Oatley K25 PWM kit - feedback and questions

Hello Phil, OK - didn't know that about PWM - so used to plonking in caps with other PSU type things - will pull it out.

I do agree that it's looking like the oscillator isn't running - am basically going to pull the circuit apart on paper and trace each pad/ trace and component to ensure no errors, then replace the op-amp chip in case it cooked off. only issue I found on the first pass over the cct was the terminal for V+ had cracked when I tightened on the 2.5mm2 cable - so it wasn't making a reliable contact. - I ended up removing the the terminals for the supply and motor and drilled appropriate sized holes in the PCB and attached short lengths of 2.5mm2 cable going off to chocolate block terminal strip for a more robust connection.

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.

the embarassing bit is that I used to be reasonably good at the hardware side of things, but too many years in programming, it've fallen out of touch

- this was/ is my foray into trying to regain some familiarity - and it's fallen into a heap.

thanks all, i'll be in touch once I get this fixed (along with the PC which died, and sort out some other issues...) too many jobs, not enough time. Thanks again for the pointers, Des

Reply to
Des Bromilow
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"Des Bromilow"

** OK.

Simple PWM motor drives are something I've spent a lot of hours on - using both BJTs and MOSFETS installed in a couple of RC model boats packed with lotsa sub C Ni-Cds. Their smooth, wide range speed control of simple permanent magnet motors combined with high efficiency is close to magical.

My first ever published article ( in the CDI pages of EA ) described my earliest effort.

.... Phil

Reply to
Phil Allison

inkscape can open and edit it, another approach is to use pdftops and edit the ps file with a text editor to remove the lines that define the undesired elelements.

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Reply to
Jasen Betts

The redactions are soft (the electronic equivalent of a label with backed with gum arabic - a little steam and it comes off),

they could have easily corrupted the yucky bitmap image that comprises the schematic if they wanted to really keep it secret.

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Reply to
Jasen Betts

This suggests that the first stage (IC2:A) isn't oscillating. check pins 1,2, and 3 of IC2 for shorts to their neighbours.

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Reply to
Jasen Betts

On Sat, 12 Mar 2011 07:58:57 +0000 (UTC), Des Bromilow put finger to keyboard and composed:

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

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Reply to
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...

Tom

Reply to
Tom

"Tom"

** 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.

  1. 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".

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...... Phil

Reply to
Phil Allison

On Mon, 14 Mar 2011 09:26:41 +1000, Tom put finger to keyboard and composed:

High-Power Reversible DC Motor Speed Controller:

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- Franc Zabkar

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Reply to
Franc Zabkar

pled

off

n

I'm impressed. (usually am by clever use of electromechanics)

Reply to
kreed

"Franc Zabkar"

** 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

Reply to
Phil Allison

"Phil Allison"

** Reason:

The heating of the motor windings is a function of the RMS value of the current flowing whereas the torque and hence power output produced is in proportion to the average current.

The ideal situation is having smooth DC current flow, where the RMS and average values are the same.

Having discrete current pulses of current means the RMS amps value is higher or even way higher than the average amps value.

... Phil

Reply to
Phil Allison

Thank you for clarifying that point.

Grant.

Reply to
Grant

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