PWM Question

2009 Views 9 Replies 4 Participants Last post by wigginsn, Aug 14, 2008

wigginsn

Discussion Starter · Aug 10, 2008

Almost but not quite traditional track power…

Does anyone know if Bachman engines are ok to run on PWM. I’ve been running my Aristo critter on a homemade setup with no problems but when I put my Heisler on the track it doesn’t like it at all.

I haven’t done any proper diagnosis but at first look it seems that its overloading the power supply every couple of seconds and causing it to drop out. Which in turn is causing control issues (full speed surges. :crying

Just wondering if I’m chasing a fault or if its simpler to cut ‘n throw the onboard electronics and do my own version.

Thanks in advance
Neil

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terry_n_85318

· #2 · Aug 10, 2008

Remove any of the Bachmann filter capacitors on the motors.

Are you sure the PWM drive can handle the added current of the Heisler?

Terry

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wigginsn

Discussion Starter · #3 · Aug 10, 2008

Thanks Terry,

Do you mean on the board or at the motors themselves?

The power supply is a 20A industrial unit, switching FET is an IRF540 and I'm using a driver chip to clean up the switching so there shouldn't be supply problems. The Heisler draws about 1A loaded using an analogue lab supply so I was surprised that the big pack was shutting down.

Cheers
Neil

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Torby

· #4 · Aug 11, 2008

My Annie runs fine on PWM. Never took the capacitors off.

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TonyWalsham

· #5 · Aug 11, 2008

Tom,
The Annie doesn't have any capacitors as far as I know.

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terry_n_85318

· #6 · Aug 11, 2008

What frequency are you running the PWM at? I have run the shay and climax with an Airwire and there weren't any problems. I believe that Airwire runs in the 20 to 30KHZ range.

Perhaps your PWM is allowing simultaneous conduction (very short short circuits of the power suppy) instead of shutting off between pulses. Just a thought. You would need an oscilliscope to look at the signals.

Terry

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wigginsn

Discussion Starter · #7 · Aug 11, 2008

Thanks guys. Switching is at about 35 kHz. I set the throttle up on a scope and it is (was) fully turning off with the throtte off but I was just using a load resistor on the bench at that stage. I'll recheck that tonite, with the loco if I can.

Its been a while since I've done this stuff but I seem to remember simultaneous conduction is having two FETs on at once, like a slight overlap on switching?? I'm just using one FET and allowing freewheel current through the built in diode. Is this something I should still look at?

The supply is a battery charger and its control system is still attached - its a bit of a beast (but free). I certainly can't rule out something haywire with that but whether its the problem or reacting to the problem I don't know. Its certainly "working" as the fault occurs.

Work has a nice 100 MHz digital scope with PC data capture, may see if I can borrow it this weekend if I can't track it down before then.

Cheers
Neil

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Torby

· #8 · Aug 11, 2008

Tom,
The Annie doesn't have any capacitors as far as I know.

Well, no wonder mine works!

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wigginsn

Discussion Starter · #9 · Aug 11, 2008

Posted By Torby on 08/11/2008 3:24 PM

Tom,
The Annie doesn't have any capacitors as far as I know.

Well, no wonder mine works!

Tom, You're making me jealous

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wigginsn

Discussion Starter · #10 · Aug 14, 2008

Problem solved.

Bit embarrasing really.. /DesktopModules/NTForums/themes/mls/emoticons/blush.gif

During this project which was to get the power supply permanently wired inside the shed and a plug for the throttle controls outside I kinda missed putting the freewheel diode back across the output terminals. /DesktopModules/NTForums/themes/mls/emoticons/whistling.gif

I somehow got it in my head that the FET diode would be doing that job. Errrm No. Not sure why the critter was ok but now the Heisler starts nicely at 15% duty and has smooth control over the whole range.

Really glad to solve this one - visitors due on Saturday with a 5 year old to "see the trains"

Cheers
Neil

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