Customising a Standard Smoke Unit

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The Great Bear
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Customising a Standard Smoke Unit

Postby The Great Bear » Sun Jul 01, 2007 9:37 pm

Having previously fitted my GWR Castle 4-6-0 loco with a Seuthe No 10 smoke unit, I foolishly ran it dry only to find that when I put fresh smoke oil in, the smoking effect had gone. The unit does still work, but will only produce smoke at higher voltages so it’s no use in a loco anymore.

Whilst removing the smoke unit I noticed that some heat damage had occurred to the chimney and having discovered that replacement chimneys were not sold as spares, I was keen to prevent this from happening again.

So the aims for smoke box conversion No 2 were to overcome some of the shortcomings of the 1st attempt.
• A larger storage tank was required so it didn’t run out so quickly. A standard No 10 unit holds only 10-15 drops of smoke oil.
• A lower voltage unit would be used so the loco would smoke at a slower speed.
• A voltage regulator circuit would be added to prevent the smoke unit from getting over-volts. It was hoped that this would run the unit cooler and give a longer life.

A Seuthe No 9 smoke unit (8-14V) and a large bottle of smoke oil were obtained from “First Class Trains” and using the supplied info “for the advanced modeller” I set about the conversion. At this point I should explain that I do not consider myself “an advanced modeller” more one that is willing to “have a go”. After all, there’s a first time for everything!

The Castle has a large lead weight that occupies most of the boiler. Once removed, a screwdriver was used to tap out the smoke box door from the inside.
I measured the available space in the front of the Castle from smoke box door to lead weight and cut a piece of 15mm copper pipe:


This was squashed oval as it’s too big otherwise.

Next, another piece of copper pipe was slotted and flattened to form the ends.


The ends were roughly cut out. Once cleaned with wire wool and pasted with plumbers flux, the two ends were soldered on using a blow torch and plumbers solder.


You need to drill a hole in the oval before soldering else the expansion and contraction of the internal air sucks holes in your solder joints and the smoke oil will leak everywhere! The hole must be where the smoke unit is going to fit.

Once soldered, the ends can be filed to shape.


Next, I cut a slot in the side of the Seuthe smoke unit using a miniature rotary cutting disk. This wants to be the internal height of the storage tank so the smoke oil can access the tank. Alternatively you can drill two holes top and bottom but I didn’t fancy the drill breaking through and damaging the central steam pipe.


Next, a suitable hole (5mm in this case) is drilled through the storage tank where the pilot hole for soldering is at one end to accept the smoke unit. The smoke unit is cut to length and epoxy resin applied to the joints. (Do not cut the central steam tube – only the outer tube).


(Top joint not glued in this photo).


Testing it will fit.


It just fits! You can see the heat damage to the chimney here!

The smoke unit was wedged in place and a small ring of Araldite applied around the outside of the outer tube. The chimney was then snapped back into place. Once dry, the chimney is sealed to the smoke unit. Note that the chimney had previously been mated with a smoke unit. The bottom part was drilled out to 5mm so the smoke unit tube fits up inside the chimney by a few mm.

The wires face forward because I discovered that if they faced backwards, I couldn’t squeeze it in. This is not a problem as they can be bent round underneath the unit.

Power Supply Unit (PSU).

Once the storage tank was made, static tests showed the unit to “puff well” at about 10V dc. A further increase in voltage gave no additional benefit except (presumably) to run the element hotter thus decreasing its life.

Sadly 10V regulators are hard to come by. Experimenting using a 5V regulator (7805) and either resistors to increase its output as detailed in its datasheet, or a zener diode to lift its ground connection, I discovered that the output voltage did not track the supply voltage closely and that the greatest error was in the 5 - 8 V range where the smoke unit just starts to create smoke. This is important in order for the loco to smoke at the slowest speed possible.

Using a simple transistor regulator circuit alleviated this problem.


The transistor voltage regulator circuit works for dc only, thus the track power has to be rectified. A full-wave bridge rectifier like this


can be used, however each diode drops a good 0.7V. By making a rectifier from 4 schottky barrier diodes, a lower volt-drop can be achieved so the unit will smoke at a slower speed by comparison.

My controller provides a full wave rectified AC output – “lumpy dc.” It uses a 16V AC transformer, so its dc output is 12V max even though the peaks are at nearly 16V. The transistor regulator therefore also outputs “lumpy dc.” By incorporating a storage capacitor, the lumpy dc can be smoothed to the peak voltage thus increasing the output voltage and making the unit smoke at a slower speed.

The finished voltage regulator circuit is shown below:


As there’s no spare space in the loco, the plan was to fit the voltage regulator in the tender and run some wires between the two. The tender acts as a partial pick-up for the loco, the coupling making an electrical connection. This was deemed the common or return. The front bogey on the loco provides (or is supposed to provide) an additional pick-up on the live. (In fact it doesn’t because the wheel sets have insulated axles – Come on Hornby get your act together!)
I decided to see if the tender could be modified to pick-up on both live and return by obtaining another pick-up strip. This was obtained from East Kent Models, and some insulators made from a piece of photographic paper.


The tender has three steel strips to add weight. One paper insulator was placed between the two pick-up strips. The top pick-up, the new one, was turned over to pick up from the wheels on the other side. The other paper insulator was used between the top pick-up strip and the steel weights.

The voltage regulator circuit was mounted on a heat-sink and bolted to the top steel strip. Although I insulated the transistor from the heatsink, I wanted to make sure there would be no short circuits – hence the top insulator.


This picture was taken during the testing phase. I drilled a small hole besides the fixing screw and fed three wires twisted together through to the underside. The wires carried the positive and negative supply for the smoke unit and the new positive track pick-up.


A small header was Araldited to the underside and the smoke unit supply fed through it. In this way the smoke box could be disabled if required. The twisted cable bundle was glued in position and routed across to the loco just below the coupling. A small hole (1.6mm) was drilled through the loco chassis to bring the wires into the motor area where they were connected up to the smoke units leads and the motors positive terminal.

No damage during the mod thankfully:


Fitting the extra pick-up in the tender adds to the loco’s slow running ability although this was already good. There is another reason. If I remove the extra track pick-up feed from tender to loco, the smoke unit will be powered entirely from the tender’s wheels. When the train is stationary, track isolation would enable just the tender to be powered. In this way the smoke unit could be kept hot and would puff earlier as the train accelerates away!


Ultimately this was a lot of work and I guess only time will tell if the smoke unit lasts longer. However:
• The storage tank I would say is a big success. It holds over 2ml of oil, enough to smoke for hours and was not too hard to make if you plan it carefully and check it fits before you glue it into position. It seems to act as a heat sink and along with the regulated power supply helps prevent heat from reaching the chimney. The chimney barely gets warm now whereas it used to get quite hot before.
• The Seuthe smoke unit produces a great puffing effect although it isn’t synchronised to the running gear.
• The oil doesn’t leak out everywhere.
• I’m hoping to get a miniature connector to allow the tender to be uncoupled from the loco although this is not an issue for me.


1. The wires leading from the smoke units are very delicate and can break off all too easily. Consider that they can be bent only once and take great care else you’ll be buying another unit.
2. Don’t look down a smoke unit when testing it. They can spit hot oil in your eye as they start up.

Further Modifications

Once the unit was finished I obtained a couple of 1 Farad 5V Super capacitors. These have very low internal resistance and therefore charge quickly and supply decent amounts of current. The two 1000uF capacitors were replaced with the two supercaps wired in series. The power supply was modified to add a current limit resistor else the controller is almost shorted until the capacitors are charged. When the smoke unit is running off the capacitors, the current limit resistor is bypassed by a diode to reduce the volt-drop.
The modified circuit is shown below:


After a suitable run at full speed, the capacitors keep the smoke unit going for 10-15 seconds longer than the previous supply. This allows the train to keep smoking whilst it slows down to a stop. A lot of extra expense for a small gain but more fun!

A short clip of the Castle in action.

Regards Tim (GB)


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Postby saslord » Mon Jul 02, 2007 10:58 am

A good comprehensive article.

Good Job


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Bushey Troughs
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Postby Bushey Troughs » Mon Jul 02, 2007 12:08 pm

Nice piece of work Tim. Interesting read too excellent.
“What experience and history teaches us is that people and governments have never learned anything from history, or acted on principles deduced from it”


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Postby headshot119 » Mon Feb 11, 2008 9:10 pm

Would this be possible with DCC???

The Great Bear
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Postby The Great Bear » Mon Apr 14, 2008 7:57 pm

The storage tank is a useful mod for any Seuth/tube type smoke unitsimply because it stores more smoke oil and acts as a heat sink but I don't know if the power supply in the tender would work at all or is even necessary with DCC. I know next to nothing about DCC but thought I read that some decoders have a high current auxiliary output that will power a smoke unit? I'm sure there's lots of info in the DCC section.
Regards Tim (GB)

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Postby santene » Mon Apr 14, 2008 10:00 pm

how much are the smoke creater things? i might put one in my super detailed building
proud modeller jake also known as Duke Jake the Bewildered of Mousehole by Sea :D

anyone got n gauge track i can buy

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Postby Martin71 » Sun Apr 20, 2008 3:23 pm


Now that was well worth the read excellent and some great ideas. Was it worth it? of course you got to practice your skills and we all learned something new!

Thank you


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Postby pete12345 » Sun Apr 20, 2008 6:50 pm

You can use smoke generators on DCC as long as you have a high current auxiliary output on the decoder. To switch the smoke unit on or off you just activate the function with the controller. The smoke will then switch on, irrespective of whether the loco is moving or not.

Once an engine attached to a train, was afraid of a few drops of rain...

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