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Simple Short Circuit Protection

My vintage power supply from Japan did not have short-circuit protection. I couldn't deal with this, as the brass models were also ancient and shorting out all over the shop. A little googling presented me with Homemade Circuit Projects: Short Circuit Protection. This article has a nice 'mechanical' short-circuit protector. A relay is energised to begin the power output and, on short, is tripped, cutting the supply.

There are no integrated circuits whatsoever inside this power supply, so I thought I'd go the mechanical way.

The circuit

A relay is used to allow power from the source to the output. This relay is initially de-energised. The momentary push-button, when activated, will provide power across the coil in the relay, energising it. There is a secondary feed to the relay's coil which goes via its switched output. As the relay switches, this secondary power is activated and the current then also flows across the coil. This provides a sort of infinite loop where the power through the relay contacts infact keeps the relay energised.

A short on the output causes a drop in voltage. This voltage is required to keep the relay energised, so a drop will actually de-energise the relay and cut the output power. The push-button will need to be activated once more to re-energise the relay.

Circuit placement inside the power supply

This circuit needs to be placed after the bridge rectifier as it requires DC power. It also needs to be placed prior to the direction switch, as the polarity needs to be constant and one-way around. This leaves us with the choice of putting it before or after the throttle potentiometer.

I had originally placed it between the rectifier and the potentiometer. I wanted the continuous voltage, otherwise I'd only be able to energise once the throttle was more than zero. Unfortunately, in this position I found that it only tripped when the throttle was at maximum. It seemed that the potentiometer was capable of absorbing the short if any resistance was provided. Prolonged exposure to a short would no-doubt heat the coil of the potentiometer and cook it.

Moving it to the other side of the throttle worked perfectly. Even at throttle setting '1' it would trip. Of course, it would only energise when on throttle '1' or higher. I considered adding more components to allow a secondary source to trigger the relay, but found that I could adjust my usage of the power pack. Now when wanting to drive trains, one must shift to low throttle, hit the trip switch and then throttle further.

Required Components

Based on the diagram above, you'll need 2 100uf electrolytic capacitors at 16v or higher, a diode capable of the amperage/voltage, and a relay. The relay should be DPDT if you also want the status light.

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As above, you can twist the diodes together in parallel to make them suffer higher currents. I used 1A diodes and wanted to make sure they could cope with the 2A transformer... but then I got bored and twisted 4 of them together. You can then see my prototype hookup to see if it all worked.

Status Light

I wanted to know when the switch was in trip mode. I didn't want to have to pull my multimeter out each time to work out where a fault was when no power was available. To do this, I used the second side of the relay to provide a 12v source to the led inside the momentary switch. This would light in the normally-closed position. As mentioned above, the relay only provided output power when energised (Normally-Open), so I wanted the light on the NC side so as to illuminate when the power was cut off.

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The switch indicated that it was rated at 12-volt and 240v. I took this to mean the internal lamp/LED? was 12v and the contacts could switch 240v. I didn't want to risk it though, so I put a 470ohm relay in series to limit the current. The resulting brightness worked perfectly well.

Case Modifications

Adding the switch wasn't going to be easy. The nicest one I could find had a huge barrel. I used the smallest drill-bit I could find and then proceeded up. I hate cutting in to metal... always worried something will go flying and take a limb off. The metal shavings aren't much fun either.

Fortunately my friend has a dremel... so we hacked away with that and made the switch fit. The best location was on the front panel. I do like the finish.

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End result

It works perfectly. Setting the throttle to '1' or higher lights the switch... it always starts in trip mode and needs to be activated. Pressing the button switches modes and the relay clicks. The LED switches off and, depending on the position of the direction switch (and the power requirements of the locomotive on the track) the locomotive will start to move.

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In fact, it shorted as soon as I tried to apply power to my ED100 on the test track. Turns out an axle had dislodged from its mount hole in the bogie frame and was happily shorting. After a realignment, all worked well. The bogies on these Tenshodo/KTM models have quite a bit of flex. The lateral screw isn't 100% tight and so the bogie frames can skew.

28Aug/150

Vintage IMAI.S.S. Power Supply Voltage Conversion

I've recently returned from Japan and have brought a loot with me. Whilst in Tokyo, there was the annual Antiques Jamboree at Odaiba and I happened across a box of HO KTM brass that I could not refuse.

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This set included a lot of track, two powered locomotives, rolling stock and a power supply. Thanks to the age of the kit, the power supply was 110v only. Preliminary testing in my Tokyo hotel proved that everything was in fantastic working order. I assume this box has been in someone's cupboard for quite a while.

Don't plug anything rated at 110v into Australian sockets

You don't want to try this at home. Our powerpoints are more than double the voltage of that in Japan... I wonder if this explains why we have a ground line. Either way, any Japanese equipment that is not dual-voltage will bake if you plug it in. This beautiful old transformer would never handle it's new home.

I had the option of chaining in a power converter, but this would have been tedious. In fact, this is my first 110-only product and I would've needed to purchase a step-down transformer. Instead I chose to pull the thing apart and replace the transformer.

What were you expecting? Integrated Circuits?

I opened up the power supply and felt that I'd opened a treasure chest. I could pick out the potentiometer for the throttle, the switch for the direction and the transformer. The final component needed to be a bridge rectifier to convert the AC to DC, but it ... didn't look like one at all.

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Following the wires, it seemed that there were 5 contacts on the device I assumed was the rectifier. A little googling informed me that this is a vintage metal rectifier with selenium insulation acting as the diodes. Very cool actually.

Purchasing a replacement transformer

Jaycar is my local electronics store of choice (if not the last available store in Australian cities) and perusing their site came up with a valid substitute.

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There were quite a few to choose from, actually. As the amperage went up, so did the weight. I assume the copper coil mass dictates how much current the transformer can deal with.

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Transformer transplant

This all went quite swimmingly. The existing wiring was labelled for safety-sake.

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It turns out that the 110v input wires were hardly twisted and taped together. I would hope that this wasn't an original factory job... but as far as I know this is a Japanese brand and this should have been the original factory plug. Quite interesting... let alone dangerous.

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Once the 110v transformer was out, the new transformer was soldered and then bolted in.

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It just fit. I mean.. a few millimeters more and we would've had to determine if Jaycar does returns.

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The 240v feeder was passed in and the wires were heatshrinked, the rest were soldered in place as per the previous construction.

Plugging it in...

With breath held, I plugged the transformer into the power board. It worked. The light lit dimly and no smoke came out.

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I rigged up a quick test track from the box of treasure purchased in Japan and tested the KTM ED100 that also came with it. After a little wheel and track cleaning the locomotive started bolting along.

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Success! This transformer is solid... although has zero short-circuit protection! Smoke started to appear during a motor short.. but I couldn't determine the source... I'll make bloody well sure in the future that shorts don't last too long.

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The directional switch is momentary unless you force it to either extremity. This is a nice touch as you can set your speed and then pulse the direction as required.

After running my brass EF62, I found a screw left on the track... the downfall of brass locomotives is that they're bound to shake things loose eventually!

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I was nothing short of ecstatic to have found this box at the antique fair in Tokyo.

5Aug/150

Tokyo Freight: Shinjuku Station

So you're in the big smoke for the day and want to see a freight train? The best advice I can give you is to head on over to Minamisenju as Shinjuku sees very little activity. On the off chance you can't do this, please refer to the table below.

Timetables

The timetable covering Shinjuku station incorporates two paths through Tokyo. The major path is the branch to Sumidagawa Freight Terminal (aka Minamisenju) and secondly the less-busy line through Shinjuku Station to Shin-Tsurumi yard. It seems that trains choose a path south from Omiya and split at Tabata Station to either destination.

I'll just cover the freight through Shinjuku for now.

Southbound Omiya Tabata Shinjuku Shintsurumi
Service Origin Depart Pass Pass Arrive Comments
▲3080 ~ (▲3071) Sapporo 0205 0226 0254 0314 ▲ = X間-Y間 月曜日運休
(Not Mondays between Aomori[Junction]-West Hamamatsu)
3086 ~ (3075) ~ (3073) Sapporo 1135 1152 1212 1227
4070 ~ (5072) Sendai 0214 0236 0256 0326
8086 Sumidagawa !! 1449 1509 1528 !! = Starts at Sumidagawa at 1402 to Tabata by 1410.
8572 Utsunomiya 0209 0231 0251 0320
8588 Utsunomiya 0454 0512 0532 0554
Northbound Shintsurumi Shinjuku Tabata Omiya
Service Origin Depart Pass Pass Arrive Comments
(▲3070) ~ ▲3081 West Hamamatsu 0246 0301 0321 0340 ▲ = 日曜日運休 (Not Sundays)
(3090) ~ (5090) ~ 4081 Nagoya 0227 0249 0309 0334
(96) ~ (5096) ~ 4083 Nagoya 0446 0502 0522 0543
8089 Yokohama Hazawa 1528 1551 1611 !! Proceeds to Sumidagawa at 1743 after Tabata. NOT Omiya.
8171 Kawasaki 0300 0320 0340 0407
8585 Kawasaki 0515 0532 0552 0618