Ok so this is hard to explain but I am multiplexing solenoids so figure about 7ohm loads running at 24VDC.
I have 20 solenoids to run split into two groups. Each group has 10 wires going to each solenoid and a common wire going each. My circuit has 10 darlington NPN (TIP120) for the low side switches and two TIP125 (darPNP) for the high side. The PNP use 3904 NPNs to pull the base low with a resistor pulling it up to the emitter voltage (24-30V). I then have 2 flyback diodes going from each NPN (the 120s) to each power rail switched by the PNPs. (see pic)
My problem is that no matter what, I get two solenoids firing together. More specifically if group one is divided into solenoids 1-10 and group two is 11-20 then 1,11 always fire together and 2,12 and so on. I have checked everything 10x over and the PNPs are not both on. I have followed every possible way current can flow and can not for the life of me figure out whats going on. It seems current goes through the correct solenoid and then follows backwards through the other but then I can't find the path to ground. I cut the connection to the flyback diodes and tried adding diodes in every place imaginable to make sure current is flowing in the right direction and nothing helps.
If I disconnect one of the PNP outputs which is the common wire for one of the groups...IT STILL FIRES BOTH!
I don't have a schematic but I have the business end of the PCB. It's very simple through hole stuff. The two groups of connections on the right are the positive rails switched by the PNPs (1-10 and 11-20) and the horizontal row is the NPNs. Each of those big holes paired together is a (1,11) and (2,12) and so on. Hope this makes sense.
View attachment 63766
I have 20 solenoids to run split into two groups. Each group has 10 wires going to each solenoid and a common wire going each. My circuit has 10 darlington NPN (TIP120) for the low side switches and two TIP125 (darPNP) for the high side. The PNP use 3904 NPNs to pull the base low with a resistor pulling it up to the emitter voltage (24-30V). I then have 2 flyback diodes going from each NPN (the 120s) to each power rail switched by the PNPs. (see pic)
My problem is that no matter what, I get two solenoids firing together. More specifically if group one is divided into solenoids 1-10 and group two is 11-20 then 1,11 always fire together and 2,12 and so on. I have checked everything 10x over and the PNPs are not both on. I have followed every possible way current can flow and can not for the life of me figure out whats going on. It seems current goes through the correct solenoid and then follows backwards through the other but then I can't find the path to ground. I cut the connection to the flyback diodes and tried adding diodes in every place imaginable to make sure current is flowing in the right direction and nothing helps.
If I disconnect one of the PNP outputs which is the common wire for one of the groups...IT STILL FIRES BOTH!
I don't have a schematic but I have the business end of the PCB. It's very simple through hole stuff. The two groups of connections on the right are the positive rails switched by the PNPs (1-10 and 11-20) and the horizontal row is the NPNs. Each of those big holes paired together is a (1,11) and (2,12) and so on. Hope this makes sense.
View attachment 63766