kostas said:
Hello
I want to design a PWM to control the duty cycle of o big fan (80watt).
I have seen a nice circuit at
http://www.cpemma.co.uk/pwm.html
The nomad's with the kick start function. The maximum current i want to
control is 6.6 Amber and in order for the kick start to work i should
use a PNP transistor, correct? Can i use a N channel MOSFET like irf540?
Do i have too use any special driver after 555 and before MOSFET or PNP?
Thanks for your time
If you are trying to build something like Nomad's 555 circuit, yes, it
uses a PNP transistor, but the design is capable of nothing like a 6
ampere output. If you use a good, high current PNP transistor, say a
TIP36 or D45H. But this circuit will not provide enough base drive to
switch one of these with a 6 ampere collector current. to get good
saturation (less than .3 volts collector to emitter) one of these may
need a base current between 1/20th and 1/10th of the collector current
(.3 to .6 amperes). The design shown has a 470 ohm resistor limiting
the base current. After you discount the 1 volt base voltage and the
1.5 volt loss through the 555, this leaves 10 volts across that
resistor for a base drive of 10/470=.021 amperes. This resistor would
have to be lowered to about 20 ohms to get the needed base drive, and
then the 555 would overheat.
So the output capability of the 555 has to be increased. Adding a
second PNP (say, a 3 amp rated TIP32) with its base connected directly
to the 555, its collector connected directly to the negative rail, and
a 15 ohm 5 watt resistor between its emitter and the big PNP's base,
you get the needed base drive. Then you need to put a 10 ohm resistor
between the base and emitter of the big PNP to suck the stored charge
out of it when the 555 output goes high, to have a clean turn off.
All these designs are also missing the diode that should be across the
motor, cathode up. Without this, the high voltage pulse you get when
you interrupt the motor current is likely to damage the switching
transistor. For a 6 ampere motor, this diode should be fast (turns
off fast when reverse bias is applied by the transistor) and rated for
at least 3 amperes, average. Candidates would be the SR503 (30 volt 5
ampere schottky) and 80SQ045 (8 ampere, 45 volt).
If you want to simplify the circuit quite a bit, replace the entire
PNP and resistor network with a big P-channel mosfet (gate directly
connected to the 555). Something like IRF5305 (0.06 ohms on
resistance, 55 volt).