Oatley K25 PWM kit - feedback and questions

[Franc Zabkar]

either way this cct is coming out of the application and being replaced with
one which doesn't have the 5s "soft start" option - the application here
is a lather and it's disconcerting for the user to press the "start button"
and then not have something happen within a second or so.

AFAICS, reducing the value of C3 (100uF) should reduce the soft start
time. I believe a value of 22uF should give you a 1 second delay.

- Franc Zabkar

 

[Tom]

I thought that Oatley were above these methods of doing things.

All this does is hurt their business and particularly support
reputation.
Their kits are normally so reasonably priced that the majority
wouldn't go to the trouble of trying to "roll their own".

Strange, they've published another kit a few months ago in Silicon Chip
that contain this schematic with addition of couple of relays to change
motor direction yet they keep this one secret...

Tom

 

[Phil Allison]

"Tom"

Strange, they've published another kit a few months ago in Silicon Chip
that contain this schematic with addition of couple of relays to change
motor direction yet they keep this one secret...

** Hmmmm .....

So Oatley have just about caught up with my design of circa 1980 for a RC
model boat.

A simple PWM controller using a 555 ( running at 5kHz) and TO3 Darlington
PNP output stage with hefty Schottky diode for freewheeling. A 10kohm WW
pot, modified so it had only 110 degrees of rotation, was directly coupled
to a RC servo for speed control. The same servo also did two more jobs:

1. At the extreme CCW ( stop) position - it operated a push on / push off
switch.

2. At in the extreme CW position - it closed a hefty microswitch.

Job 1 energised or de-energised a 10 amp DPDT relay for motor reversing.

Job 2 bypassed the Darlington output stage for "overdrive".

Power came from 12 sub-C Ni-Cds and drove a 5 pole geared motor - a German
made " Decaperm".

http://www.cornwallmodelboats.co.uk/acatalog/decaperm_vlrg.jpg




....... Phil

 

[Franc Zabkar]

Strange, they've published another kit a few months ago in Silicon Chip
that contain this schematic with addition of couple of relays to change
motor direction yet they keep this one secret...

High-Power Reversible DC Motor Speed Controller:
http://www.siliconchip.com.au/cms/A_112070/article.html

http://us1.webpublications.com.au/static/images/articles/i1120/112070_4mg.jpg
http://us1.webpublications.com.au/static/images/articles/i1120/112070_5mg.jpg

- Franc Zabkar

 

[Phil Allison]

"Franc Zabkar"

High-Power Reversible DC Motor Speed Controller:
http://www.siliconchip.com.au/cms/A_112070/article.html

http://us1.webpublications.com.au/static/images/articles/i1120/112070_4mg.jpg
http://us1.webpublications.com.au/static/images/articles/i1120/112070_5mg.jpg

** Why on earth does the PWM circuit run at 300Hz ??

This is far too low a frequency for good efficiency.

There will be large current pulses in the motor at almost any setting while
the brushes and motor windings will suffer from additional heating with no
benefit. Simply raise the frequency to say 3kHz and put a heatsink on that
Schottky diode and it will be far better job all round.

Using a higher frequency makes the current flowing in the motor become
nearer to steady DC at all duty cycles of the PWM. The Schottky diode will
then pass more average current (and less peak) during the "off " times in
the PWM so will dissipate more heat.

A correctly operating PWM drive provides the same kind of speed control over
a permanent magnet motor as variable DC voltage control does - but with
the high efficiency essential for battery powered vehicles and the like.

I note the circuit provides no " braking " function and this could be an
issue for some vehicles. Braking means placing a short on the motor to bring
it to a rapid stop.


..... Phil