cupoftea
Advanced Member level 6
Hi,
We went into an office and they had been using trailing edge dimmer units with incandescants.
Now they want us to replace them but for use with (literally!!!) whatever lamps the customer may decide to connect up.
(eg LED, CFL, whatever)
They cannot say what lamps the customer may connect up, cannot say how much input capacitance those
lamps may have.! But they tell us that sometimes the customer may severely overload the lighting circuit.!
They also tell us that the customer may simply short circuit the lighting circuit.!
The power level wanted for each dimmer circuit is 3kW.
Now..they took the spec to a consultancy and asked for "cheap as possible" solution.
I have that solution in front of me now.
Its just using back to back 650V TO247 FETs in the mains line.
They switch these on and off with opto's. That's pretty much it!
There is no snubber, no turn-off diversion, nothing to protect against instances where the circuit
may (due to noise) accidentally get switched ON as a leading edge dimmer at mains peak.
In fact, there's pretty well no protection against anything!
What do you say about this?
There is a "cheap as chips" overcurrent circuit involving a small thyristor whose gate-to-cathode
is across a 0.025R sense resistor, and the thyristor should pull the gate down in case of overcurrent.
One thyristor protector for each of the back-to-back FETs ..and one works for positive
half cycle, one works for negative half cycle.
The whole thing absolutely reaks of "cheap and cheerful and may the failure returns build high".
What do you say about these lack of protection levels? What do you say about not even knowing what
the lamps (loads) are going to be.?
We went into an office and they had been using trailing edge dimmer units with incandescants.
Now they want us to replace them but for use with (literally!!!) whatever lamps the customer may decide to connect up.
(eg LED, CFL, whatever)
They cannot say what lamps the customer may connect up, cannot say how much input capacitance those
lamps may have.! But they tell us that sometimes the customer may severely overload the lighting circuit.!
They also tell us that the customer may simply short circuit the lighting circuit.!
The power level wanted for each dimmer circuit is 3kW.
Now..they took the spec to a consultancy and asked for "cheap as possible" solution.
I have that solution in front of me now.
Its just using back to back 650V TO247 FETs in the mains line.
They switch these on and off with opto's. That's pretty much it!
There is no snubber, no turn-off diversion, nothing to protect against instances where the circuit
may (due to noise) accidentally get switched ON as a leading edge dimmer at mains peak.
In fact, there's pretty well no protection against anything!
What do you say about this?
There is a "cheap as chips" overcurrent circuit involving a small thyristor whose gate-to-cathode
is across a 0.025R sense resistor, and the thyristor should pull the gate down in case of overcurrent.
One thyristor protector for each of the back-to-back FETs ..and one works for positive
half cycle, one works for negative half cycle.
The whole thing absolutely reaks of "cheap and cheerful and may the failure returns build high".
What do you say about these lack of protection levels? What do you say about not even knowing what
the lamps (loads) are going to be.?