Why is there DC voltage coming from variable TIU channels

The tiu has 4 channels, Fixed 1.. fixed 2... var 1 & var 2.

When you mention channel 2 I'm not sure what you mean?

What are your power supplies?

Gregg,

TIU has 4 channels, 1 - 4, labelled as you describe.  Channel 1 is Variable output 1, Channel 2 is Fixed output 1, Channel 3 is fixed output 2, Channel 4 is Variable output 2.  I am using a ZWL to power each channel from each output of the ZWL.  I do use a common ground.

Can you be a bit more specific when you say two channels read DC and 2 AC?  I use a Fluke but not the model you mention.  I have nothing that says AC or DC, so if I don't know what I've got, I try both AC & DC settings.

What transformer(s) configuration are you using?

Fix channels pass input power directly through to the output.  THe VAR channels us power FET to control both halfs of the input power.  That is why the output is controllable for VAR and Not for Fixed.  What level is the DC component?  Do you have voltage control of the variables?  If you swap a Var input source to a fixed channel, does the syptom stay on the VAR channel or move to the fixed with that power supply.  G

Steam Loco Greg posted:

I noticed a few early legacy engines losing sound in the tender on my inside loop which is on tiu ch 1, variable 1. 

Some of our Legacy steam lose sound if it has a momentary power interruption, either hot or common, which I was told is caused by Lionel changing the sound software. Are they crossing a switch from loop to loop?  (Our old  TMCC equipment just picks up again as if nothing happened.) Does it come back if you do a quick reset? Not a change I like, had to buy some of John's "Last Battery" solutions from Hennings!

The other 2 loops on ch 2 and 3 fixed voltage.  I checked voltage on all tracks.  The fixed channels read AC and the variable read DC.

You are saying the TIU is changing AC into DC?

  Changed the inner loop to a fixed channel and legacy tender had sound, just like the fixed loops.   And the track now read A.C..   

 

Thanks for the responses.  Some more info based on questions:

• I am using a ZW-L Lionel transformer.  
• The 2 Variable channels on the TIU (Rev L) are the ones exhibiting the DC measured voltage (note, this does not necessarily mean the voltage is pure DC, but modified by the variable channels in such a way that the Fluke Meter registers as DC Voltage).
• I have gone from one track to another (tracks are isolated) and lose the sound on the transition.  Reset does not bring the sound back (note, this only happens with 2 engines I have, both Legacy, Santa Fe Northern and UP Challenger).  Also, if I run the engine on the inner loop (the one with the issue) I do not get sound at all.  Note, I put a Berk Tender on the Santa Fe Northern and the sounds were fine.  So something in the circuit on the Northern tender does not like the variable voltage on the TIU variable channel.
• GGG, the issue follows the variable channel of the TIU.  If I remove the TIU from the system entirely, the sound of the engine is fine.  If I run on one of the fixed channels the sound is fine from the tender.
• GGG, the Variable channel is reading 20 V DC with no or little AC component (.1 or so).  On the fixed tracks I read 20 V AC with no DC component.
• GGG, I can vary the voltage on the variable channels, what I have not tried is using the handheld to set it to a fixed channel (not sure electronically if that would have an impact).

Any more insight is greatly appreciated.

I'm not sure what engine you have, but some can run on DC and some can't. If you have DC on the track, I would not run the engine, unless you are sure it supports DC. It sounds like it doesn't, since you get no sound.

This is an odd problem. I thought the input power dictated what power came out of the outputs of the TIU. Have you checked the input power from the ZW-L?

George

Why not try setting the var to fixed with Menu/System/ dcs set up/ select channel and then FXD.

There is a slight delay powering up.

GGG posted:

Fix channels pass input power directly through to the output.  THe VAR channels us power FET to control both halfs of the input power.  That is why the output is controllable for VAR and Not for Fixed.  What level is the DC component?  Do you have voltage control of the variables?  If you swap a Var input source to a fixed channel, does the syptom stay on the VAR channel or move to the fixed with that power supply.  G

George,

Is the FET a MOSFET? Can it be that the FET went bad and is converting AC to DC? MOSFETs are capable of doing that. I may be way out in left field here...

George

To me this is weird get full DC voltage.  If you can raise and lower the output of the variables and turn voltage off with the DCS remote with the Input at full AC voltage, the FETs are working.  There is a TVS across these FET, I an not sure just speculating if a TVS Some how failed and became a diode passing rectified DC.  But I think you would not have full voltage.  WHat happens if you read the output on a AC setting?  G

G, if I read it on AC I get about .1 V ish.  I am thinking about purchasing a cheap hand help oscilliscope.  Any good recommendations for a cheapie that can help me investigate this?

Thanks,

G, All, did some more sleuthing this evening and I believe I isolated the issue.  I deleted and re added my variable tracks.  My Var2 track works fine, is on AC power and I can control thru the DCS remote.  On the other hand, Var1, I cannot control with the DCS remote and it reads 10 V AC and 10 V DC.  I believe the diode may be bad on channel 1 causing the issue.  Looks like I need to get Var 1 channel repaired.  G, I will be getting in touch in a while to ask for your services.

Thanks all,

Ok, Now that makes sense.  G