What fuse/circuit breaker for 12VDC500MA

[mwalther]

Okay, I just shorted out another 500MA-12VDC power supply ($3.95 at partsexpress.com) as I install my Tortoise control panel and add additional Tortoise and switches. I am going to have to repace it and track down the short, but before I do I want to put a fuse/circuit breaker in line to protect it. What type of fuse/circuit breaker should I get? I suspect that I want to get something higher than 500MA, but how high and does the voltage matter? Also I am assuming that I put the fuse on the DC output line, does it matter which polarity? I can't get trains running until I can operate turnouts. Thanks

[rrinker]

Fuses are not polarized. If this is a plug-in type power supply (wall wart), they usually have a fuse internally, as you've found out. There's no way to repalce it though. The problem with putting an external fuse on is you need some value that will blow BEFORE the internal one. You're not likely to find one at Radio Shack for this purpose. How many Tortoises are you powering? Each one draws about 12ma, so 40 will draw about half an amp, or 500 ma. If you are running 20 or less, try finding a 250 ma fuse, that will blow before the power supply fuse.
Or just find the short first - hook a battery and buzzer, or light bulb, across the wires where the power supply would connect. If it beeps, or lights, you have a short. Systematically work out from the power supply end disconnecting the switch controls until the beep stops or the light goes out, your short lies beyond this point.

--Randy

[mcouvillion]

mwalther,

500MA = 1/2 amp. Use a 1/2 amp fuse rated for 12 volts in the positive side of the circuit. Get a small fuse holder and position it so that you can get to it easily. You may be replacing fuses for a while.

Without knowing how you wired the Tortoises, and how many, it is kind of difficult to offer constructive advice. I would suggest that you demonstrate to yourself that you can properly wire and operate one Tortoise on the bench before tackling a yard full. There may be a fundamental problem with how you wired the panel or you may have too many switch machines on the one power supply. Tortoise tells you how many milliamps it draws during movement and when stalled. I don't remember the numbers, but you will need to add these up and see if you have exceeded either of those values. It may be that too many of the turnouts motors are moving at the same time, overtaxing the power supply. Try to stay below about 60-70% of the power supply's rated capacity and you should never have a problem.

Mark C.

[mwalther]

Thanks. I have 23 slated for this area and 1 power supply. I am only using wiring to power the Tortoise without using any accessory contacts. I am using 10-position cards and have had trouble shorting if card not positioned correctly ( I would recommend using position 1-8 leaving 9&10 unused where possible). I had all installed and working individually and was just wiring in the control panel. I'm suspecting a short at one of the 20 odd DPDT switches due to amatuer soldering, but it could be anywhere. I had been adding them in groups of 5 and problem occurred at 11-15 so I will start with them. I need another power supply, but will want to protect it.

[mwalther]

Thanks. I have 23 slated for this area and 1 power supply. I am only using wiring to power the Tortoise without using any accessory contacts. I am using 10-position cards and have had trouble shorting if card not positioned correctly ( I would recommend using position 1-8 leaving 9&10 unused where possible). I had all installed and working individually and was just wiring in the control panel. I'm suspecting a short at one of the 20 odd DPDT switches due to amatuer soldering, but it could be anywhere. I had been adding them in groups of 5 and problem occurred at 11-15 so I will start with them. I need another power supply, but will want to protect it.

[mwalther]

Thanks. I have 23 slated for this area and 1 power supply. I am only using wiring to power the Tortoise without using any accessory contacts. I am using 10-position cards and have had trouble shorting if card not positioned correctly ( I would recommend using position 1-8 leaving 9&10 unused where possible). I had all installed and working individually and was just wiring in the control panel. I'm suspecting a short at one of the 20 odd DPDT switches due to amatuer soldering, but it could be anywhere. I had been adding them in groups of 5 and problem occurred at 11-15 so I will start with them. I need another power supply, but will want to protect it.

[rrinker]

This is as good a reason as any to NOT use the edge card connectors. ALthough if you get the blanking plug inserts for them you won;t be able to install them incorrectly and cause a short. I prefer a more positive connection so i solder a 8-wire pigtail to the Tortoise and connect that to an 8-pin terminal strip. The rest of the wiring attaches there. There is also another idea which onvolves soldering pins for Molex-type connectors to the Tortoise, these again offer a much more solid connection than the edge card connectors.

--Randy

[rogerhensley]

I use a 9v 500MA AC Adapter – Radio Shack 273-1651D to supply power for my home made Torty and LED power supplies. I drive my LEDs at just short of 4v DC and the Torties at about 7v. This produces a great slow motion and I have never had any problems with either the LEDs or the Torties. I have currently 8 Torties in operation with 4 more to go. The advantage of the power supply other than control over the voltage? The source is isolated from the usage and can be easily fused at 1/2 Amp.

[rrinker]

Nice power supply there! I dig the 'retro' knobs. Looks like your parts bin is even older than mine [:D]. The 'No' marking is a nice touch too. There was a time when I would do exactly the same, but now I'm having a hard time reconciling the parts cost witht he fact that I have 3 or 4 perfectly good computer power supplies just sitting here that put out a nicely regulated 5v and 12v, with enough power to drive mre Tortoises than I could possibly fit in my basement.

--Randy

[rogerhensley]

I know myself. The 'No' is to remind me that LEDs can be turned into flash bulbs in spite of dropping resistors!

Yes, my parts bin goes back to the early 60s with some parts from the 50s.

[jkeaton]

One alternative to using a fuse or circuit breaker would be to use a 12V light bulb wired in series, as you would a fuse. I know with a homebuilt one amp throttle an 1156 taillight bulb works well. Doing this protects the power supply and gives you a nifty "overload" lamp without the need to keep replacing fuses. When the amp draw is low, the light bulb's filament is cool and has very low resistance, so you don't lose much power for the Tortoises - but have a short or an overload, the amp draw increases, the bulb's filament heats up and starts to glow, and it's resistance rises dramatically, limiting the total amp draw from the power supply. With a 0.5 amp supply you may need a 12v lightbulb that draws less current than an 1156 - someone else maybe able to comment on this.

Jim

[mwalther]

Thanks for that suggestion. I have a similar bulb wired in parallel across the leads of the power supply at a connector that I use as I fit/test newly installed Totoises. Wired this way the light is on until there is a short and the light goes out, but if you aren't watching to see this .... there goes another power supply. So to do it your way I still have to pay attention, but I get some protection. I'll try it.

[rrinker]

Get yourself a buzzer and clip leads and a battery at Radio Shack. Hook one side of the battery to a clip lead. Hook the other side of the battery to one side of the buzzer. Hook the other side of the buzzer to the clip lead. Disconnect your power supply. Connect clip leads to panel where the power supply used to hook up to. Now if you get a short while working under the layout you'll HEAR it.

--Randy