evandude
New Member
I have a small circuit built where a PIC uses MOSFETs to switch a couple of solenoids. the solenoids use quite a bit of power (one is a 3 watt/6 volt, running it at 9v though... the other is much larger, no rating though)
I have the mosfets switching the solenoids directly from the 9v battery that acts as a power source. I have a 6800uF 16v capacitor in parallel with the battery, which is as large a cap as i can physically fit into my project... The PIC and all control electronics are supplied by a 78L05 voltage regulator...
I had some problems earlier, where the PIC would lock up on power-up, i found that it was browning out because of how long that large cap took to charge on power-up. so i enabled brown-out reset, and that took care of that.
however, now every time the PIC trips the solenoids, it is resetting itself. I know this is because the solenoids are using so much power, it is dropping the voltage levels. I added a 330uF cap in parallel with the OUTPUT of the voltage regulator, to help stabilize just the logic power levels... but this hasn't helped... I even tried upping it to 3300uF, and that still didn't cut it. i know there is no way that the PIC alone is using enough power to drain a 330uF (much less a 3300uF) decoupling capacitor... this is also especially odd considering the original board only had a 100uF decoupling cap after the regulator.
my question is: can current flow BACKWARD through a 78L05? that seems to be the only way that the voltage could be dropping that badly even with such a large decoupling cap after the regulator... I compared this board to the original one that it replaced, and there looks to be a diode between the +9v and the input of the voltage regulator. I didn't put one in when i designed it, because i never thought there would be any danger of current flowing backward through the regulator... All i have lying around are 1N914's, but the diode on the original board looks much beefier, probably rated at several amps... I don't imagine that the PIC would need that much power... but i'm also a little leery of sticking a tiny glass diode in there and running the risk of having it blow up in my face...
any thoughts?
thanks,
-evandude
I have the mosfets switching the solenoids directly from the 9v battery that acts as a power source. I have a 6800uF 16v capacitor in parallel with the battery, which is as large a cap as i can physically fit into my project... The PIC and all control electronics are supplied by a 78L05 voltage regulator...
I had some problems earlier, where the PIC would lock up on power-up, i found that it was browning out because of how long that large cap took to charge on power-up. so i enabled brown-out reset, and that took care of that.
however, now every time the PIC trips the solenoids, it is resetting itself. I know this is because the solenoids are using so much power, it is dropping the voltage levels. I added a 330uF cap in parallel with the OUTPUT of the voltage regulator, to help stabilize just the logic power levels... but this hasn't helped... I even tried upping it to 3300uF, and that still didn't cut it. i know there is no way that the PIC alone is using enough power to drain a 330uF (much less a 3300uF) decoupling capacitor... this is also especially odd considering the original board only had a 100uF decoupling cap after the regulator.
my question is: can current flow BACKWARD through a 78L05? that seems to be the only way that the voltage could be dropping that badly even with such a large decoupling cap after the regulator... I compared this board to the original one that it replaced, and there looks to be a diode between the +9v and the input of the voltage regulator. I didn't put one in when i designed it, because i never thought there would be any danger of current flowing backward through the regulator... All i have lying around are 1N914's, but the diode on the original board looks much beefier, probably rated at several amps... I don't imagine that the PIC would need that much power... but i'm also a little leery of sticking a tiny glass diode in there and running the risk of having it blow up in my face...
any thoughts?
thanks,
-evandude