Interacting elements. Confused... How to isolate?

Thread Starter

mriksman

Joined Aug 31, 2010
38
I have the following circuit on a board;

1590063158996.png

If I leave DIM2 floating, then when I ground DIM1 - it only dims from 240VAC to 151VAC.

If I ground DIM2, then when I ground DIM1, it will dim down fully to off.

How is DIM2 pin affecting DIM1...?
 

AlbertHall

Joined Jun 4, 2014
10,031
Do you have the GND connections from the top circuit connected the GND connections of the bottom circuit?
I don't think they should be connected.
 

ci139

Joined Jul 11, 2016
1,559
? GND -s in between the SCR switches (at the MOSFET-s' sources)
. . . ? Put 1GΩ (10⁹Ω) from sorces to GND (if the simulator requires referencing them)
. . . see the FL5150 d/s you likely have something more wrong there
 

Thread Starter

mriksman

Joined Aug 31, 2010
38
Thanks for the replies. I have the following observations;

With DIM CONTROL2 floating, and DIM CONTROL 1 grounded, GATE1 voltage is 0V RMS and GATE2 voltage is 16.6V (which is fully ON; internally the IC has a 17V zener to use for the gate driver).
Which is to be expected. If DIM CONTROL1 = 0V, duty cycle is 0% so the RMS of the GATE1 voltage is 0V.
Additionally, DIM CONTROL 2 is actually C3 charged to 5V (internally Vdd of the IC generates 5V). 2.5V is 100%, and anything over 4V is actually a special case to let the IC know to turn off switching and go full ON. As such, the GATE2 has RMS voltage of ~17V.

Yet I am seeing LIGHT1 partially ON (154V)!? Yet GATE1 is OFF. Somehow it's being partially lit from the GATE2/LIGHT2 MOSFET
 

Thread Starter

mriksman

Joined Aug 31, 2010
38
.... which is why I queried the Vdd values, since the maximum allowed voltage on the dim control pin is 6V.
Internally it generates Vdd of 5V.
I have a 3.3V PSU onboard as well - I use that to pull DIM CONTROL high. DIM CONTROL 0-100% is 0-2.5V (it has an internal ADC with 2.5V reference)
 

Thread Starter

mriksman

Joined Aug 31, 2010
38
Yes, see post #2.
Ahh. That's what that meant...

The two circuit Gnds also need to be isolated from the mains supply ground, otherwise the MOSFETS will short Line to ground.
I don't have mains supply in my circuit. But if I did, that's good to know.


So - How would I then drive the DIM CONTROL? The GND from the controller's 3.3V couldn't be tied to either of the FL5150's GNDs right?

Appreciate the help guys.
 
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