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Tortoise Switch Machine, Power Pack "Buzz" MRH product support
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Prolific poster 2,094 posts musgrovejb Prolific poster 2,094 posts
musgrovejb
Have my Tortoise switch machines setup on a DC bus configuration using toggle switches to control the switch machines. The bus is powered using an MRC model train power pack connected to the DC output. Getting feedback, (buzzing sound), from the power pack that increases as the power, (throttle), is increased. (Power to the switch machines is the only thing the power pack is used for) Switch machines seem to be working fine but the buzzing is a little bit annoying. What is causing this feedback and is there a way to correct this?

Modeling Missouri Pacific Railroad's Central Division, Fort Smith, Arkansas

https://youtube.com/playlist?list=PLENIMVXBDQCrKbhMvsed6kBC8p40GwtxQ

 
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Pelsea Prolific poster 2,398 posts
Pelsea

Does the power pack have pulse power?

Some power packs feature pulse width control. This usually gives better low speed performance on DC locos. Some other power packs just have poor rectification. Either way, you might be happier with a simple wall wart. pqe
Temporarily inactive due to annoying but non life threatening medical issues.
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steamfan1211 Regular poster 156 posts
steamfan1211

Slight AC leak

I had the same problem. I read how some DC power supplies leak a slight amount of AC causing the Tortoise motor to try and "bounce" back and forward. Hence the buzz. Solved the problem by using a wall wart which puts out pure DC. No more buzz.

Michael

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LKandO Prolific poster 3,839 posts
LKandO

Buzz

Could be the windings in the transformer. http://www.federalpacific.com/literature/drytrans/10transformernoise.pdf

Quote from All About Circuits web page:

Quote:

Audible noise is an effect primarily originating from the phenomenon of magnetostriction: the slight change of length exhibited by a ferromagnetic object when magnetized. The familiar “hum” heard around large power transformers is the sound of the iron core expanding and contracting at 120 Hz (twice the system frequency, which is 60 Hz in the United States) -- one cycle of core contraction and expansion for every peak of the magnetic flux waveform -- plus noise created by mechanical forces between primary and secondary windings. Again, maintaining low magnetic flux levels in the core is the key to minimizing this effect, which explains why ferroresonant transformers -- which must operate in saturation for a large portion of the current waveform -- operate both hot and noisy.

Another noise-producing phenomenon in power transformers is the physical reaction force between primary and secondary windings when heavily loaded. If the secondary winding is open-circuited, there will be no current through it, and consequently no magneto-motive force (mmf) produced by it. However, when the secondary is “loaded” (current supplied to a load), the winding generates an mmf, which becomes counteracted by a “reflected” mmf in the primary winding to prevent core flux levels from changing. These opposing mmf's generated between primary and secondary windings as a result of secondary (load) current produce a repulsive, physical force between the windings which will tend to make them vibrate. Transformer designers have to consider these physical forces in the construction of the winding coils, to ensure there is adequate mechanical support to handle the stresses. Under heavy load (high current) conditions, though, these stresses may be great enough to cause audible noise to emanate from the transformer.

Alan

All the details:  http://www.LKOrailroad.com        Just the highlights:  MRH blog

When I was a kid... no wait, I still do that. HO, 28x32, double deck, 1969, RailPro
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musgrovejb Prolific poster 2,094 posts
musgrovejb

DC Power Supply

Thanks for the feedback. Sounds like swapping out the power pack with a DC Power plug is the way to go. "Any special configuration needed or can you attach the DC plug wires directly to the bus supplying power to the switch machines"? (Direct solder or terminal strip)

Modeling Missouri Pacific Railroad's Central Division, Fort Smith, Arkansas

https://youtube.com/playlist?list=PLENIMVXBDQCrKbhMvsed6kBC8p40GwtxQ

 
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LKandO Prolific poster 3,839 posts
LKandO

DC Power Plug

Attach the plug output wires directly in place of the power pack. If the wall wart has a connector on the end of the wire, the center pin receptacle is positive and the ring around the outside of the plug is negative. Make sure the wall wart has sufficient amperage to do the job. Not a good idea to continuously operate a wall wart at or near its maximum output.

Alan

All the details:  http://www.LKOrailroad.com        Just the highlights:  MRH blog

When I was a kid... no wait, I still do that. HO, 28x32, double deck, 1969, RailPro
nsparent.png 
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Thomas Hillebrant potlatcher New member 23 posts
Thomas Hillebrant potlatcher

Need a Big Capacitor

I had the same buzzing problem with the single Tortoise I have on my layout that I power with a scavenged wall wart.  Apparently, wall warts are notoriously "noisy" - that is the DC is not fully rectified and continues to alternate somewhat.  I got a suggestion from the MR discussion board to solder a large capacitor across the positive and negative input pads on the Tortoise.  I don't remember the actual rating of the cap, but the one I bought at Radio Shack was about 3/4" diameter x 1-1/2" long.  This made a world of difference.

Tom
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Logger01 Prolific poster 1,786 posts
Logger01

Power Supplies

Many cheep DC power packs are very poorly regulated or filtered if at all and produce an output similar to the signal on the right.

The ripple in this signal is what causes the buzz in these supplies. Ripple can be reduced with appropriate filters (e.g., capacitors) but not eliminated. Wall wart power supplies with poor filtering and or no regulation will exhibit the same problem. To prevent this problem look for a "regulated" DC supply with adequate current capacity.

As Tom mentioned you can try a large filter capacitor, Just make sure that you get the polarity right (positive to positive). But I would go with a regulated 12 VDC wall wart. As noted in Circuitron literature a 500 mA power supply could drive about 30 tortoise, but I would not recommend driving more than about 15 with a 500 mA supply.

Ken K

gSkidder.GIF 
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Pelsea Prolific poster 2,398 posts
Pelsea

Don't count on

the center pin being positive. There's a diagram on the bottom that looks sort of like this - --C-- + Or + --C-- - If it is the latter, the center pin is negative. pqe
Temporarily inactive due to annoying but non life threatening medical issues.
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Thomas Hillebrant potlatcher New member 23 posts
Thomas Hillebrant potlatcher

Capacitor Rating

I looked under the layout and can see that I used a 4700 microfarad, 35V rated capacitor.  This was the better solution for me simply because the wall wart was free (it went to the charger for a cordless drill that had long since died), so I got my power supply for only the cost of the capacitor.

Good luck with whatever solution you choose.

Tom
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