I would like to preface this post by saying I have no formal education or training with electronics and am completely self-taught. I really enjoy designing home brew computers from the late 70's and early 80's as my hobby. I have made a couple in the past (6800, 6502, and now 8080) and they have all be successful, but all on two-layer PCBs. For this project, I have created an 8080 based computer (don't ask why lol) on a four-layer PCB but am having a problem with the power input.
It is my first four-layer PCB and everything works perfectly except on the power input. For some reason, when I apply a constant 5 volts with my bench power supply, the input voltage is always lower, depending on how many components are installed on the board. With no components, there is no voltage drop. When I install all the components into their sockets, it drops by about 1.2 volts. To make the circuit run properly, I have to increase the power supply to 6.2 volts so that the board power is exactly 5 volts.
I know that voltage drop is usually caused by resistance in the circuit, so that was my first point of testing. I measured several points across the entire board, and at any point between any two VCC pins or two ground pins, the resistance was always zero ohms. Between the power input pins and any pins are zero ohms. And in the wires from the power supply to the board is also zero ohms. (I know there is SOME resistance, but my multimeter read 0.01 at the highest point.)
I am not sure what else could be causing the board. The board is quite large at 8in x 10in. Luckily PCB prices have come down a bunch from what I have heard because I only had to pay $65 for it! Anyways, I did a lot of research when designing it so I could make the best board possible. The layers are as follows:
- VCC (5 volts)
I have several larger 100 uf capacitors at the power input and 0.1 uf bypass capacitors on all ICs on the board. There is also a -5v generator and a 12v boost converter, if that makes any difference. I am not sure what else to test here. Maybe the power supply is wonky, but I doubt it because the voltage is correct when read with the multimeter. Again, 6.2 volts at the power supply, 5.0 volts on the board.
Thank you so much for reading my post and any help would be greatly appreciated!