spuffock said:
You have built an oscillator. OOPS :shock:
Go back to the first idea, save money and gnashing teeth.
haha no wonder there seems to be an 800 mV swing on that 5v line. I'll strongly suggest that we put that BJT circuit on a breadboard if we determine that the 800 mV ripple does exist and we cant greatly reduce it. However, the cellular modem is able to operate off of this power supply.... I'm not completely certain that the 800 mV ripple is there.
As for if I'm
trying to build a switching supply, no... just want something that works. The 7805 should dissipate VERY little heat. Its ouput ONLY goes to an opamp, which of course has very high input impedance. The ouput of the NFET goes to the other input of the opamp, and the output of the opamp drives the NFET until there is 0 difference between the 7805 and the FET drain.
I actually did put it into PSPICE last night, those are ideal parts in PSPICE, and it showed 0 ripple. I don't know how to change those things so that they're not ideal.
Here is a better shcematic of it though... if anyone can help me add some ripple on the 13.8v DC source I'd appreciate it. I tried several AC sources, but couldn't figure out how to do an appropriate AC sweep on it. Instead of a sweep, I'd rather just select the frequency and then run it at the frequency for some set amount of time though.
On the graph, the teal line (wait, I'm a guy... pretend I said blue! I don't know what teal is I swear!) is the 5v rail, and the mauve (I MEAN PURPLE! :shock
is the output of the op amp.
Also, the higher the gain of the opamp the closer it gets to 5v (well, 4.96 since that is what our 7805 outputs).
Edit: The resistors are meant to just be a load on the circuit. R2 is the load from 0s to .1s, R1 || R2 is the load from .1 to .2, and R2 is the load from .2 to .3 seconds. This is just the approximate load for when the modem is or is not transmitting.