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First model train and using MTH HO DCS exclusively.

Reference Barry’s DCS Companion 3rd Edition.

Page 57, 3rd Paragraph from bottom suggests that “Variable channels may not use DCv and is not suitable for MTH PS3 HO.”

Question: Is that true when a Variable Channel is changed to Fixed Chanel?  That is, if I change both of my TIU Variable Channels to Fixed Channels can I use 18vDC in/out as in normal Fixed Channels 1 &2?

Page 67 shows a Geographic Track Block Diagram powered by all four TIU Channels. 

Question:  I have two DC power supplies with adjustable DCv and Amp controls VOLTEQ 1810HD ( ).  I can set both power supplies to 18volts DC at 10amps and monitor their outputs with DCCSpecialities RRampMeters.  Can I use one power supply for two channels, Variable 1 and Fixed 1, and the other power supply for Fixed 2 and Variable 2 (BOTH VARIABLE Channels 1 &2 software changed to FIXED Channels 1 & 4)?  Do I need to be concerned that a locomotive, crossing from one block to another, might cause “bridging” between power supplies and result in a disaster?  I will also have DCCSpecialties PSX Circuit Breakers between Power Supplies and TIU inputs.




Last edited by Douglas Duncan
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1. The fixed channels pass-thru the DC power from input to output - red-in to red-out, black-in to black-out.  The variable channels use transistors switched on/off synchronized to the incoming 60 Hz AC cycle to chop the input voltage to create a lower output voltage.  When you set it to fixed (no variable reduction), the TIU must still turn on its transistors on each cycle; there is no relay or whatever to simply pass-thru the input to output.  OK.  Now the question is what the TIU does if it does not see the input AC cycle and only sees DC.  Since it is looking for the AC cycle, will it wait forever and never turn on its transistors?  I don't know the answer but I suspect it would do just that and you'd get no output whatsoever. 

However.  You might look into the passive mode where your power supply is connected to the TIU variable channel output connectors (nothing connected to the TIU variable channel inputs).  You'd have to confirm passive mode works for DC voltages (I don't see why it would not but one of the MTH gurus might enlighten us).

2. As I see it the "bridging" issue has nothing to do with the TIU, DCS, etc..  It's simply a matter of how your particular power supplies behave if connected together.  As you say, there will be instances where an engine with multiple power pickups momentarily straddles two power districts.  The engine may even be intentionally stopped with pickups on each side.  So it's like shorting the two power supplies with 10 ft or 20 ft wire (the length of track to the straddle point).  Power supplies can be designed to handle such situations differently - though if you have identical designs both dialed in to the same voltage this no doubt be the least stressful scenario.


I REALLY appreciate your replies!!  Looks like my best starting strategy should be to use one Power supply and just Fixed 1 & 2.   I have seen diagrams and text for passive mode in Barry's books and elsewhere.  If there is confirmation that Variable, set to Fixed, will work with DCV in passive mode, perhaps I would use one power supply for Fixed 1&2 and the other power supply for Variable 1&2 set to Fixed.  As you might guess, I always do things the hard way.

Thank you and Happy New Year!!!!!!!!!!!!!!

As you study passive mode, you'll see the main reason guys do this is to support higher currents that you can't run thru (from Input jacks to Output jacks) the TIU channel.  It occurred to me that one application where guys need higher currents is in large-scale which is primarily a DC environment.  This may be weak reasoning but does give some hope that passive mode DC on a variable channel is OK.  Otherwise, DC operators are essentially paying double for a TIU since they would only be getting 2 DCS channels instead of 4.  Perhaps one of the large-scale operators might see this thread and comment.

Separately, I notice you're using a PSX breaker.  I thought one claim to fame of this device is it's ability to handle the complex AC power waveform encountered in DCC.  Do you really need this capability if using DC?  Also, if you end up using passive mode, see various threads (search OGR for threads with terms "PSX" "choke" "passive mode") about inserting a 22 uH high-current inductor/choke between the PSX and the track.  Apparently the PSX when connected directly to the track can materially affect the DCS signal from a TIU output.  The choke demotes this effect.  You do not need this choke if the PSX is placed on a TIU Fixed channel since the choke is effectively built-in to the TIU.

Also, it looks like you are planning to use meters that are capable of reading true-RMS on DCC signals.  This is fine but somewhat overkill if you have pure DC.  There are many many Volt and Amp meters that work on plain DC for a fraction of the cost of a true-RMS meter.


Hello Stan:

I will continue to seek clarity on use of DCV, going in, around or through Variable/Fixed channels 1&2.  I'll have to try to diagram a passive mode configuration using one power supply for the pair of Variable channels set to Fixed, using a Passive wiring around the Variable Inputs, and use the second Power Supply for normal DC in/out of the Fixed 1&2 Channels. 

I am in contact with an engineer of PSX gear regarding use/need for a choke (I am not electrically competent/connected to know anything about circuits, etc.).  It is my understanding that there is no DCS signal degradation issue if the PSX breaker is placed between the Power supply and the TIU Input.  Apparently there is a potential issue if a PSX breaker is placed after the TIU Output.  The PSX engineer was under the impression that MTH HO DCS was an ACv system because he has (OGauge?) clients using ACv for which he has PSX AC breakers.  He does have PSX for DCv as used for other HO DCC systems.  (I think I am reporting correctly?)  He is researching the use of PSX on DCV MTH HO DCS systems as regards use of breakers and auto reversers.  If/when I have any info I'll report here.

My thought on the use of the RRampMeter was/is simply a caution to make sure the power supplies at set at "identical" values, thinking that would mitigate any issues?  Again, I claim complete ignorance!!


Douglas Duncan posted:


My thought on the use of the RRampMeter was/is simply a caution to make sure the power supplies at set at "identical" values, thinking that would mitigate any issues?  


I'd think the digital readout on your power supplies would be good enough to set the different supplies to "identical" DC voltage values.  Just my 2 cents...


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  • GPR-1810HD

Hello Again:

Stan noted "where guys need higher currents is in large-scale which is primarily a DC environment.  This may be weak reasoning but does give some hope that passive mode DC on a variable channel is OK."  

However on page 120 of Barry's 3rd Edition Companion, middle of page, there is the following:

"Since TIU Variable Channels operate by regulating voltage from transformers to the tracks, it is not possible to use Variable Channels when wired for Passive TIU Mode of Operation.  Again, this is because track power does not pass through the TIU under Passive mode of operation."

I have to say, as a novice, I guess I do not understand the purpose of setting Variable channels to Fixed.  My thought was using a Variable Channel as Fixed meant they replicated the function of normal Fixed Channels and, as I would be using using a fixed output at 18VDC from my power supplies, I do not need to vary the voltage.

So, my remaining question:  Is there a method for using all four TIU channels as DCV sources for DCS MTH HO layout (using two power supplies)?


Note that I have only used AC on my TIU's in passive mode.  I use DC on a fixed channel on the bench for loading sound and chain files.  I don't know how they're react to passive connections with DC, though I suspect it should be the same as AC.

In passive mode, the setting of the variable channel to fixed mode should have no effect, so I don' think that's an issue.

FWIW, I think I'd have a 22uh choke between the power supply and the track, the TIU output should NOT go through the choke.  I'm guessing the DC supply will have a pretty significant capacitance, and that likely will affect the DCS signal since you're impressing it on the output of the DCS signal generator.  Here's three that should be suitable: High Power 22uh Chokes

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