to make use of a finger "noise".

If the triac is triggering OK with a simple switch (no matter the mains polarity), the touch plate could be easily adapted.

You may be right, it could be done if the data sheet had provided additional application information or at the very least given further details on the input pin's structure.

And indeed it is not your problem. I said your problem as a figure of speech.

As it stands though, and without actually having the circuit in the lab bench, all we can do is to speculate.

@schmitt trigger: I have 3 versions of the LS723x datasheet, 2 of them have no page 8 and on page 8 of the third one I could not directly find your quoted statement. Do you maybe have a link to your version of the datasheet?

@all:
Suppose it is a matter of not having a voltage difference to generate the tiny current; would something like this adaption do the trick then:

Have you tried to change those resistors, to increase the sensitivity?

You could also try that extension (the bipolar transistor).

Sure, here it is:

https://www.alldatasheet.com/datasheet-pdf/pdf/166901/LSI/LS7232.html

Page 8 clearly indicates: "operation is independent of line plug polarity".
And Page 6 doesn't have an explicit note that it must be polarized, although the inputs are clearly labeled (P)hase and (N)eutral.

As I mentioned in my previous post where I admitted to Red_Alert that his proposal could be a way to achieve it, I also complained that the datasheet is not very well written and open to interpretation.

For instance, on page 6 there is this little note 4: "Use connection A when neutral is not available. Use connection B when neutral is available".

What does this exactly mean? The way I understand it is that connection A is for the classic 2-wire dimmer in series with the light bulb. And that connection B is for a three wire connection: the phase, neutral and load.
That is my understanding. It may be wrong. It may also mean that if there is no "real Neutral" connection, you must only use connection A.
My gut feeling, based in your symptoms, is that the first meaning is the correct one.

But without having an IC in my possession to play with, and with the scant information from the datasheet, all I can do is to speculate.

EDIT: Looking at their website, I found an app note (for low voltage halogen lamps employing a transformer) which does have this note:

NOTE : Touch Sensitive Operation requires AC MAINS and Transformer Polarity
as shown. The connection to AC HOT may be required for transformers
with small values of interwinding capacitance

Which again, may point out the need for current to flow from the phase to earth...but I don't know.

SENS (Pin 5)
A low level activates this input (See Table 1) which controls the turn-on, turn-off and conduction angle Ø (See Figure 3) of the TRIG signal with respect to the SYNC input.


EXT (Pin 6)
The EXT input is functionally identical to the SENS input except that a high level activates this input. It is recommended that the EXT input be used instead of the SENS input when long extension wires are employed between the IC and the Sensing Circuitry (See Figure 5 and Figure 6) because the EXT input is less susceptible to “Noise”.

Click to expand...

Thus all you need is a pulse signal (positive or negative) that could be achieved by a finger touching the base of a darlington bipolar transistor, by example.

The finger touch detection isn't a breaktrough feature of this circuit; it main purpose is to generate a custom pulse to drive the triac (thus aquiring the dimming function) by using a single input (button/electric pulse).

One could adapt any other finger touch detection circuit to control the SENS/EXT pins.

Still experimenting: when connecting a switch between pin 5 and ground (pin 6/7) the circuit is working in both polarities L/N. So the circuit on page 8 is working as depicted by the manufacturer.

I am still intrigued by the application note on page 6: the choice between connection A or B (note 4) depending on 'when neutral is available'. I tried this but I overlooked that the values of R1/C2 should have been changed when using connection A. I could try that again with the right values. But my guess is that 'when neutral is available' means that when you for example replace a wall switch you only have a 'semi-neutral' wire because the ceiling lamp connects it to the real neutral.

Also a separate finger touch detection circuit should solve the problem but the problem is that teh circuit board is very small, it should fit in a table lamp base.

Thanks for your hints, I'll keep you informed...