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PWM

Postby Reith01 » Thu Jul 30, 2015 4:31 pm

Hello there,

I've just joined because I should be receiving my first t-gauge bits and pieces tomorrow...I hope. I didn't buy a starter set, just some track, rolling stock and accessories.

I didn't buy a controller because I have some PWM controllers that can be set to fixed voltages. However, does anyone know the pulse frequency preferred by the motors? If I have to I can construct a new controller around two 555 timers, one at the basic pulse frequency triggering the other as a monostable with a varying pulse width, with something to deliver power on the output.

Thanks in advance for help you can give.
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Re: PWM

Postby dkightley » Thu Jul 30, 2015 5:06 pm

Hi Reith

Welcome to the forum. I'm sure you're going to have fun with T...

What a stonker of a first question!! And I'm not sure what the answer is What someone may have to do is to take a look at the official PWM controllers and measure the frequency throughout the range...to see what they use and to see if the frequency varies with input setting.

I might get chance to do so in the next day or so. If I do, I'll post my findings.
Doug Kightley
Webmaster here and volunteer at the National Tramway Museum http://www.tramway.co.uk
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Re: PWM

Postby hedge » Thu Jul 30, 2015 6:10 pm

I remember there was a huge amount of discussion of this subject on the 'old' forum. There were a LOT of experiments and some detailed measurements and tests including scope traces of the factory PWM controllers. I guess there is no way access that mine of information now. :(
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Re: PWM

Postby martink » Thu Jul 30, 2015 9:18 pm

The standard PWM controllers use a 100Hz square wave with a peak voltage of 4.2V on batteries. I ended up using between 150Hz and 180Hz with 5.5V for my own designs, limiting the maximum duty cycle to 80%.

My first design also used a pair of 555s (the second for the overload protection)....
http://modelrailmusings.weebly.com/t-ga ... oller.html

Here is an oscilloscope trace of the standard unit driving a short HST - note the back EMF and how much the motors decelerate between pulses.

Image
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Re: PWM

Postby Reith01 » Thu Jul 30, 2015 9:34 pm

Thank you. Yes, I hope to have some fun.

I built the PWMs I have to run at a frequency of 10kHz and about 16kHz. The 10kHz one ended up driving a small fan.

I really should buy another powered car to experiment on rather than risk the rolling stock I bought. I'll do so tomorrow.

I doubt the frequency would have an adverse effect as long as the voltage is kept at 4.5v but best to err on the safe side!
I might reduce the frequency a bit. I'll report back.
These circuits aren't difficult to build so I can spend any savings on more stock!
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Re: PWM

Postby Reith01 » Thu Jul 30, 2015 9:40 pm

martink wrote:The standard PWM controllers use a 100Hz square wave with a peak voltage of 4.2V on batteries. I ended up using between 150Hz and 180Hz with 5.5V for my own designs, limiting the maximum duty cycle to 80%.

My first design also used a pair of 555s (the second for the overload protection)....
http://modelrailmusings.weebly.com/t-ga ... oller.html

Here is an oscilloscope trace of the standard unit driving a short HST - note the back EMF and how much the motors decelerate between pulses.

Image


Uh-oh. Just noticed your post. It looks like a completely new controller is needed. One of the advantages of making one apart from saving money is
the possibility of voltage control: starting and stopping, changing speed on the track and things like that.

I'll make the pulse frequency variable from about 75Hz to 200Hz to experiment. What surprises me is that there's no noise with it. I tried lower frequencies with
a fan (used in an amplifier) and though slight, the frequency was detectable as audio).

Thank you for the tips and information.
Reith01
 
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Location: Sussex.

Re: PWM

Postby martink » Fri Jul 31, 2015 12:54 pm

The low frequency really took me by surprise too. For the larger scales, 200Hz or so is about optimum for low speed control but the price is noisy, growling motors. At higher frequencies they even sing! Given the tiny T motors, plus the observed rate at which they decelerate after each 100Hz pulse, I had expected that something in the high hundreds would be the best choice. However, after a few experiments I found that performance really fell off for anything above 200Hz, so I stuck with something around 150Hz. In the larger scales I use PWM pulses with rounded or sloping edges to minimise noise and stress, but haven't found the need in T.

My current control system uses a single PIC to generate 4 distinct (and synchronised!) PWM outputs, one for each section of the layout, with the automation following each train as it advances from section to section.
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Re: PWM

Postby Reith01 » Sun Aug 02, 2015 6:00 pm

From your information I put together a veroboard circuit using the triangle output of the 555. The timing components give 140Hz. I could change it but I might as well start with that. The result feeds into a medium power transistor BD139 with a maximum collector current of 1.5A. Hopefully two motor units won't exceed an amp.

There's a motorised chassis coming to try things out. All that remains is overvoltage protection but I need to see how much spiking there is once the motor is running before I can do much. Your oscillograph doesn't appear to be spikey so maybe a crowbar circuit would be an overkill.

I've given some thought to automating acceleration and deceleration, sensing and things. The comparator side of the circuit can receive any control voltage within range so a capacitor charge/discharge could be wired to it. - More experiments! I don't know how many controllers I'll eventually need but these things can be run up in a couple of hours.

Anyway, it's due to start happening very soon. Again, thanks.

A bit spacious but it was developmental!

Image
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Re: PWM

Postby Reith01 » Sun Aug 02, 2015 7:10 pm

It looks like I'm past the editing window. Just to borrow your idea - a master PWM generator! I could pass the output of the 555 into a unity gain buffer to feed as many controllers as necessary. I'll try that mod for something to do tonight!

Cheers.
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Re: PWM

Postby mattd10 » Sun Aug 02, 2015 7:48 pm

Fascinating to follow as I'd love to build my own PWM controller with added simulation (I also looked at having a capacitor across the control pin to achieve this) but my electronics knowledge is pretty limited so I resorted to using other designs found online which were all designed for larger scales. This could be the answer I'm looking for!
Matt
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