Constantin
Well-known member
Hi all,
I have a conceptual question regarding a voltage-sampling circuit I've previously discussed. The circuit is designed to have a wide inout range of 115-230VAC.
Below is a slightly modified version, the voltage source is actually a transformer with a no-load voltage of 9.2V @ 230V input. I am considering improving the resolution of the ADC output by bypassing one of the resistors on the transformer leg whenever the system detects that it is attached to a 115VAC circuit.
I propose doing this by shorting resistor #2 with a MOSFet solid state relay like this clare unit. Besides a drop resistor for the LED inside the SSR, I presume it would also be a good idea to feature a pull-down resistor on the input side of the SSR to prevent accidental engagement?
The above (if it is a workable solution) would effectively add a bit to the resolution at 115VAC, right? My circuit simulation tool suggests that this is a workable solution but I also wonder if there would be any kind of damaging inductive kickback from the transformer secondary... that said, since the relay would be either permanently on or off, the damage potential from switching should be pretty minimal.
FWIW, the maximum current through the resistor proposed being shorted is on the order of 150uArms. The maximum Vrms voltage through the resistor is 18VAC. Under maximum 115VAC conditions (i.e. 115VAC x 1.25 safety factor) the current going through the relay peaks at 470uA.
I have a conceptual question regarding a voltage-sampling circuit I've previously discussed. The circuit is designed to have a wide inout range of 115-230VAC.
Below is a slightly modified version, the voltage source is actually a transformer with a no-load voltage of 9.2V @ 230V input. I am considering improving the resolution of the ADC output by bypassing one of the resistors on the transformer leg whenever the system detects that it is attached to a 115VAC circuit.
I propose doing this by shorting resistor #2 with a MOSFet solid state relay like this clare unit. Besides a drop resistor for the LED inside the SSR, I presume it would also be a good idea to feature a pull-down resistor on the input side of the SSR to prevent accidental engagement?
The above (if it is a workable solution) would effectively add a bit to the resolution at 115VAC, right? My circuit simulation tool suggests that this is a workable solution but I also wonder if there would be any kind of damaging inductive kickback from the transformer secondary... that said, since the relay would be either permanently on or off, the damage potential from switching should be pretty minimal.
FWIW, the maximum current through the resistor proposed being shorted is on the order of 150uArms. The maximum Vrms voltage through the resistor is 18VAC. Under maximum 115VAC conditions (i.e. 115VAC x 1.25 safety factor) the current going through the relay peaks at 470uA.
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