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Power Supply Question

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Alan:
This nice little article arrived in my Inbox today. Very pertinent to your test track power selection. Have a read: https://www.electronicspoint.com/resources/power-supplies-common-terms-and-limitations.55/?utm_source=newsletter&utm_medium=email&utm_campaign=February

nodcc4me:
Interesting read. I hooked my meter onto the test track. The old MRC Trainpack was initially putting out 13v. I placed a locomotive on the track and the voltage remained the same until I turned on the prime mover. Then it dropped to 10-11v. The HC Sim was showing a low voltage error, as it should. I'm thinking the regulated wall wart won't drop like that and will solve the problem.

The Trainpack has a speed control lever that has no effect on the track voltage, So it must change the current. Yet it has no effect on the RP equipped loco, which is being controlled by the LM module. Does that sound about right?

Alan:

--- Quote from: nodcc4me on February 02, 2017, 12:01:12 PM ---Interesting read. I hooked my meter onto the test track. The old MRC Trainpack was initially putting out 13v. I placed a locomotive on the track and the voltage remained the same until I turned on the prime mover. Then it dropped to 10-11v. The HC Sim was showing a low voltage error, as it should. I'm thinking the regulated wall wart won't drop like that and will solve the problem.

The Trainpack has a speed control lever that has no effect on the track voltage, So it must change the current. Yet it has no effect on the RP equipped loco, which is being controlled by the LM module. Does that sound about right?

--- End quote ---

Other than a few Tyco train set power packs, the only MRC power pack I have experience with is the old Model 500 of which I had several. My response is based on that experience.
 
Sounds right... to a point. An unregulated and unloaded 12v rated power supply will measure 13v or more often 13.8v. As long as a minimum voltage is maintained (somewhere around 9v), the LM functions. Unlike your Trainpack, a LM uses pulse width modulation to control speed. Pulse frequency, not voltage (directly) dictates speed. The LM is compensating, so to speak, for the position of the Trainpack lever. I would expect to see the voltage fall below the minimum LM requirement when the speed lever is moved to zero or near zero. Old school train power packs had rheostats with not much resistance difference between full on and almost full off. This was because of the required minimum starting voltage to get trains moving. The rheostat wiper would be completely off the winding when in the full off position. It's possible that even at the lowest speed lever setting there is still not sufficient resistance in the rheostat to prevent the LM from falling below its minimum working voltage.

Fun fact: My old 500's had impulse power. The switch simply changed the diode arrangement from bridge rectifier configuration to single diode half wave configuration. That baby really buzzed (at 60Hz to be exact) when the switch was set to impulse. As a kid I used to imagine all the buzzing going on were my prime movers working hard. Ah, simpler times.  ;D

A regulated wall wart will have some minor voltage fluctuation with a change in loading. They are cheap consumer devices, not precision instruments. However, the change will be minimal compared to your unregulated Trainpack and should have no measurable effect on loco operation. The key advantage for you is wall warts are available in 15v which is picture perfect for RP.

Alan:
Found a picture online. Conjured up great memories for me from a long time ago. Reminded me again why I play with trains today.

Alan:
Sorry, forgot to address one of your comments...


--- Quote ---The Trainpack has a speed control lever that has no effect on the track voltage, So it must change the current.
--- End quote ---

Your Trainpack is an unregulated power supply. It is a passive device. The voltage and current change (inversely) depending upon load and rheostat position. The article is about regulated power supplies only. The article applies to wall warts.

Electrically, this is your Trainpack and a locomotive:



With R1 being the speed lever and R2 the locomotive. R1 changes with lever position, R2 changes with loco speed and load. Ohm's law states the circuit current is determined by R1 + R2. It also states voltage drop across R2 = V - R1 drop. Since both R1 and R2 continually change as you operate the train there will also be changes in voltage across and current through R2 - your locomotive.

A regulated power supply will either: A) maintain a constant voltage regardless of R2 value; or B) limit the total circuit current to the value produced by R2. Voltage regulation is important to RP. Current regulation is not needed in the power supply as the LM already does this (loco stall setting on your HC). It is a misnomer of sorts to say current regulation. It is more accurate to say current limiting. Any device will only pass as much current as its internal R determines. Current regulators are for use with devices that have low R but not the ability to handle the current (heat) that such a low R would produce. LEDs are prime examples. Model train motors are bulky sturdy devices with the ability to dissipate a lot of heat. They only have a problem dissipating heat from excessive current when the R falls to very low. And even then it takes time for the heat to build up. The HC stall setting, AKA power supply current regulation/limiting, keeps the current below the critical point for the loco motor during low R instances (stalled or extreme loading).

Does that help?

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