Capacitors, chokes & mysteries - Bachmann "Hall"

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George Stein
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Capacitors, chokes & mysteries - Bachmann "Hall"

Postby George Stein » Tue Jul 23, 2019 3:24 am

Gentlemen:

On the rear of the motor of the Bachmann Rood-Ashton "Hall" is a PCB board with capacitors & spring-like things [chokes??]. Most DCC installation advice (e.g., YouChoos & Buffers) recommends removing the whole assembly to permit mounting the DCC chip vertically behind the motor. Also suggest providing jumper wires to replace chokes.

So, while I understand removing the standard EU required capacitors, "chokes" are new to me. What are they for? Must the wires by replaced? Why? Looking at the whole PCB thing, it seems it's there for a reason, but I'm confused. Help please.

George
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Ironduke
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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Ironduke » Tue Jul 23, 2019 8:11 am

Hi George,
a choke is another name for an inductor. Inductors pass DC but block high frequency noise (or choke it, I guess). Since the motor output of a DCC decoder is a high frequency PWM signal, the chokes are likely to interfere with it.

The PCB is there to stop the motor noise feeding back to the supply and also making lots of radio noise. The chokes are in series with the motor to block electrical noise coming from the motor. The capacitors are in parallel with the motor to "short out" electrical noise from the motor. Basically manufacturers of electrical things just add components until their product passes EMC testing (and still works).

Hopefully the decoder will perform this function on its own so the PCB is not required if you install a decoder.
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Rob

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Bigmet » Tue Jul 23, 2019 8:41 am

I was interested to test this last aspect when DCC arrived on the layout. The decoders were very efficient at blocking any motor noise from reaching the rails, far more effective than the supplied suppression on those same locos in original form running on DC.

If the loco is going to be 'DCC forever' might as well strip all the suppression components out when fitting decoders.

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Gordon H » Tue Jul 23, 2019 12:42 pm

Bigmet wrote:I was interested to test this last aspect when DCC arrived on the layout. The decoders were very efficient at blocking any motor noise from reaching the rails, far more effective than the supplied suppression on those same locos in original form running on DC.

Just wondering what type of test you performed for this?
The subject has been discussed ad nauseum on various forums, yet (to my knowledge), no-one has been in a position to quantify the effect.

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Ironduke » Tue Jul 23, 2019 1:20 pm

I imagine you could use an old fashioned AM transistor radio to detect EMI and probably an oscilloscope on the rails and the power supply would give you a visual indication. It would at least give you the ability to make a comparison.

Proper EMC testing really requires a lab environment. Here is an article. I work at an electronics manufacturer and we have to send our products elsewhere to get tested. It usually takes a few attempts and minor design tweaks to pass. Not really my area though.
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Rob

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Bigmet » Tue Jul 23, 2019 3:56 pm

Gordon H wrote:
Bigmet wrote:I was interested to test this last aspect when DCC arrived on the layout. The decoders were very efficient at blocking any motor noise from reaching the rails, far more effective than the supplied suppression on those same locos in original form running on DC.

Just wondering what type of test you performed for this?
The subject has been discussed ad nauseum on various forums, yet (to my knowledge), no-one has been in a position to quantify the effect.

My Pa is an electronics engineer, shared his oscilloscope with me when I wanted to tinker. Subtract the output of the DCC system track feed from the power leads of the decoder with the motor going full chat, practically no residual, just millivolts of random noise. In DC form the worst of the locos tested was putting 25V spikes on the rail!

There's minimal technical interest in the UK hobby, outside circles like MERG. That would be the organisation to join if you want to explore this aspect.

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RAF96
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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby RAF96 » Wed Jul 24, 2019 9:52 am

I had read somewhere about using an AM radio to listen for circuit noise (unfortunately I mislaid the frequency I used but easy enough to twiddle the dial).

It definitely worked as I came near to my PSUs with the radio there was obvious ‘leakage’ heard.

One wag on the forum said it was a good way to see if your cheap Chineses electrical devices were genuine or knock off by the noise they transmitted.

Try it also with you your plug in Powerline type ethernet adaptors, famous in the amateur radio world for spoiling their hobby and which they are campaigning to have banned.
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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Gordon H » Wed Jul 24, 2019 12:56 pm

Bigmet wrote:My Pa is an electronics engineer, shared his oscilloscope with me when I wanted to tinker. Subtract the output of the DCC system track feed from the power leads of the decoder with the motor going full chat, practically no residual, just millivolts of random noise. In DC form the worst of the locos tested was putting 25V spikes on the rail!

OK.
Presumably you used a two-channel oscilloscope, with the two channels connected to the red and black decoder power input wires, with 'Subtract' enabled, to see the difference signal. I was just wondering if you had access to a spectrum analyser to get a more quantifiable result.
There is, of course, a considerable difference between a DCC set-up like this and the 'conventional' DC scheme, namely the track power source. The DCC booster driving the track has a very low impedance output which is effective bi-directionally and therefore attenuates RFI well, whereas most DC controllers might only appear as low impedance in the selected direction of current flow, i.e. any interference coming back of the opposite polarity would not see so much of a load, and therefore not be suppressed so well.

There's minimal technical interest in the UK hobby, outside circles like MERG. That would be the organisation to join if you want to explore this aspect.

I have been a MERG member for well over 30 years, hence my interest in anyone else's findings on the subject.
I do have access to an accredited EMC testing facility at work, but the opportunities to use it for things like this are few and far between unfortunately. :(

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Bigmet » Wed Jul 24, 2019 1:56 pm

Gordon H wrote:
Bigmet wrote:... I was just wondering if you had access to a spectrum analyser to get a more quantifiable result...

That would be fun. I had access to 'all the gear' when working, but retired now; and the labs have closed too, so no getting in on a day pass...

Worse yet, my Pa's excellent storage scope has gone missing. He abruptly developed vascular dementia and has had to be placed in a care home: and when I went to look for it when shutting the house up 'there it wasn't'. It's a little hard to miss half a hundredweight of Cossor on a lab stand, so I guess he had loaned it out to someone - which he did quite frequently. Who though? He couldn't remember and I couldn't find out...

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Gordon H » Wed Jul 24, 2019 4:13 pm

Forgot to ask - did you have the loco on a rolling road when doing your measurements, as I suspect that is the only practical way of having the loco in action with scope probes attached. :)

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Bigmet » Thu Jul 25, 2019 9:03 am

Gordon H wrote:Forgot to ask - did you have the loco on a rolling road when doing your measurements, as I suspect that is the only practical way of having the loco in action with scope probes attached. :)

Heavens no, I dislike rolling roads as a complete waste of money. I always want to see performance on the rails, because that's where the loco has to operate. Yard of track and flying leads to the mechanism slung overhead. Only need a few milliseconds for the sample.

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RAF96
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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby RAF96 » Thu Jul 25, 2019 1:08 pm

The problem with a rolling road is you cannot easily load the loco, except by pressing down on it. Running a loco on a flat track up against a buffer will load it but at risk of overheating the motor.

An example of how little load a rolling road imparts is my old Class 56, which would run sweet on the rolling road but couldn’t summon the power to drag itself along a track.
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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby End2end » Thu Jul 25, 2019 1:51 pm

I agree when it comes to rolling roads. Too damned expensive for what they are.
It's the other reason I bought this off ebay FOR 99P!!!
Image
The initial reason being that I was going to build an end to end layout and would need to run-in new loco's, although now my plans have changed.
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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby Peterm » Thu Jul 25, 2019 11:11 pm

End2end wrote:I agree when it comes to rolling roads. Too damned expensive for what they are.
It's the other reason I bought this off ebay FOR 99P!!!
Image
The initial reason being that I was going to build an end to end layout and would need to run-in new loco's, although now my plans have changed.
Thanks
End2end

Didn't you do well. :) I haven't a clue how much that amount of track would be, but definitely more than 99P.
Pete.

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Re: Capacitors, chokes & mysteries - Bachmann "Hall"

Postby End2end » Thu Jul 25, 2019 11:14 pm

Sometimes you just have to embrace "collection only" especially when it's just down the road. :lol:
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