I am using a ULN 2003 (multi darlington array relay driver IC) to drive several relays for an audio bypass application. I'm getting some popping when the relays switch, and was wondering if ramping up the control voltage would help to reduce or eliminate this problem.
To achieve this, could I simply put a cap (say 0.1uF) across the across the input and output of each Darlington pair? I have a data sheet of the ULN if you need to see one.
"for non-mechanical switches the switch elements may couple a little of their control signal into the signal path. This is what happens with JFET, CMOS and relay bypass switches. If any of these are driven with a sudden-change control voltage, the unavoidable capacitances inside them can couple the fast wavefront into the signal lines, and this makes a pop. Some CMOS switches couple only a nanosecond or two of sound in, but several volts of it. The amplifier could not normally respond to this, but it overloads other circuits in the effect, and that overload is then heard as a pop."
It's a little confusing there when he starts talking about 'non-mechanical switches' and later mentions relay bypass switches (I'm taking that to mean relays, which are mechanical. Is there some type of silicon relay bypass switch that isn't JFET or CMOS?). He writes the solution later in the page as ramping up the control voltage:
"Ramp the control voltage up/down slowly (over a mS or two) to keep it from being coupled through the small capacitances."
So, (whether it will get rid of the pop problem or not) will connecting a cap across the input and output pins ramp the voltage this way?
The relay is electro-mechanical. The coil is the "electro" part - the switching part is mechanical. "Ramping up" the control voltage will do nothing for you - except maybe burn out the chip. Drive the relays properly - which includes connecting pin 9 to +. The Geofex site may have a fix for the popping noise that switching via relays causes - but it does not involve increasing the control voltage.