Class 67 TTS Decoder in Bachmann Loco Fail

[BRGreenDieselPaul]

Hi all

I normally use Zimo & ESU sound decoders in my locos but thought I would give the Hornby Class 67 TTS a chance in my new Bachmann Class 66, I know the 67 is not quite the same as the 66 but the sound is close enough for me and under £40 for something that sounds close enough is better than £100 plus for the normal ones, especially as the Non sound 66 Evening Star now costs over £160, I needed to save a little money, hence the reason to go over to TTS which for Diesel sounds is supposed to be as good or near enough to make the huge cost saving worth while.

So to my problem, I used a 12 to 6pin adaptor so the fitting was straight forward, on test the motor function and sound worked well but I noticed an issue with the Loco lights, when set to forward motion all the front and rear lights were on, both red and back but in reverse they worked fine.  Is this a result of trying to install it in a bachmann loco or is it a problem with the decoder.

I did notice that I had used a bit of unshrunk heatshrink to isulate the decoder from the loco's metal body and when I removed the decoder to test it in my decoder tester I found the the heatshrink had started to shrink so its getting quite hot, is that normal for TTS Decoders.

Does it sound like mine is faulty (Not a good sign for my forst go with TTS) or something else I have missed, like TTS not playing well with Bachmann. Although it should be compatable.

Thanks for any help

Paul

[96RAF]

Most likely cause is the lighting load is too much for the TTS decoder functions output limit (100mA). These are not protected so will not shut down but just damage the decoder.

 

Try disconnecting the lights and see if the decoder recovers for motor control and sound. I got lucky with a Class 37 DIY lighting kit that was over the limit  by adding an extra 1K resistor in the blue feed.

 

My TTS67 decoder has been happily running loco and lights for many months now without any trouble.

 

Rob

[2e0dtoeric]

I used a 12 to 6pin adaptor - ??

Bachmann are usually 21 pin, and TTS decoders are 8 pin. That incorrect adaptor might be the problem! (or was that a monster typo?)

[BRGreenDieselPaul]

Oops

Yes monster typo

21 to 8 pin adaptor obviously  😳

[Johnboy99]

Sorry but what do TTS stand for?

thanks

[Chrissaf]

Twin Track Sound.

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As an analogy think of it like stereo audio where you have two separate channels. However in TTS, one track (channel) is used for the background engine sound. Whilst the other track (channel) is used for the spot sounds like toots, whistles and beeps etc.

[Johnboy99]

Thanks Chris. I am getting there I now have 7 locos with dcc decoders fitted,just ordered chips for two ones with sound.

[BRGreenDieselPaul]

Re the suggestion its the LED's in the bachmann loco overloading the decoder, does the Hornby Class 66 not have the same number of LED's at each end as the Bachmann ie 3 white and 2 red at each end meaning no more than 5 on at any one time, unless they all come on when they shouldn't like on mine.

Wouldn't have thought 5 LED's would come anywhere near 100mA or would the decoder be damaged if it was put in a Hornby Class 66?

Just getting my head around the logic

Paul

[Howbi]

LED's are rated at 20ma each with a peak forward current of 30ma so 5 on at a time is at least 100ma therefore it is possible to compromise the TTS decoder........as RAF96 suggests, a small resistor in the common return could solve the problem..........HB

[BRGreenDieselPaul]

So I assume the same issue would arrise if its fitted to a Hornby Class 66 then, as that has the same number of LED's

What size resistor do you recomend to be inserted in the blue common return

Paul

[Chrissaf]

Start with 1K (1,000 ohms) and work up in about 1K increments until you get the LED brightness level you are happy with.

.

Although HB is correct that the written specification for a typical LED is a nominal current of 20mA with a peak current of 30mA.......it is rare for a manufacturer to design a LED circuit to operate at or near maximuum LED specification parameters. I would expect actual LED currents to be nearer a nominal 10mA per LED in a well designed circuit. My own custom circuit designs are based on a nominal 5mA with a peak of 10mA per LED.

.

However, that said. These TTS decoders do seem to drive attached LEDs at the higher end of the LED current range.

[96RAF]

I would have thought that if the spec of an led is 20mA then it will drve at that if it is available, else drive at the restricted current if other restrictors ar ein circuit. As the TTS decoders can supply up to 100mA on a function then that is what the leds will try to pull if so allowed.

I know from experience that 5  x 3mm leds on one function output can cause a TTS decoder to throw a wobbly.

Rob

[96RAF]

Thinking about what I wrote earlier, maybe I should have said that leds obey Ohm’s Law in that the current they draw depends upon the voltage applied and this applied voltage is derived by the supply voltage modified by any resistors in circuit.

 

Hence a supply voltage of say 12 volts from the output of a decoder function needs to be tamed by a resistor or resistors to bring the led voltage within that shown on the spec sheet, this voltage/current varies with led colour. Once the specified led voltage has been achieved then the spec current will be seen.

 

How you get within these spec values is up to you to derive based upon the number of various leds in circuit and whether they are in series or parallel or both, all within Ohm’s Law.

 

I presume you are now totally confused.

Rob

[tnttrucker]

Hello, I have the Bachmann 66 Evening Star and I have fitted a 67 TTS sound chip using a 21 to 8 pin adaptor and everthing works fine including the Cab light using function 25. Can I ask if there is a possiblity that the 8 pin chip is in the wrong way round?

[96RAF]

If the decoder is reversed then the blue common positive will be on the green aux negative wire and the yellow and white wires will be reversed therefore the lights won’t work and whatever is on the Aux function will only work if it is not polarity sensitive, e.g. bulb, smoke or micro-motor, etc.

 

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[Fishmanoz]

Rob and all, when applying Ohms Law, you’d best keep this in mind:

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So if the maximum LED current is 30mA, this is the current value to use.  When Chris says he limits the current to 5mA or 10mA, then you plug that current in.

[Fishmanoz]

Also remember that where you have a resistor in series with the LEDs and a resistor in the blue common circuit, R is the sum of the values of these resistors.  And the power rating of the resistor in the blue circuit must be sufficient to handle the total current flowing in each of the LED circuits added together.

[Chrissaf]

This YouTube video explains Fishy's post using practical experimentation with one scene showing what happens when you don't limit the current through a LED, as explained in the text part of the screen capture in Fishy's post.

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It is the best YouTube video I have yet found explaining LEDs in a voltage / current circuit.

.

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PS - I would never recommend running a LED at its maximum current i.e 30mA. There is no discernable reduction in brightness running it at 20mA or at 10mA for that matter, but the life span of the LED at lower currents is vastly improved, to the point where it will probably outlive both you and I. Apart from which, the less current the LED is consuming, the less the electrical strain is on the component providing the current. In this case the DCC decoder.

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To put both Fishy's post and my own into perspective. Most will be aware that LED products for model railways typically are specified as 12 volts (or thereabouts) with a 1K (1,000 ohm) current limiting resistor. The forward bias voltage for a White LED is nominally 3 volts and a Red LED 2 volts give or take a bit. If you apply the Ohms Law formula in Fishy's post to those parameters, then on a 12 volt supply you get currents of 9mA and 10mA respectively for the White and Red LEDs. These 9mA and 10mA currents fit in with my documented LED current recommendations that I have posted many times.

[Fishmanoz]

QED

[96RAF]

All would have been clearer had I added the word forward to my post above when talking about led voltages.

I was going to post the same diagram but it wasn’t on the pc I was posting from.

interesting what Chris says about brightness as I run my street lights from the speed knob of an old analogue controller (HM2000) and I can dim them right out.

Rob

[Fishmanoz]

May I suggest still not quite Rob.  LEDs, just like all Ds (diodes) are switching devices.  They are open circuit to reverse voltage and remain so until forward voltage exceeds their threshold, after which they are short circuit.  That is until the circuit current exceeds the maximum they can handle, at which point they fail.

 

The circuit resistance you put in series with them is not there to tame voltage, it is there to limit current to within the LED‘s current rating.  You can avoid using resistors for this job but only if you power them from a properly designed constant current supply, not possible (practical) if you are using a decoder function circuit.

[96RAF]

I stand corrected Fishy. The dividing resistor brings the supply voltage down to that required by the led, all in accordance with th circuit diagram above.

Rob

[BRGreenDieselPaul]

Hi all

Replacement decoder back and installed, resistor added as suggested above and all working well now.

Thanks guys

Paul

[RDS]

Is that a recommendation then? 

A resistor in the blue lead because I am about to install a Class 47 TTS chip in my Bachman Class 47 (when my 21 to 8 pin converter arrives)

[Fishmanoz]

It is if you would otherwise exceed function output current limits by having a lot of LED lighting connected.