555 timer calculation

[Nathan L,]

I have DSE kit ZA-9178, and quoting from the text:
"The 10k ohm and 5.6k ohm resistors, along with the 0.1uF capacitor at pins
2 and 6, set the output frequency of the 555 to about 6.8kHz, although this
will vary according to the speed setting."

However, 1.44 / ( (R1+R2+R2) * C) gives me 679.2Hz.

The "speed setting" involves a 10k pot across the supply, with the wiper
connected to pin 5 of the 555. How much does that effect the frequency?
Does the center setting (5k) of the pot set the frequency to 6k8Hz?

I'm wanting to use it to control fans, but at the lower speed settings the
PWM becomes audible. I need to keep the output frequency preferably above
20kHz for the lower speed setting.

Supply is 12V (less protection diode).


Cheers in advance for any suggestions.

 

[Phil Allison]

"Nathan L,"

I have DSE kit ZA-9178, and quoting from the text:

I'm wanting to use it to control fans,

** What sort of fans ????

Not brushless DC fans ( process or fans ) I hope.



.............. Phil

 

[Martin]

I have DSE kit ZA-9178, and quoting from the text:
"The 10k ohm and 5.6k ohm resistors, along with the 0.1uF capacitor at
pins 2 and 6, set the output frequency of the 555 to about 6.8kHz,
although this will vary according to the speed setting."

Does if the cap was 0.01uF... typo in the instructions???

 

[Nathan L,]

Does if the cap was 0.01uF... typo in the instructions???

The parts list of the kit includes a "0.1uF 63VW MKT polyester". The cap
supplied has "100nJ100" labeled on it.

 

[Nathan L,]

"Nathan L,"


** What sort of fans ????

Not brushless DC fans ( process or fans ) I hope.

The circuit has good filtering and back-EMF protection built in.
In amendment to my previous post, the 555 is run at 9.1 volts, regulated by
a zener.

 

[Phil Allison]

"Nathan L,"
"Phil Allison"

The circuit has good filtering and back-EMF protection built in.
In amendment to my previous post, the 555 is run at 9.1 volts, regulated
by a zener.

** Oh dear - he is just not listening.

A "brushless DC" fan is ***NOT** a simple DC motor - it is an
electronic device and must be driven from a voltage source.

PWM drive is not on.




........... Phil

 

[Nathan L,]

"Nathan L,"
"Phil Allison"


** Oh dear - he is just not listening.

A "brushless DC" fan is ***NOT** a simple DC motor - it is an
electronic device and must be driven from a voltage source.

PWM drive is not on.

Why exactly? I'm seeing plenty of examples around the net doing exactly
that - so long as the frequency isn't above 30kHz?

 

[Phil Allison]

"Nathan L,"

"Phil Allison" .

Why exactly? I'm seeing plenty of examples around the net doing exactly
that - so long as the frequency isn't above 30kHz?

** It is certainly not the recommended nor accepted method - it may just
work
with some brushless fans but not all.

BTW

You are using the wrong PWM topology - see the back pages of any DSE
catalogue
( p 290 of the 2002/3 cat ) for the proper ( ie constant frequency) way to
get PWM drive from a 555 IC.




............. Phil

 

[Franc Zabkar]

I have DSE kit ZA-9178, and quoting from the text:
"The 10k ohm and 5.6k ohm resistors, along with the 0.1uF capacitor at pins
2 and 6, set the output frequency of the 555 to about 6.8kHz, although this
will vary according to the speed setting."

However, 1.44 / ( (R1+R2+R2) * C) gives me 679.2Hz.

You're right. That must be a typo.

The "speed setting" involves a 10k pot across the supply, with the wiper
connected to pin 5 of the 555. How much does that effect the frequency?
Does the center setting (5k) of the pot set the frequency to 6k8Hz?

The 555 has an internal 3-stage potential divider. AFAICS, the
"neutral" setting of the pot is when its wiper is set to 2/3 Vcc, ie
the frequency will not be affected at this setting.

I'm wanting to use it to control fans, but at the lower speed settings the
PWM becomes audible. I need to keep the output frequency preferably above
20kHz for the lower speed setting.

Supply is 12V (less protection diode).


Cheers in advance for any suggestions.

Your brushless DC fan motors use integrated Hall effect sensors/ICs
for electronic commutation. AFAICS, straight unfiltered PWM control is
not possible for this type of design unless the PWM frequency is much
lower than the fan RPM, or unless there is a smoothing capacitor
across the fan, and/or a choke in series with the fan to convert the
pulses to flat DC. The commutation ICs can typically operate from
3VDC, although the motor may stall at this speed.

See these datasheets for examples:
http://www.anpec.com.tw/products/download/pdf/APX9141a7.pdf
http://www.anpec.com.tw/products/download/pdf/APX9140a8.pdf

This picture illustrates the electronic commutation principle:
http://www.ece.msstate.edu/~hagler/May2001/02/FIG2.JPG


- Franc Zabkar

 

[Richard]

Try typing '555 timer calculator' into your favourite search engine.....

Richard.

 

[Nathan L,]

Try typing '555 timer calculator' into your favourite search engine.....

Richard.

Hehe I did exactly that - the first one I tried gave me an incorrect figgure
I didn't trust these to start with -- rightly so as I still had to check
it myself.