Electronic circuit breaker

TWRackers

Joined Dec 29, 2008
41
For non-model-railroaders reading this... the FWBR is needed because the polarity of the supplied power can be reversed. This way the LED will always be forward-biased whenever the polyfuse goes effectively open-circuit.
 

GopherT

Joined Nov 23, 2012
8,009
For non-model-railroaders reading this... the FWBR is needed because the polarity of the supplied power can be reversed. This way the LED will always be forward-biased whenever the polyfuse goes effectively open-circuit.
What are the typical voltages and current draws on a model railroad set? Is 12-0--12VAC common voltage range?
 

TWRackers

Joined Dec 29, 2008
41
What are the typical voltages and current draws on a model railroad set? Is 12-0--12VAC common voltage range?
The power packs I use, which are made by KATO and MRC, put out 0 to 12V with a reversing switch to swap the polarity. Neither output terminal can be considered to be the common side. Using the term DC to describe the output is iffy, because different throttle models use waveforms with a varying amount of pulse content to let the locos move slowly at low settings. I'm building my own which will use straight PWM.

But for the scales I have some experience in (HO and N), 12 volts (let's call it unipolar ^_^) is the max output for throttles, and the loco motors are designed for that, as well as any lights inside passenger cars and such. Even DCC (direct command/control, let's don't go there) is based on 12 volts on the track. I think Z scale (1:220) uses 12 v too, but I'm not certain. I've never seen Z-scale-specific throttles. T-scale (1:450, hard to find) is very specialized and runs on only 8 volts. It definitely uses special throttles. For bigger than HO, I don't know.

For current draws, I think my N/HO MRC throttles will source at least 2 amps at full throttle before internal limiting kicks in. The KATO throttles have a physical breaker. In practice, on our club layout, we can get by on the non-shinkansen tracks with 0.6 A PTC polyfuses, partly because we run the trains, even "fast" passenger trains, at prototypical scale speeds, nowhere near full throttle. That even handles one member's train with three steam locomotives probably running at scale 80 kph (50 mph).

On our shinkansen tracks, we have a couple of 16 or 17 car shinkansen with two motor cars (which are never the end cars). Those forces us to go up to 1.3 A for those tracks because they're run up to scale 320 kph (200 mph) speeds. During switching moves in and out of the shinkansen yard, we sometimes for short periods will have two trains on opposite sides of the same loop, but they're moving at low speed so one doesn't catch up to the other.

So far it's been working well for us, especially after upgrading the breaker box to polyfuses from glass-cartridge fuses. No more melted bogies.
 

GopherT

Joined Nov 23, 2012
8,009
Thank you Alec_t
I don't know how much current the train draws. The output of the transformer is 12-0-12 at 2 amps. I have several 12VDC relays so if that's a better way I would be grateful for any assistance in using them. Incidentally would 1 amp diodes be sufficient for D1 and D2?

You got me thinking so I pulled out an old train set and a simple 12-0-12 transformer (since my power supply was dead). Here is my plan, simulation says it latches and reset button must be pressed if currents above 2 amps occur.

image.jpg
 

Thread Starter

boatsman

Joined Jan 17, 2008
187
@ Gopher T
Thank you for your reply. From what I understand in your train circuit the function of the relay on the rhs is to change track direction. I don't recognize the circuit symbol next to the 680m resistor. Can you please explain to me how the circuit works? Thanks.
 

GopherT

Joined Nov 23, 2012
8,009
@ Gopher T
Thank you for your reply. From what I understand in your train circuit the function of the relay on the rhs is to change track direction. I don't recognize the circuit symbol next to the 680m resistor. Can you please explain to me how the circuit works? Thanks.
1) Bridge diode converts AC voltage to DC (with help of capacitor 3300uF).

2) dpdt relay reverses track direction (switch above relay determines direction)

3) 0.68 resistor is sensing current. (Current x 0.68 = voltage)

4) left op amp outputs what ever the voltage across the 0.68 ohm resistor may be.

5) a) right op amp outputs "high" left op amp outputs less than zener reference
b) right op amp outputs low when voltage across 0.68 ohm resistor exceeds zener reference.

6) device to left of 0.68 ohm resistor is an N-channel mosfet transistor (essentially a switch in this case).
When the gate is fed a high signal from op amp, the switch is on, when fed a low signal, switch is off.

7) the circuit should latch once the current across the 0.68 ohm resistor is exceeded, my simulator has been glitchy since the last update so I assume it all works but no promises.

8) remove the short circuit and press the reset button to start again. The red LED shows when the right op amp is low and needs to be reset.


A second N-channel mosfet can be put in series with the short circuit n-channel mosfet to serve as a Pwm speed control.

You can google Pwm 555 timer circuit if you want to add that feature.
 

Thread Starter

boatsman

Joined Jan 17, 2008
187
Thank you Gopher T for the explanation. I have never worked with mosfets so the diagram was strange to me. Also your use of millioms and millifarads.
 

GopherT

Joined Nov 23, 2012
8,009
Thank you Gopher T for the explanation. I have never worked with mosfets so the diagram was strange to me. Also your use of millioms and millifarads.
Use an LM324 or similar (maybe mor modern) op amp that can handle input / output to supply ground.

Also, make sure n-channel mosfet is reasonably low ON resistance and can handle 13v gate-source voltage. Most will be less than one ohm

The mOhm and mF are automatically converted by the simulator - not my preference.

You can use a darlington NPN in place of the mosfet but you will give up some drive voltage across the track.
 

stancomm

Joined Jun 21, 2010
5
Gopher T
Thank you.
Hello gents;

I think that I might be able to offer another possibility.

Working on school buses, I was occasionally asked to repair the external light monitor. In the U.S. it's a panel just above the driver's head.
The older, just about bullet-proof monitors, used a reed switch with a coil of wire around it - a current relay - to turn on an LED indicating the condition of that particular circuit. If the external lamp (turn signal, brake light, etc.) was powered and not blown out, an indicator LED for that circuit would be lighted indicating that all was OK.
I would suggest using a similar current relay set to pull in at about something just over the typical locomotive startup current. The reed switch manufacturers data sheets include the ampere-turns required to pull the contacts closed.
Use those contacts to pick the DPDT relay of your choice. One set of N/O points will short the lead from the speed pot's wiper to circuit ground. The other set of N/O points will be wired in parallel with the reed relay's points through an N/C Reset Switch. The "OOPS, we've got a short" lamp is wired across the relay's coil. The reed switch will open as soon as the output voltage goes to zero.
This will stay in safe mode until the short is removed and the reset pushed.
Yea, it is very "old-world", but the bipolar current sensing and control circuitry tends to make things complicated in a unipolar silicon world.

Stan
 
Top