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Author Topic: Bachmann controllers dead spot?  (Read 443 times)

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Offline Jonathan Prince

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Re: Bachmann controllers dead spot?
« Reply #15 on: February 09, 2018, 04:30:17 pm »
No problem! I think I'll just keep them around until I actually need better controllers.

Let me get this right: the wall plug transforms to DC, then all the controllers do is vary the resistance? (ie. the output is the same as the input when fully on) Does this mean I could (in theory) wire the controller to an uncontrolled DC output from another transformer? Perhaps that would provide more current as Snowwolflair said.

Offline MalcolmAL

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Re: Bachmann controllers dead spot?
« Reply #16 on: February 09, 2018, 04:39:44 pm »
the wall plug transforms to DC, then all the controllers do is vary the resistance?
Almost :) !
The Bachmann  wall plug gives AC out and there is a bridge rectifier on the input of the controller,
and it is a bit more than just a resistance, it uses some electronics to generate a set voltage out.
« Last Edit: February 09, 2018, 04:41:29 pm by MalcolmAL »

Offline Jonathan Prince

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Re: Bachmann controllers dead spot?
« Reply #17 on: February 09, 2018, 05:15:55 pm »
the wall plug transforms to DC, then all the controllers do is vary the resistance?
Almost :) !
The Bachmann  wall plug gives AC out and there is a bridge rectifier on the input of the controller,
and it is a bit more than just a resistance, it uses some electronics to generate a set voltage out.
Ok thanks - so I can't just use it to control a DC output then.

Online ntpntpntp

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Re: Bachmann controllers dead spot?
« Reply #18 on: February 09, 2018, 06:03:12 pm »
They're a feedback design which implies PWM
I thought feedback could be applied to straight DC (?).

A typical feedback controller works by sensing the back EMF generated by the motor when the power is cut for a fraction of a second (effectively the motor becomes a generator).   Consequently this needs gaps in the power which is precisely what PWM gives you (until you're at 100% duty cycle anyway). 

You can also implement feedback with fixed width pulse power (eg. derived from the AC cycle) and rely on increasing/decreasing the actual output voltage whilst the pulses remain the same. I think some older feedback controllers work this way rather than using PWM (and actually my KPC controllers may do this, I need to put a scope on the output and take a look).

Either way, the controller applies a negative feedback circuit to try and maintain constant speed - so if the motor isn't going as fast (generating enough BEMF) as the circuit expects then the controller needs to increase the voltage. Likewise if the motor is going faster than wanted the voltage needs to be reduced.  This requires that the feedback circuit be tuned to expect the correct amount of BEMF, and of course this can vary with different motors and mechanisms which is why a given feedback controller works better with some locos than others. The controller is tuned to a "typical" expected BEMF, unless there are some posh controllers out there which have variable tuning?

In the digital world PWM gives you a relatively simple way of applying the increase or decrease.  DCC decoders certainly use PWM and feedback this way, and of course with decoders you often get CV options to adjust feedback tuning and the PWM characteristics.


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