Question about TTS decoder. Possible malfunction?

[DCayón]

Hello everyone,

Back in 2015 I bought BR class 8P "Duke of Gloucester" with TTS sound (Ref. R3244TTS), and I have decided to install a Seuthe 22 smoke generator activated by a 12V subminiature relay.

But before that, I tested voltage between different pins and the common (nº7, blue one) to know which one was the best. This was the outcome:

Pin 6, front light F0, white cable -> 17.4V both activated and deactivated.

Pin 2, rear light F0, yellow cable -> 17.4V both activated and deactivated.

Pin 3, aux. hardware F18, green cable-> 17.4V when activated, 14.3V when deactivated.

I expected all of them to be 0V when deactivated, or at least pins 6 and 2 to change like pin 3 (I have also checked them changing the loco's direction). What I wanted to know is if this behaviour is normal or not, or if I am missing something obvious.

Besides that, no matter which function pin I connect the relay to, it will not activate its coil. And I know the relay works correctly because I have checked it with a potentiometer and a simple LED circuit.

Thank you very much in advance.

[96RAF]

DCC voltage entering the decoder is usually around 15vAC (not real AC but a bi-polar square wave potential).

Blue is a common positive provided by the decoder usually at 12V. White, yellow and green are switched floating negatives sitting at 0V. These become true negatives for the Blue wire when activated. Current limits across each of the switched circuits is 100mA with a total limit imposed across the Blue wire in conjunction with the added motor load. The total decoder load is 800mA.

White and Yellow are used for directional lights, hence when activated by F0 they are also direction dependant. White will drive the loco front white lights and rear red lights when the loco is going forward, Yellow will drive the loco front red lights and rear white lights when the loco is going in reverse.

Green is activated by F25 on a TTS decoder.

Be aware that motor loads are self-protecting against overload at 500mA, but the function outputs are not and any overload on them is likely to damage the decoder.

It is unwise to apply a direct voltage to the motor circuit when a decoder is installed as this can damage the H-bridge drivers in the decoder.

For your application you should connect the relay coil between Blue and Green (switched by F25) and run the smoke unit from the relay main contacts using the 15vAC rail voltage seen on the red and black pins suitably rectified. It is presumed the relay pull in is less than 100mA.

[Chrissaf]

Just to add to Rob's reply.

In it he wrote:

Quote "White, yellow and green are switched floating negatives"

The key word here is "floating". This means that when taking voltage measurements you need a load to pull the power line down to the zero voltage rail. This zero value, of course, can only happen when the appropriate F command has been issued to enable that function in conjunction with the physical load being present. Without a load very odd voltages can be measured as your experiments indicate.

Personally, I would also put a bridge rectifier between the DCC track voltage and the smoke unit that is being switched by the relay contacts. Putting the smoke unit directly across the DCC track voltage could corrupt your DCC signals.

[ColinB]

The function outputs of the TTS decoder when they are switched on basically put a ground on to what ever you have attached to it. So in the case of leds the positive goes to the common which is a voltage, via a resistor and the negative is switched to zero by the function. When the function is switched off, the connection to ground no longer exists so the led goes off.

[Rog RJ]

It is also recommended to fit a diode across the relay coil connections to protect the decoder from high voltage bemf spikes when the relay operates and releases.

[DCayón]

Thank you all for your replies and your advise.

I think I clearly know how to connect the smoke generator via rectifier and the relay main contacts (I intended to follow this scheme:

https://www.brian-lambert.co.uk/DCC_Page_1.html#Bookmark15)

but my problem remains in the pins, and it is getting a little bit frustrating.

I have repeated the experiments with a 2Mohm resistor between the blue common and the lights and aux pins (acknowledging they are floating voltages), getting the same values as before. I have also connected a LED between blue and green in series with a 330ohm resistor, which glows no matter the aux function is activated or not, but showing 3.35V between the pins when active, and 3.15V when not.

In addition, I have connected the relay coil the same way (but without resistor), and nothing happend. I am using a ROCO Z21 system, and this relay:

https://www.farnell.com/datasheets/2626620.pdf

Is this a problem with my decoder, or am I just missing something?

Thanks again.

[96RAF]

It is possible your previous attempts at measurements have damaged the decoder.

You can try a full reset by writing value 8 to CV8 which may get the decoder back on its feet.

This diagram tells of the basic block flow of a decoder fitted with a stay alive. A sound decoder simply has an amplifier and speaker added interfaced with the main processor to operate sound and external functions.

[DCayón]

It is possible your previous attempts at measurements have damaged the decoder.

You can try a full reset by writing value 8 to CV8 which may get the decoder back on its feet.

 

 

Just did that, but nothing has changed except for the address number which returned to default nº3. (I previosuly changed it to 4). To clarify the things a bit, if I test green and blue with a 2Mohm, and the function is deactivated, should I get 0V on my tester?

[Chrissaf]

A 1K to 5K resistor is more likely to work than a 2Meg Ohm one, that value is just too high.

With the function deactivated you're unlikely to read 0 volts because the semi-conductor for that function line (green) will not be turned on, thereby the green wire will be a floating voltage and give strange readings and not providing a path to the zero volt rail.

You need to activate the green wire function [F25 as previuosly advised for a TTS decoder]. This on a fully working decoder will switch on the semi-conductor associated with the green wire function [see the image in Rob's previous post].

With the green wire function enabled and on a good working decoder the green wire will be connected to the zero volt rail of the decoder via the semi-conductor switching function. You should then read a voltage of about 12 volts, but no higher than 14 volts and no lower than 10 volts between the green wire and the blue wire with the blue wire being positive. If this test fails, then the indications are that the TTS decoder is faulty.

[DCayón]

A 1K to 5K resistor is more likely to work than a 2Meg Ohm one.

 

 

I have tried, it does the same but between 4V and 4.7V

[96RAF]

I would be setting this up using a breadboard for the external components with a decoder test rig in circuit. Most test rigs have terminals for attaching free tails so you can slave in some leds to prove the function circuit(s) then patch in your relay in lieu. Once the relay is pulling in reliably from the decoder then lash up your smoke unit with its bridge rectifier from a DCC supply to fully prove the circuit.

[DCayón]

I am on a trip tomorrow, when I return home in several days, I will try more things.

Kind regards.