I'm looking to use the dimmer circuit at Elliot Sound Products as the basis for designing a digitally controlled dimmer.
This makes use of an op amp running as a comparator to drive an SI875x optically isolated mosfet driver.
I'm curious as to why the author chose an op amp rather than a comparator. I've read the excellent TI paper "Op Amps and Comparators - Don't Confuse Them". This explains how the typical output stages differ: op amps run in the linear region, where as comparators run saturated and warns against using op amps in the way described in the circuit. "Texas Instruments applications have received a number of inquiries about using op amps as comparators in an open loop. The best advice [we] can give is to not do this. The very best a designer can expect is very poor performance; the worst is a circuit that does not work or even burns out" Ouch.
The snippet of the circuit is here
My guess is that it allows the simpler isolater part which runs as a simple diode emulator and does not require a separate decoupled 5v supply unlike the SI8751.
Here's my first cut at modifying the circuit to use a LM339 comparator and the Si8751.
The problem I can see is that the pull up resistor required by the open collector output from the comparator will turn the isolator on, at times where this may not be required. A transistor switch to invert the output logic could easily fix this.
Anyway looking forward to some feedback regarding the pros and cons of both options.
This makes use of an op amp running as a comparator to drive an SI875x optically isolated mosfet driver.
I'm curious as to why the author chose an op amp rather than a comparator. I've read the excellent TI paper "Op Amps and Comparators - Don't Confuse Them". This explains how the typical output stages differ: op amps run in the linear region, where as comparators run saturated and warns against using op amps in the way described in the circuit. "Texas Instruments applications have received a number of inquiries about using op amps as comparators in an open loop. The best advice [we] can give is to not do this. The very best a designer can expect is very poor performance; the worst is a circuit that does not work or even burns out" Ouch.
The snippet of the circuit is here
My guess is that it allows the simpler isolater part which runs as a simple diode emulator and does not require a separate decoupled 5v supply unlike the SI8751.
Here's my first cut at modifying the circuit to use a LM339 comparator and the Si8751.
The problem I can see is that the pull up resistor required by the open collector output from the comparator will turn the isolator on, at times where this may not be required. A transistor switch to invert the output logic could easily fix this.
Anyway looking forward to some feedback regarding the pros and cons of both options.