Hornby R8249 decoder circuit diagram

[Julian B]

Hi All

 

This is my first post so please excuse any mistakes i make.

Im a returnee to the hobby after rediscovering my childhhood model railway equipment in the loft durning lock down , Anyway to cut a long storry short and a lot of Peco code 75 Bullhead track , endless replacment wheels and the culdron of fire that is the change from DC to DCC i have a working layout    Some of my converted Lima and Hornby models really struggled to work with the combinaltion of the scale track and their poor pickups but i finally have them behaving . 

 

A few days ago i fitted a 4700uf cap to the R8249 decoder and it worked a treat.  The converted loco would crawl across un powered Frogs ( thats a term you dont hear too often ) without problem    but on day 2 of testing it failed .  I think ive recked the rectifier as i didnt put a current limiting resistor in series with the cap, i know, daft mistake .  The dc output  of the rectifier is only measuring 5 ish volts accross the cap   and i seem to think it was measuring arount 17volts DC when it was working.

I will probably replace the decoder but at £25 a go i want to see if i can fix the broken one . I do have some electronics knowledge and looking at the board it looks like one side is the power supply and the other is dedicated to black majic and voodoo  

 

Any help much appreciated.  If you have a circuit diagram for the R8249 could you let me know ?

[Butler Henderson]

Use a decoder that is designed for a stay alive to be fitted to it. The R8249 is a cheap nasty decoder not worth any where near it's price.

[Izzy]

First off you really need both a resistor and diode in circuit and in addition a Zener will help limit the current to the capacitor. If you look around online several circuit diagrams and component suggestions will be found. I use a 100R SMD resistor combined with a Schottky SMD 40v diode and a 16v Zener.

 

As to decoders well, I now only use Zimo. The MX600 at £25 is still available and will be far better. 
 

Bob

[RAF96]

Maybe you should have looked on Strathpeffer Junction site to see how to build a stay alive and see how he hooks it up to a TTS decoder which is essentially an R8249 with sound.

Hornby do not publish any of their kit wiring schematics in the public domain.

 

Edited February 8 by RAF96

[NIK]

Hi,

 

Just for a bit more detail, without a suitable current limiting resistor between bridge rectifier a typical DCC decoder bridge rectifier will be damaged by the initial current as the capacitor charges. A diode is needed between the capacitor positive back to the +ve on the bridge rectifier so the charge from the capacitor can be supplied to the appropriate bits of the decoder when contact is lost with the track.

 

Regards

 

Nik

 

 

[Pete the Elaner]

5 hours ago, Butler Henderson said:

Use a decoder that is designed for a stay alive to be fitted to it. The R8249 is a cheap nasty decoder not worth any where near it's price.

 

I agree. When I started with DCC, the Hornby decoder was £8 , which I felt was ok value when more 'standard' decoders from, the like of Lenz & Zimo were about £20. It has never been a vary feature-rich decoder though.

Now its price has crept up to £22-£24 & better spec decoders are around £25, I would much rather pay the (little) extra.

[Julian B]

Hi All 

Thanks so much for all the info 

 

Yes to the bit about limiting the current ( school boy laziness on my part )  although im not sure what the revesed biased diode across the resistor does although it wont hurt i guess ,   also i see the valid point of the zenner across the cap just to keep the supply limited   to a sensible level.

The one thing that i didnt see much mention of on the internet and forums was the fact that if you have a massive batttery like "stay alive" then the DCC decoder chip will happily work away regardless whether there is any connection to the host devivce , this means in effectt that if you issue a speed setting and then connection is lost, that speed setting is the one that is acted upon and any other command like" stop" is  obviously not recieved and therfore not processed.  Am i right with this or are the decoders looking for a heart beat signal ? 

With my very limmited knowledge i would therefor suggest that going for the smallest possible value of capacitor is the way to go.   Not sure if you will agree with that 

 

Also thanks for the info on the decoder chips  , i see that there is a lack of love for the Hornby boards   I think the comment that they are nasty rather harsh  but again i am new to this so alternate information on  non nasty electronics boards that you might reccomend is greatly appreciated

 

Thanks again

[WIMorrison]

19 minutes ago, Julian B said:

Am i right with this or are the decoders looking for a heart beat signal

 

For the decoder to get a 'heart beat' signal from the command station then that would be in the DCC signal and if it could 'hear' the 'heart beat' from the command station then you wouldn't need 'stay alive' 😉

 

Decent decoders allow you to limit the length of time a decoder will run without a DCC signal which you limit to 1 or 2 second - I couldn't say if the Hornby decoders support this common setting though.

[Nigelcliffe]

15 minutes ago, Julian B said:

Hi All 

Thanks so much for all the info 

 

Yes to the bit about limiting the current ( school boy laziness on my part )  although im not sure what the revesed biased diode across the resistor does although it wont hurt i guess ,   also i see the valid point of the zenner across the cap just to keep the supply limited   to a sensible level.

The one thing that i didnt see much mention of on the internet and forums was the fact that if you have a massive batttery like "stay alive" then the DCC decoder chip will happily work away regardless whether there is any connection to the host devivce , this means in effectt that if you issue a speed setting and then connection is lost, that speed setting is the one that is acted upon and any other command like" stop" is  obviously not recieved and therfore not processed.  Am i right with this or are the decoders looking for a heart beat signal ? 

With my very limmited knowledge i would therefor suggest that going for the smallest possible value of capacitor is the way to go.   Not sure if you will agree with that 

 

Also thanks for the info on the decoder chips  , i see that there is a lack of love for the Hornby boards   I think the comment that they are nasty rather harsh  but again i am new to this so alternate information on  non nasty electronics boards that you might reccomend is greatly appreciated

 

Thanks again

 

The diode allows full current to flow from capacitor into decoder during "discharge".   The limit on current will be the max current draw of the decoder (ie. the same current that it draws when running normally on DCC power).   
If you don't have the diode, but only the charging resistor, then the maximum current is limited by that resistor.   If the voltage on the capacitor were, say 15v, and the charging (and now discharging) resistor 100ohm, your discharge max current is 0.15A.   As quite a few motors need 0.2 or 0.3A, or more, there's not enough current to move the motor, which kind-of defeats the point.   

 

Better decoders (people keep mentioning those :-) )  allow for settings in the decoder to say "1 second without a DCC packet, then stop running because we've lost track signal".    Inferior decoders don't and just run-away until the capacitor empties.   

Better decoders can tell the difference between "DC from the track for DC running" and "DC from a capacitor for stay-alive" and respond accordingly,  others can't so you have to disable analogue (DC) running in those decoders.  

 

Buy yourself one Zimo decoder and see what it does:   some of them come with the charging circuit built in, so all that's needed is an appropriate capacitor.   Others you add the circuit yourself.    All of them are documented by the maker.   

 

 

 

[Julian B]

Hi All

 

Thought i would post a little update on the Hornby decoder i managed to break

 

Firstly i removed the rectifier diodes hopeing that these might be the cause of the issues . Alas this wasnt the case  .  When removed they read correctly . Anyone doing the same, these tiny thing have a very low foward bias voltage , only 0.2-0.3V

Whilst the diodes were off the board i hooked in a bench power supply and set it to 12 volts .  No significant current draw was noted

 

On the decoder board there is a surface mount 7805 voltage regulator and the 12 volts were being correctly regulated down to 5 volts for the processor chip on the other side of the board and that is exactly what i measured there .

 

So thats about it . i did return the model to the track with the bench supply still coneected ( rectifier removed )  but it still failed to respond to the id number that was previously programmed

 

Ive put the rectifier diodes back on the board and put the chip into the " requires more attention " plie .   Unfortunately i only have a Hornby Select controller ( funds are tight ) and that doesnt allow you to get to the cvs i believe  . Ive read that CV8 needs to reset to reset the chip?   

 

 

Anway  its been a learning experience and again thanks for all the help oh and if anyone can reccommend a dcc controller that can program CVs and that isnt the price of a new kitchen i would be interested to hear

 

Thanks again

[smokebox]

Get the Select upgraded to the latest firmware, v1.6, and you can then write to cvs.  It cannot read cvs unfortunately. 

Cost is £15 plus you postage cost to send it to Hornby.  You need to get a returns number before you send it.

[RAF96]

11 hours ago, smokebox said:

Get the Select upgraded to the latest firmware, v1.6, and you can then write to cvs.  It cannot read cvs unfortunately. 

Cost is £15 plus you postage cost to send it to Hornby.  You need to get a returns number before you send it.

The Select effectively programs on the main as it doesn't have a dedicated programming outlet, and without Railcom cannot read-back.

Edited February 10 by RAF96

[Nigelcliffe]

12 hours ago, smokebox said:

Get the Select upgraded to the latest firmware, v1.6, and you can then write to cvs.  It cannot read cvs unfortunately. 

Cost is £15 plus you postage cost to send it to Hornby.  You need to get a returns number before you send it.

 

For about the same amount of money, someone who is up to unsoldering surface mount parts, could probably put together a DCC-EX system - keep the costs down by using a 12v supply rather than needing two voltages.   That has a proper programming track.  

 

- Nigel

[Butler Henderson]

An alternative would be use Decoder Pro https://www.jmri.org/help/en/html/apps/DecoderPro/index.shtml and a SPROG https://www.sprog-dcc.co.uk/ on a decidated programming track - mine is on a short piece of wood along with the SPROG and its power supply allowing it be used convienetly with a PC / laptop