Resistor required for TTS decoder in a Heljan OO loco

[digger62]

Hi.

I'm looking to fit a Hornby TTS decoder to a Heljan loco and understand that it is likely to fry the chip. However I have read that you can use the TTS decoder to piggy-back another decoder so that it just provides sound and the original powers the motor and lights.

My question is around the resistor that is required on the TTS decoder to provide resistance against the motor wires. I have no idea what value it would need to be.

Does anyone have any idea?

As always many thanks for looking.

Mark.

[Crosland]

The resistor is only needed to allow CV reading on a programming track (service mode). Writes can still be done without the resistor but your system will tell you there was an error. You would need to disconnect one or other decoder every time you want to change anything in service mode, unless both decoders support decoder locking.

 

If you can cope without reading anything back from the decoders then give the two decoders different addresses and use "ops mode" or "on the main" programming to tweak the sounds.

 

If you really want to permanently fit a resistor then make sure you disable the motor drive by setting Vmax to a low value, or setting appropriate values in the speed table. If you ever inadvertently try to drive the loco this will stop the resistor getting too hot and possibly causing damage. The decoder responds to programming commands by briefly turning the motor output on full. To be able to read CVs you need a current of at least 60mA to flow.

 

Using ohms law with, e.g., a 15V track voltage you will need 15/.06 or 250 ohms. I would use 100 - 150 ohms to increase the current and give a better chance of success. 100 ohms would give 150mA.

 

That will lead to a power dissipation in the resistor of (I*I*R) 0.15*0.15*100 or 2.25W with 100 ohms. A lower power 1W resistor will be fine for the very brief pulse use during programming, but not if you attempt to drive the decoder at full speed, hence the comment about Vmax, above.

 

 

[digger62]

Many thanks for your very informative response. I'll have a think and decide which way to go. 

[RAF96]

Unfortunately TTS does not support changing CV5 - Vmax, it is essentially a basic Hornby R8249 decoder with sound.

Note that any resistor across the motor pins will get hot as soon as you throttle up so unwise to leave one fitted after programming (if used).

[Izzy]

Can I ask if this is a steam or diesel loco? I presume the latter. I ask because you can’t really piggyback the TTS steam decoders, they need a motor in circuit to work properly. No Sync of the chuff etc if you do, I’ve tried.

 

As far as diesels go it’s fine, and with most you can adjust the notch levels if there is too much disparity with the motor control in the decoder driving the loco. You do need to give the TTS decoder the same acceleration and deceleration rates as the other one. 
 

You do not need a motor in circuit for the TTS to do Prog on main. With so few cv’s that can be altered anyway I can’t see any real requirement to adjust them once initially done, so based on my experiences I would suggest you hook up the TTS first to the track feeds, set what you want by POM including address, and then add into the loco hooking up the track connections once you have set up the ‘main’ decoder.
 

[RAF96]

Generally you cannot set an address using PoM, although knowing the existing address I have managed to write blind to CV1 effectively setting a new short address.

[digger62]

Thanks Izzy and RAF96. It's a class 47 and I've plugged the TTS decoder into a class 31 set and matched the address, acceleration and deceleration to the decoder in the class 47. I take what you say and will not bother with the resistor. Does that mean all I need to do is connect the red and black power pick up wires on the TTS decoder to the ones on the motor decoder?

Once again really appreciate your kind help and advice.

Mark.

[Izzy]

On 30/05/2021 at 09:04, RAF96 said:

Generally you cannot set an address using PoM, although knowing the existing address I have managed to write blind to CV1 effectively setting a new short address.

Ah. Maybe this is because I use a MRC/Gaugemaster Prodigy and with that you can only POM to a known address, an individual loco, and not the system as a whole, which I believe is often the case with other makes.

[Izzy]

19 minutes ago, digger62 said:

Thanks Izzy and RAF96. It's a class 47 and I've plugged the TTS decoder into a class 31 set and matched the address, acceleration and deceleration to the decoder in the class 47. I take what you say and will not bother with the resistor. Does that mean all I need to do is connect the red and black power pick up wires on the TTS decoder to the ones on the motor decoder?

Once again really appreciate your kind help and advice.

Mark.

Yes. In fact you can run a TTS (diesel) decoder without it being in anything, just hooked up to the power. Need the speaker attached of course.........    Generally, you can POM to adjust many of the TTS cv’s, sound level etc, because they use numbers higher than are used in many other non-sound decoders AFAIK, certainly the ones I have. You don’t of course have to bother with faffing around with the motor control ones. So you can write to them using the same address as the other decoder without also affecting it. Having the same address IMHO, is far easier than consisting the two decoders. 

[spamcan61]

I would think the challenge here is that TTS decoders are rated at 500mA maximum continuous current and a Heljan Western might well exceed that. I don't see how a resistor helps unless you're happy with a slow running loco.

[JohnR]

2 hours ago, spamcan61 said:

I would think the challenge here is that TTS decoders are rated at 500mA maximum continuous current and a Heljan Western might well exceed that. I don't see how a resistor helps unless you're happy with a slow running loco.

 

The OP is wanting to add the TTS decoder as well as a conventional decoder - the resistor was to give the TTS decoder a motor load for programming. 

[RAF96]

5 hours ago, Izzy said:

Yes. In fact you can run a TTS (diesel) decoder without it being in anything, just hooked up to the power. Need the speaker attached of course.........    Generally, you can POM to adjust many of the TTS cv’s, sound level etc, because they use numbers higher than are used in many other non-sound decoders AFAIK, certainly the ones I have. You don’t of course have to bother with faffing around with the motor control ones. So you can write to them using the same address as the other decoder without also affecting it. Having the same address IMHO, is far easier than consisting the two decoders. 

 

Example is their Vent Van TTS which have the motor control disabled and a hefty resistor fitted as standard.

[digger62]

Just to let everyone know that I fitted the decoder yesterday and it works really well. I upgraded the speaker and I'm really pleased with it as I haven't tried anything like this before. Normally just plug in a decoder and away I go so this has given me the confidence to try other projects.

A big thank you to all who contributed and helped me to complete it.

Mark