MEASURE RESISTANCE
Posted: Fri Jan 07, 2022 1:41 pm
hi all
I've been trying for months to read 3 resistances in the format I've attached i have used 1 analogue input to try to read in turn each resistance ie 1-2, 1-6 and 2-6 with the latter number in each being grounded I've used relays (relays are 5v 370 ohm coil non latching) driven by transistors both pnp and npn switching, to switch between them and although there is only 0.3 ohm between pole and NC of the relay the reading seems to be around 2 ohms higher or more some times, i have also used 4052 multiplexor but readings are off still, also used a ULN2003 relay driver but this makes the readings +20 ohms, if i measure just one resistor straight from the analogue input and reference resistor the reading is bang on compared to a fluke, my reference resistor is 267.3 ohms and 0.1% tolerance so i know my software is bang on the problem seems to be when running the analogue signal through the pole to N/O or N/C on the relay sometimes if the ground for the transistor is moved i get better results but not always. i really don't know how to proceed with this and run out of switching options i would appreciate any input to progress this further
bob
ps sorry if this is in the wrong section
I've been trying for months to read 3 resistances in the format I've attached i have used 1 analogue input to try to read in turn each resistance ie 1-2, 1-6 and 2-6 with the latter number in each being grounded I've used relays (relays are 5v 370 ohm coil non latching) driven by transistors both pnp and npn switching, to switch between them and although there is only 0.3 ohm between pole and NC of the relay the reading seems to be around 2 ohms higher or more some times, i have also used 4052 multiplexor but readings are off still, also used a ULN2003 relay driver but this makes the readings +20 ohms, if i measure just one resistor straight from the analogue input and reference resistor the reading is bang on compared to a fluke, my reference resistor is 267.3 ohms and 0.1% tolerance so i know my software is bang on the problem seems to be when running the analogue signal through the pole to N/O or N/C on the relay sometimes if the ground for the transistor is moved i get better results but not always. i really don't know how to proceed with this and run out of switching options i would appreciate any input to progress this further
bob
ps sorry if this is in the wrong section