# 555 Timer - less then 50% duty cycle question.

Discussion in 'Electronic Basics' started by John Popelish, Jul 28, 2004.

1. ### John PopelishGuest

Either should work. The silicon diode will cause slightly more change
in duty cycle as the battery voltage falls, but whether this is good
or bad depends on what you want.
Either can work well. The mosfet is simpler to connect, because you
don't need to limit the current and this lowers the total battery
current, but the difference is small.
Sounds like the switch is on during the 2/3 part of the cycle, instead
of the 1/3 part. Try using the opposite polarity transistor and
switching the opposite side of the supply to the motor. That should
invert the part of the pulse that is turning the switch on.
A schematic would be very helpful. Please post to A.B.S.E. or email
me one.

2. ### cpemmaGuest

I've built the simpler single 555 PWM circuit
http://www.cpemma.co.uk/555pwm.html & found schottky diodes (BAT42) gave
slightly nearer 0-100% duty than silicon (1N4148). But I wouldn't make a
special purchase of them to get the advantage.

3. ### RubiconGuest

Hello.

Using a calculating program for a 555 timer operating at less then 50%
duty cycle the diagram that the program displays has two diodes. One
from pin#7 to pin#6 and the other from pin#2 with R2 to pin#7. I
understand that the formulas work through these diodes but what
exactly should they be? Signal IN4148, Schottky or another type?

In addition when switching a motor via the 555 what's the best choice
of switch? A transistor or a mosfet or some other device? I have tried
both transistor and mosfet to switch a motor at approx 33% duty cycle
but failed to do so correctly. At the moment it runs faster than it
should when it shouldn't. That is a 555 monostable output through an
NPN transistor switches another 555 astable (555 V+ to V+, V- to
collector) that switches another NPN transistor or mosfet (Motor V+ to
V+, V- to collector/drain) to control the motor. At the moment the
motor runs as soon as power is applied. Not sure what's wrong as the
wiring seems O.K.

Regards,

Andrew.

4. ### John FieldsGuest

---
Check "Duty cycle controller" on abse and

http://pdfserv.maxim-ic.com/en/ds/ICM7555-ICM7556.pdf

for the timing equations so you can figure out the RC's

5. ### RubiconGuest

John,

Thanks for the help. I'm in the process of making a basic schematic of
what I've done so far and when completed I will make it available for
you to have a look at.

Regards,

Andrew.

Thankyou,

Andrew.