Hi,
I have some trouble with a circuit I try to make, and i can't figure what's wrong. Maybe it will be really obvious to real electonic engineers.
I'm trying to make a charge pump to raise a 5v to 12v. And I'd like to be able to switch quickly the tention. That's for providing a VPP for programming a PIC. Found this little project very interesting to begin with electonic. I managed to get the pump working (got around 13v) and make a switching mechanism with two npn transistors. (I attached the cricuit picture to the post don't know if it shows up).
command input : pin marked L on the schematic
T1 : transistor at the bottom linked to the ground
T2 : transistor on top liked to the charge pump output
This is what I was expecting to get :
When command input is Low, T1 is opened, T2 is closed. Tention on T2 emiter is charge pump tention minus T2 base-emiter forward voltage (up to zener tention).
When command input is High, T1 is closed, T2 is opened. Tention on T2 base is close to zero and current goes down the ground through T1. With the 470k resistor the current should not be enought to dry out the 100u capacitor of the pump. So when we switch back, the tention on the 100M resistor raises directly to 12v instead of a slow incrementation.
The simulation gives me exactly what I expected. When I switch the command input, the tention on the 100M resistor switches directly and the tention in the pump is preserved. In practice, I have a different comportement. With my implementation when command input is Low I have the same results, but then command input is high the tention on the 100u capacitor drops around 8 volt. So when I switch back to low input the tention takes time to go back from 8 to 12v.
There is really something I'm missing here. Is anyone has any hints ? thx
If you want to play with the simultation click here :
http://www.falstad.com/circuit/index.htmlClick on menu File>Import, and then copy paste the following code :
$ 1 5.0E-6 5.459815003314424 65 5.0 43
R 192 144 160 144 0 0 40.0 5.0 0.0 0.0 0.5
d 192 144 240 144 1 0.805904783
d 240 144 288 144 1 0.805904783
d 288 144 336 144 1 0.805904783
c 240 144 240 224 0 1.0E-6 4.863609993506333
c 288 144 288 224 0 1.0E-6 1.357337975778239
c 336 144 336 224 0 9.999999999999999E-5 6.2212372591500555
R 240 224 240 272 1 2 1220.0 2.5 2.5 0.0 0.5
R 288 224 288 272 1 2 1220.0 2.5 2.5 3.141592653589793 0.5
g 336 224 336 272 0
p 240 144 336 224 0
p 288 144 336 224 0
g 480 288 480 320 0
r 480 144 480 208 0 470000.0
r 432 272 384 272 0 1000.0
L 384 272 384 336 0 0 false 5.0 0.0
w 480 208 480 256 0
w 480 144 528 144 0
w 528 144 528 192 0
r 560 224 560 288 0 1.0E8
w 528 288 480 288 0
t 432 272 480 272 0 1 -6.1977021288495076 4.999999999801981E-11 100.0
t 480 208 528 208 0 1 -0.023535130250547986 0.4434744078508954 100.0
z 528 288 528 224 1 0.805904783 12.0
w 528 224 560 224 0
w 528 288 560 288 0
w 336 144 480 144 0
o 10 64 0 34 20.0 9.765625E-5 0 -1
o 11 64 0 34 20.0 9.765625E-5 1 -1
o 6 64 0 35 20.0 102.4 2 -1
o 19 64 0 35 20.0 9.765625E-5 3 -1
