# Starting a decoder equipped loco sooner?



## Jerry McColgan (Feb 8, 2008)

I have a LGB 21791 Corpet-Louvet (with a factory installed decoder) that I am using with a LGB 4135s (analog) sound car under analog track power. 

The sound car starts chuffing before the loco starts moving. 

If I reprogram the decoder to start moving the loco at a lower voltage will the loco also start at a lower analog track voltage? 

Since the sound card does not have a decoder or chuff control I cannot reprogram it (except that I may try changing the chuff start rate by adding a resistor). 

The simplest solution seems to be that if I can get the loco to start at a lower voltage it might better match the chuff rate of the sound card. 

The instructions for the 21791 Corpet-Louvet show: 


CV2 Starting voltage (0-255) [5] 
voltage for speed setting 1 - if loco starts only at a higher speed setting, increase value. 


This suggests to me that changing CV2 to a higher number would start the loco at a lower voltage and that setting CV2 to 255 would start it at the lowest possible voltage. Is this correct or do I have it backward? 

Is there a negative side to doing this and would it affect operations in both analog and MTS? 

I don't want to "just try it" because similar efforts with other locos have resulted in unexpected problems that were hard for me to get rid of. 

Thanks, 

Jerry


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## ntpntpntp (Jan 14, 2008)

Hi Jerry 

sorry, but I think you have got it backward a little! CV2 is all about setting the minimum voltage for dcc speed step 1, as a fraction of the maximum volts supplied to the decoder. A motor needs a certain minimum voltage to turn over, and so you can adjust this value until the loco starts moving smoothly at dcc speed step 1. Increasing the value of CV2 will simply make the motor turn faster at the slowest speed step. 

Your problem with the sound unit starting before the loco moves under analog dc power is more likely down to the loco decoder needing to see a certain voltage in the track before it "wakes up" and starts to drive the motor, and in this case the sound unit is already getting enough volts to start chuffing. 

Probably not much you can do about this in terms of adjusting the loco decoder. Some brands of decoder seems to "wake up" at slightly lower dc voltages than others but there's not a lot in it. 

More likely you need to introduce some sort of voltage reduction into the power feed to the sound unit so that this then requires more volts before it starts running? 

Nick


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## Jerry McColgan (Feb 8, 2008)

Hi Nick, 

That is what I suspected since all decoder equipped locos seem to need a higher startup voltage. 

My next step will be to add a resistor to the input of the sound unit which should require more volts to start it. I'm just concerned that while I may start the chuffing at a higher voltage I don't know what to expect at the high end of the chuffing. 

I bought some resistors at Radio Shack today to play with. 

Thanks, 

Jerry 


Posted By ntpntpntp on 06/17/2008 2:54 PM

More likely you need to introduce some sort of voltage reduction into the power feed to the sound unit so that this then requires more volts before it starts running? 
Nick


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## SlateCreek (Jan 2, 2008)

Jerry, 

You may find it easier to use diodes ... a pair of diodes arranged back to back (which allows current flow in both directions) will drop current measurably (usually about a volt and a half) per diode (pair... depending on direction, you're really only using one or the other.) So, a set of two (pair) gives you three volts drop, three gives you four and a half, etc. 

I've forgotten the reason, but many folks seem to prefer this method. 

Matthew (OV)


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## Greg Elmassian (Jan 3, 2008)

if you only use diodes in one direction, than the loco will only operate in one direction, because power will only flow in one direction (polarity). 

you need 2 parallel strings of diode "drops". 

Jerry, measure the voltage that the loco starts moving at, and the voltage the sound unit starts at, and you can try to build up the difference with diode drops. 

Unfortunately, you may have some difficulty isolating the power inputs to the sound decoder. You only need the diodes on one "side" of the power input. 

Regards, Greg


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## Jerry McColgan (Feb 8, 2008)

Hi Greg, 

Only the loco has a decoder. I am running on analog track power and the sound unit is from a LGB 4135s sound boxcar and is analog (chuff only) with no decoder. 

Thanks, 

Jerry 

Posted By Greg Elmassian on 06/21/2008 9:06 PM
if you only use diodes in one direction, than the loco will only operate in one direction, because power will only flow in one direction (polarity). 
you need 2 parallel strings of diode "drops". 
Jerry, measure the voltage that the loco starts moving at, and the voltage the sound unit starts at, and you can try to build up the difference with diode drops. 
Unfortunately, you may have some difficulty isolating the power inputs to the sound decoder. You only need the diodes on one "side" of the power input. 
Regards, Greg


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