| Electronics > Projects, Designs, and Technical Stuff |
| Dumb Question on Current Source |
| (1/3) > >> |
| engrguy42:
I threw together a simple constant current source shown below. I did something stupid but can't figure out what. If I use a single power supply to power the MOSFET/OpAmp input it works fine. But if I have a separate power supply for the input and another for the MOSFET drain/source output it doesn't work. The MOSFET won't turn on. BTW, this isn't an LTSpice question (both work fine in LTSpice), it's an actual breadboarding question. And I'm sure the breadboard wiring matches the diagrams in both cases. |O Anyone? Thanks. |
| langwadt:
--- Quote from: engrguy42 on May 26, 2020, 09:46:39 pm --- And I'm sure the breadboard wiring matches the diagrams in both cases. --- End quote --- and you are probably wrong ... |
| engrguy42:
--- Quote from: langwadt on May 26, 2020, 09:54:18 pm --- --- Quote from: engrguy42 on May 26, 2020, 09:46:39 pm --- And I'm sure the breadboard wiring matches the diagrams in both cases. --- End quote --- and you are probably wrong ... --- End quote --- Really? So the two supplies should work like that? It's weird because all I did was move a couple wires to go from the dual supply to the single supply and suddenly it worked fine. So the underlying wiring should be okay... |O |
| schmitt trigger:
Take readings of voltages at every opamp pin and label them in your schematic. Which supply is powering the opamp? |
| engrguy42:
--- Quote from: schmitt trigger on May 26, 2020, 09:57:39 pm ---Take readings of voltages at every opamp pin and label them in your schematic. Which supply is powering the opamp? --- End quote --- Vcc for the opamp is coming from the supply on the left. EDIT: And for the nonworking one (2 supplies) the supply on the left is actually 12v, and the output to the gate is 7.3v, the - input is 0.11 volts, and the + input is 7.3 volts. So the MOSFET isn't going on (it has a 3 volts threshold). And with the single 10v supply it's all as expected...load volts is 6V. |
| Navigation |
| Message Index |
| Next page |